Ex Parte Schaller et al

Patent Trial and Appeal Board

Feb 25, 2015

11530338 (P.T.A.B. Feb. 25, 2015)

UNITED STATES PATENT AND TRADEMARK OFFICE
____________

BEFORE THE PATENT TRIAL AND APPEAL BOARD
____________

Ex parte RAIMUND SCHALLER,
ARMIN HOLZNER, RICHARD EHRENFELDNER,
MICHAEL HOECHTL, WOLFGANG KERN,
FRANZ STELZER, and ARMIN TEMEL
____________

Appeal 2013-002146
Application 11/530,338
Technology Center 1700
____________
Before CATHERINE Q. TIMM, GEORGE C. BEST, and
MICHELLE N. ANKENBRAND, Administrative Patent Judges.

BEST, Administrative Patent Judge.
DECISION ON APPEAL
The Examiner has finally rejected claims 1‒10, 48‒51, 64‒69, 72‒80,
and 82 of Application 11/530,338 under 35 U.S.C. § 103(a) as obvious.
Final Rejection (FR) (Nov. 25, 2011). Appellants1 seek reversal of these
rejections pursuant to 35 U.S.C. § 134(a). We have jurisdiction under
35 U.S.C. § 6(b).

1 Semperit Aktiengesellschaft Holding is identified as the real party in
interest. App. Br. 6.
Appeal 2013-002146
Application 11/530,338
2
For the reasons set forth below, we REVERSE.
BACKGROUND
Reactions between thiols and unsaturated organic compounds were
observed more than a century ago.2 Photocatalysis of such reactions was
observed by 1928,3 and a free radical chain-reaction mechanism was
proposed in 1940.4 At least as early as 1948, persulfate-catalyzed reactions
between thiols and a variety of unsaturated polymers (including
polybutadiene, polyisoprene, and emulsion-polymerized styrene-butadiene
rubber) were reported.5 More recent work has included, for example, the
photochemical crosslinking of polystyrene-polybutadiene-polystyrene block
copolymers with multifunctional thiols.6

2 N.B. Cramer & C.N. Bowman, Kinetics of Thiol-Ene and Thiol-Acrylate
Photopolymerizations with Real-Time Fourier Transform Infrared, 39 J.
Polymer Sci.: Part A: Polymer Chem. 3311 (2001) (citing Posner, 38 Ber.
646 (1905)).
3 F. Ashworth & G.N. Burkhardt, Effects Induced by the Phenyl Group.
Part I. Addition of Polar Reagents to Styrene and the Behavior of the
Halogenated Ethylbenzenes, J. Chem. Soc. 1791, 1797 (1928).
4 F.R. Mayo & C. Walling, The Peroxide Effect in the Addition of Reagents
to Unsaturated Compounds and in Rearrangement Reactions 27 Chem. Rev.
351, 387‒394 (1940).
5 G.E. Serniuk et al., Study of the Reaction of Buna Rubbers with Aliphatic
Mercaptans, 70 J. Am. Chem. Soc. 1804 (1948).
6 See, e.g., C.E. Hoyle et al., Thiol-Enes: Chemistry of the Past with
Promise for the Future, 42 J. Polymer Sci.: Part A: Polymer Chemistry
5301, 5324‒26 (2004) and references cited therein; C. Decker, Light-
Induced Crosslinking Polymerization, 51 Polymer Int’l 1141 (2002) and
references cited therein; C. Decker, Kinetic Study and New Applications of
UV Radiation Curing, 23 Macromol. Rapid Communications 1067, 1085‒91
(Cont’d)
Appeal 2013-002146
Application 11/530,338
3
The ’338 Application describes methods for producing cross-linked
elastomers through photoinitiated cross-linking of a latex with a thiol or a
selenol. Spec. Claim 1 is representative of the ’338 Application’s claims
and is reproduced below:
1. Method for producing a cross-linked elastomer
comprising:
producing a mixture of at least one latex and at least one
starter component for initiating a cross-linking reaction or at
least one latex that bears at least one starter component or
starter group,
the starter component or starter group comprising
at least one photoinitiator which through irradiation with
electromagnetic radiation in the ultraviolet and/or visible
spectral range forms a reactive species,
and the mixture includes at least one cross-linking
auxiliary comprising at least one of a thiol and a selenol; and
irradiating the mixture with UV light and/or visible light
to at least partially cross-link the at least one latex.
App. Br. 40 (Claims App’x) (some paragraphing and indentation added).

(2002); C. Decker & T. Nguyen Thi Viet, High-Speed Photocrosslinking of
Thermoplastic Styrene-Butadiene Elastomers, 77 J. Applied Polymer Sci.
1902 (2000); C. Decker & T. Nguyen Thi Viet, Photocrosslinking of
Functionalized Rubbers IX. Thiol-Ene Polymerization of Styrene-
Butadiene-Block-Copolymers, 41 Polymer 3905 (2000); C. Decker &
T. Nguyen Thi Viet, Photocrosslinking of Functionalized Rubbers, 8: The
Thiol-Polybutadiene System, 200 Macromol. Chem. Phys. 1965 (1999); C.
Decker & T. Nguyen Thi Viet, Photocrosslinking of Functionalized
Rubbers, 7: Styrene-Butadiene Block Copolymers, 200 Macromol. Chem.
Phys. 358 (1999).
Appeal 2013-002146
Application 11/530,338
4
REJECTIONS
On appeal, the Examiner maintains the following rejections:
1. Claims 1‒3, 8‒10, 48‒51, 64‒69, 75, 79, 80, and 82 are rejected
under 35 U.S.C. § 103(a) as unpatentable over the combination of
Dove7 and Warner.8 FR 2; Ans. 3–4.
2. Claims 4‒6 are rejected under 35 U.S.C. § 103(a) as unpatentable
over the combination of Dove, Warner, and McGlothlin.9 FR 5;
Ans. 3–4.
3. Claim 7 is rejected under 35 U.S.C. § 103(a) as unpatentable over
the combination of Dove, Warner, McGlothlin, and Park.10 FR 6;
Ans. 3–4.
4. Claims 72‒74 and 76‒78 are rejected under 35 U.S.C. § 103(a) as
unpatentable over the combination of Dove, Warner, and
Lipinski.11 FR 7; Ans 3–4.
DISCUSSION
Each of the Examiner’s rejections is based upon the combination of
Dove and Warner as describing or suggesting the elements of independent
claims 1 and 9. FR 2‒4, 5, and 7. As Appellants argue, App. Br. 18, the

7 US 5,691,446, issued November 25, 1997.
8 US 3,338,810, issued August 29, 1967.
9 US 6,329,444 B1, issued December 11, 2001.
10 US 2005/0171282 A1, published August 4, 2005.
11 US 2006/0253956 A1, published November 16, 2006.
Appeal 2013-002146
Application 11/530,338
5
Examiner has not provided sufficient facts and explanations to support the
combination of these references. We, therefore, reverse rejections 1‒4.12
The Examiner found:
Dove teaches a method for forming a cross linked
elastomeric material by mixing a natural rubber latex
(polyisoprene) with crosslinking starters and accelerators such
as thiol compounds and heating (at least partial crosslinking)
the compounds to produce a suitable dip forming latex (col[.]
12 ln[.] 33‒44) followed by curing the latex after article
formation using normal manufacturing procedures (col[.] 13
ln[.] 13‒22). Dove does not teach the use of radiation to cure
or the use of a photoinitiator for the accelerant.
FR 2‒3 (emphasis added).
The Examiner further found:
Warner teaches a method for crosslinking non-conjugated
polymers prepared from dienes such as isoprene (col[.] 2
ln[.] 31‒40) by a reaction of a thiol with the diene polymer
irradiated by UV light (col[.] 2 ln[.] 1‒15, col[.] 1 ln[.] 50‒66).
Warner teaches the inclusion of photoinitiators (col[.] 2
ln[.] 55‒60). It would have been obvious to one of ordinary
skill in the art at the time the invention was made to use the
thiol and photoinitiator of Warner to crosslink the non-
conjugated rubber of Dove, because both relate to crosslinking
irradiation treatment of nonconjugated rubbers with a thiol
compound presenting a reasonable expectation of success, Dove
teaches using known processing techniques for curing
prompting one of ordinary skill to look to related art, and doing
so presents a simple substitution of known curing techniques
(UV substituted for heat) yielding predictable results.
Id. at 3.

12 In view of the disposition of this appeal, the Examiner may wish to
consider the effect, if any, of the references cited in footnotes 2‒6 of this
opinion on the patentability of the ’338 Application’s claims.
Appeal 2013-002146
Application 11/530,338
6
The evidence in Warner does not adequately support the Examiner’s
finding that substituting UV curing for heat curing in the process of Dove
would have been reasonably expected to be a simple substitution yielding
predictable results.
Warner describes the material produced using its curing method as
producing clear, hard, solid materials. E.g., Warner, col. 1, ll. 29‒42; col. 3,
ll. 6‒11 and ll. 27‒28. Indeed, Warner states that “[t]he clear, colorless,
solid product of this invention due to its rigidity and inertness is superbly
suited for can coatings, pints [sic, paints], organic glass applications and the
like.” Id. at col. 1, ll. 39‒42.
Appellants also argue that a person of ordinary skill in the art would
not use a curing method that produces hard, rigid material in a process for
production of elastomeric materials. App. Br. 18; Reply Br. 5‒7.
The Examiner responds:
The combination of Dove and Warner is based on the similar
reaction taking place in the thiol crosslinking of natural rubber
in Dove and the thiol crosslinking of polyunsaturated dienes in
Warner. One of ordinary skill presented with the choice of UV
curing a polydiene or heat curing a polydiene would find it
obvious to perform the crosslinking with either method. That
the end use product of Warner is different than the end use
product of Dove does not detract from the teachings of the
crosslinking reaction.
Ans. 11 (emphasis added).
We are not persuaded by the Examiner’s argument. The Examiner
assumes that the reactions taking place in Dove and Warner are similar.
This assumption is not adequately supported. Given the vast difference in
the properties of the resulting materials, a person of ordinary skill in the art
likely would believe that the curing reactions in Dove and Warner are very
Appeal 2013-002146
Application 11/530,338
7
dissimilar. For example, the person of ordinary skill in the art might think
that the reactions might be proceeding through different mechanisms.13 The
Examiner has provided neither evidence nor a convincing argument to the
contrary. Thus, we cannot agree with the finding that a person of ordinary
skill in the art at the time of the invention would have used Warner’s cross-
linking method in Dove’s process for making an elastomeric material.
CONCLUSION
For the reasons set forth above, we reverse the Examiner’s rejections
of the each of the pending claims in the ’338 Application.
REVERSED

sl

13 Early work on reactions between thiols and unsaturated compounds
suggests that the acid-catalyzed and peroxide-catalyzed reactions have
different mechanisms. E.g., R. Brown et al., The Addition of Thioacetic Acid
to Unsaturated Compounds J. Chem. Soc. 2123 (1951); V.N. Ipatieff & B.S.
Freidman, Reaction of Thiol Compounds with Aliphatic Olefins 61 J. Am.
Chem. Soc. 71 (1939); V.N. Ipatieff et al., Reaction of Aliphatic Olefins with
Thiophenol 60 J. Am. Chem. Soc. 2731 (1938); S.O. Jones & E.E. Reid, The
Addition of Sulfur, Hydrogen Sulfide and Mercaptans to Unsaturated
Hydrocarbons 60 J. Am. Chem. Soc. 2452 (1938).
Notice of References Cited



 








 









 

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11/530,338 2013-002146
1700
N.B. Cramer & C.N. Bowman, Kinetics of Thiol-Ene and Thiol-Acrylate Photopolymerizations with Real-Time Fourier
Transform Infrared, 39 J. Polymer Sci.: Part A: Polymer Chem. 3311 (2001) (citing Posner, 38 Ber. 646 (1905)).
F. Ashworth & G.N. Burkhardt, Effects Induced by the Phenyl Group. Part I. Addition of Polar Reagents to Styrene and the
Behavior of the Halogenated Ethylbenzenes, J. Chem. Soc. 1791, 1797 (1928).
F.R. Mayo & C. Walling, The Peroxide Effect in the Addition of Reagents to Unsaturated Compounds and in Rearrangement
Reactions 27 Chem. Rev.351, 387394 (1940).
G.E. Serniuk et al., Study of the Reaction of Buna Rubbers with Aliphatic Mercaptans, 70 J. Am. Chem. Soc. 1804 (1948).
Notice of References Cited



 








 









 

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11/530,338 2013-002146
1700
C.E. Hoyle et al., Thiol-Enes: Chemistry of the Past with Promise for the Future, 42 J. Polymer Sci.:
Part A: Polymer Chemistry 5301, 532426 (2004).
C. Decker, Light-Induced Crosslinking Polymerization, 51 Polymer Int’l 1141 (2002).
C. Decker, Kinetic Study and New Applications of UV Radiation Curing, 23 Macromol. Rapid
Communications 1067, 108591 (2002).
C. Decker & T. Nguyen Thi Viet, High-Speed Photocrosslinking of Thermoplastic Styrene-Butadiene Elastomers, 77 J.
Applied Polymer Sci. 1902 (2000).
Notice of References Cited



 








 









 

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C. Decker & T. Nguyen Thi Viet, Photocrosslinking of Functionalized Rubbers IX. Thiol-Ene Polymerization of
Styrene-Butadiene-Block-Copolymers, 41 Polymer 3905 (2000).
C. Decker & T. Nguyen Thi Viet, Photocrosslinking of Functionalized Rubbers, 8: The Thiol-Polybutadiene System, 200
Macromol. Chem. Phys. 1965 (1999).
C. Decker & T. Nguyen Thi Viet, Photocrosslinking of Functionalized Rubbers, 7: Styrene-Butadiene Block Copolymers, 200
Macromol. Chem. Phys. 358 (1999).
R. Brown et al., The Addition of Thioacetic Acid to Unsaturated Compounds J. Chem. Soc. 2123 (1951).
Notice of References Cited



 








 









 

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a step supposed to be followed by either (1) the addition of water,
or (2) isomerisation to styrene, or (3) addition of phenylmethyl-
carbinol. He comments on the mechanism previously discussed by
Phillips (J., 1925, 127, 2567) dependent on the production of
Rl -C- from a halogen compound by the removal of the halogen
ion, but he retains the conception that, in such an ionisation, the
hydrogen atom It, accompanies the halogen ion.
Rl\ 4-
R;/'
R2 .. R, .. .. K2 .. ..
R,:C:H -+ R,:C* + H:C1: not R,:C:K -I- :C1: .. .. ..
: Cfl : ..
The inversion in the formation of phenylmethylcarbinol could be
accounted for (compare Fischer, Ber., 1907,40,489 ; Lapworth and
Mottram, Hem. Munchester PhiE. Xoc., 1927, 71, 63) if the removal
of the halogen ion were dependent on one of the frequent approaches
of the water or other solvent molecules, to the side of the a-carbon
atom away from the halogen, occurring simultaneously with a rare
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1796 ASHWOR!PE BND BmEfCHARDT :
polariaation of the molecule sufficient to ionise the halogen under
these conditions. This mechanism would also account for other
observations which Ward discusses, namely, that solutions of potass-
ium cyanide, ammonia, and potassium acetate in ethyl alcohol all
give a-phenyldiethyl ether as the main product.
Many investigators (compare Conant and Kerner, J. Amer. Chem.
Soc., 1924,46,232) have observed that phenyl and carbonyl groups
have similar effects on the halogen atoms in systems of the form
R*[CH,],*Cl (R=phenyl, carbethoxyl, etc .), phenyl being less
powerful than any carbonyl group, and the results of the present
investigation also show such similarities. It is to be noted from a
theoretical point of view, however, that while the general effect of
phenyl and carboxyl is in the same direction relative to hydrogen,
the conjugative effects will depend on the nature of the side chain
and the various factors may operate in different ways in different
reactions of halogen atoms in the side chain. The a-halogen in
alkylbenzenes forms with the nucleus a type of allyloid system
- d9--N (V) (Allan, Ozford, Robinson, and Smith, Zoc. cit.)
in which the polarisation of the nucleus assists the tendency of the
halogen atom to separate with a negative charge. This is in aocorcl
with the results of the investigations on the hydrolysis of substituted
benzyl halides (Ann. Reports, 1927, 24, 156), and also with the
capacity of the phenyl group for stabilising a positively charged
a-carbon atom which is clearly shown in the positive triphenylmethyl
ion. The opposite polarisation (type A, p. 1793) and the general
effect may account for its less powerful effect in stabilising a negative
charge in the sodium derivative of triphenylmethyl.
I n the case of the p-halogenated ethylbenzenes the effect of the
phenyl group on the halogen atom, here separated from it by two
single bonds, appears to be slight, judged by the low reactivity to
potassium iodide in acetone (Conant and Kerner, Zoc. cit.). Assuming
that the separation of the halogen ion is the first process in the
production of styrene by the action of aqueous-alcoholic potassium
hydroxide on w-bromostyrena, this reaction will be dependent on
the ease with which the electrons of the a-carbon atom co-ordinate
with the w-atom to form the double bond, as well as on the ease with
which the halogen atom is removed. The conjugation with the
nucleus (type C, p. 1793) will facilitate such co-ordination and the
removal of the proton will follow. On the other hand, this reaction
may be dependent on the tendency of the a-hydrogen to ionise,
which will be increhsed by the opposite type of polarisation and the
general effect.
cBr
I=/
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EFFECTS INDUCED RY THE PHENYL GROUP. PART I. 1797
The repetition of Posner's experiments gave the products which he
obtained, and the snlphone from the addition of phenyl mercaptan
to styrene was identical with that obtained by the action of sodium
benzenesulphinate on p-bromoethylbenzene. The sulphide obtained
from the addition was also shown to contain less than 2% of the
a-isomeride, by comparing the boiling point and refractive index with
those of the two synthetic sulphides. It is therefore clear that
phenyl mercaptan adds to styrene in the opposite sense to hydrogen
bromide and other polar reagents, and the theoretical considerations
which satisfactorily correlate those normal additions will not serve
in this case if the reaction takes place between the plarised ethylene
bond and the hydrogen and thiophenoxide ions.
I n a search for other evidence in favour of a special mechanism for
this reaction the course of the addition of phenyl mercaptan to
styrene was folIowed by observations of the very large contraction
in volume which takes place during the reaction and also bydeter-
minations of the depression of the freezing point of benzene caused by
iuixtures of phenyl mercaptan and styrene. The rate of addition is
greatly dependent on the intensity of illumination, and the increase
in rate produced by exposure to sunlight continues for some time after
the reaction vessel is returned to diffused light ; but the reaction was
not stopped in the dark and the product was always phenyl p-phenyl-
ethyl sulphide. A small percentage of piperidine had such a
powerful retarding action on the addition that no change in volume
was observed in diffused daylight even after several days, although
a contraction took place in sunlight. The action of light is well
known to produce or enhance abnormal polarisations and depolaris-
ations in a. wide range of reactions, and experiments are in progress
with a view to obtaining evidence in favour of one or other of the
possible alternative mechanisms for the interaction of mercaptans
with ethylenic hydrocarbons.
A remarkable isomerisation was observed in an attempt to pre-
pare phenyl- p-phenylethylsulphone (111) by reducing phenacyl-
phenylsulphone (VI) with zinc amalgam and hydrochloric acid : a
liquid sulphide was obtained which on oxidation with hot chromic
acid was converted into phenyl-a-phenylethylsulphone (I). A
similar migration of groups takes place when u-bromoacetophenone
is heated with yellow ammonium sulphide (Willgerodt, Ber., 1889,
(vT.) COPh*CH,*SO,Ph -+ (11.) or CHPh(CH,)*SPh (VII.1 -3 (I.)
E X P I< R I M E N T A L.
22,534).
Preparation of the Monohalogen Derivatives of Ethylbenzene.-
Phenylmethylcarbinol (b. p. 100"/15 mm. ; 3 : 5-dinitrobenzoate
Pu
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1798 ASHWORTH AND BURKHARDT :
m. p. 95O), made by the action of magnesium phenyl bromide on
acetaldehyde, was converted by gaseous hydrogen bromide into
a-bromoethylbenzene. This, by the action of silver acetatc
suspended in ether, readily yielded phenylmethylcarbinyl acetate
which on hydrolysis gave the carbinol free from any detectable
proportion of p-phenylethyl alcohol, as was shown by the properties
of its 3 : 5-dinitrobenzoate and the entire absence of the fragrance
associated with the isomeric compound.
The P-chloroethylbenzene used in some of the experiments
described later mas made by converting glycol chlorohydrin into its
p-toluenesulphonate and leaving this in contact with a cold ethereal
solution of magnesium phenyl bromide for 36 hours; about an
equal weight of a liquid, which was probably ethylene chlorobromide,
was also produced (see Ferns and Lapworth, J., 1912, 101, 273).
p-Bromoethylbenzene was made by heating 8-phenylethyl alcohol
with constant-boiling hydrobromic acid for 24 hours; it had b. p.
97"/12 mm., and was quite free from the pungent odour characteristic
of the a-bromo-compound.
Action of Aqueous-alcoholic Potassium Hydroxide on the Isomeric
Brmethy1benxenes.-The two bromo-compounds, when heated on the
water-bath under precisely similar conditions with aqueous-alcoholic
potassium hydroxide (about 0-4N), lost all the halogen (as deter-
mined by the decrease in alkaline titres) within 30 minutes. I n the
product from the a-bromo-compound, no styrene whatever could be
detected. The product from the p-bromo-compound was almost pure
styrene and absorbed bromine (1 mol.), yielding stlyrene dibromide.
Addition Reactions of #tyrene.-The styrene was made by
Howard's method from cinnamic acid (J., 1861, 13, 136).
(A.) Addition of hydrogen bromide. A mixture of styrene and a
cold saturated solution of hydrogen bromide in glacial acetic acid was
kept over-night and then poured into water, and the oil obtained was
distilled (b. p. S5O/lS mm.). The product was nearly pure a-bromo-
ethylbenzene, as it was converted, by means of silver acetate and
alkali successively, into phenylmethylcarbinol which gave the
characteristic 3 : 5-dinitrobenzoate (m. p. 95") and had no detectable
odour of the isomeric p-phenylethyl alcohol (3 : 5-dinitrobenzoate,
m. p. 108").
(B.) Addition of ammonium hydrogen sulphite. An emulsion of
styrene (20 g.) with N/Q-ammonium hydrogen sulphite (2 1.) and
kieselguhr (100 g.) was kept with frequent shaking for 10 days.
After filtration the liquid was boiled with excess of barium hydroxide
until ammonia was expelled and worked up for soluble salts (com-
pare J., 1925, 127, 307); 8 g . (yield, about 16%) of colourless
barium a-phenyZethanesuZphmate were obtained in bundles of small
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EFFECTS INDUCED BY THE PHENYL GROUP. PART r, 1799
needles [Found: H20, 6.8; Ba (in anhydrous salt), 27.1.
(C8H,SO,),Ba,2H2O requires H,O, 6.6%. (C8H,S0,),Ba requires
Ba, 27-0%]. No alkyl sulphite could be detected (compare Zoc. cit.).
A portion of the salt was converted into the amide, which separated
from alcohol in feathery crystals, m. p. 121".
The sulphonic derivatives were identical with those obtained from
the ammonium salt produced when a-bromoethylbenzene was heated
with aqueous ammonium sulphite for several hours (Found for the
barium salt : H,O, 6.9 ; Ba, in the anhydrous salt, 2643%. Amide,
feathery crystals, m. p. 121", mixed m. p. 121").
As a-bromoethylbenzene shows little or no tendency to become
converted into styrene, the foregoing results indicate clearly that
the sulphonic salt from styrene and hydrogen sulphite is the ac-deriv-
ative of ethylbenzene. The conclusion, however, was checked by
preparing the p-sulphonic acid.
p-Bromoethylbenzene (8 g.) was boiled with aqueous ammonium
sulphite, and the solution worked up as usual with barium hydroxide.
Barium p-pheng1ethanesuJphonate (5 g.) was isolated in large, glisten-
ing plates [Found : H,O, 3.4; Ba (in anhydrous salt), 26.8.
(C8H9S0,),Ba,H,0 requires H@, 3.4%. (C8H,S0,),Ba requires
Ba, 27.0%].
( C . ) Addition of phenyl m,ercaptan. The product obtained from
styrene and phenyl mercaptan by Posner's method (Zoc. cat.) gave,
on oxidation with cold dilute potassium permanganate solution and
excess of sulphuric acid, the pure p-sulphone, m. p. 56-57", identical
with that prepared from P-bromoethylbenzene.
Phenyl a- and p-phenylethyl sulphides and the sulphide obtained
by fractionating the addition product mere unattacked by hot or
cold neutral potassium permanganate, even when it contained
manganese dioxide in suspeiision. The addition of bromine or of
excess of dilute sulphuric acid caused rapid reduction of the per-
manganate. If the oxidation was carried out hot, a mixture of
phenacylphenylsulphone (VI), m. p. 94-95", and phenyl-p-phenyl-
ethylsulphone was obtained.
Phenyl-P-phenylethylsulphone (111) was prepared from P-bromo-
ethylbenzene by boiling with an alcoholic solution of sodium
benzenesulphinate for 12 hours and separated from carbon tetra-
chloride in feathery crystals, m. p. 56-57' (Found : s, 13.2. Calc.
for Cl4HI4O2S : S, 13.0%). The sulphones could not be prepared
from the chloroethylbenzenes, as the a-chloro-compound was con-
verted into styrene and the p-isomeride was unchanged.
Pure phenyl 01- and p-phenylethyl sulphides were prepared by
heating sodium thiophenoxidc (1.1 mols.) in alcoholic solution with
6- or p-bromoethylbenzene (1.0 mol.), pouring the mixture into
The sulphonamide melted at 124".
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1800 ASHWORTH AND BUILZ(HARDT:
water, and extracting the product with carbon tetrachloride. The
sulphides were distilled, heated for 3 hour under reduced pressure at
100" in a stream of carbon dioxide, and redistilled. The a-compound
had b. p. 163-164"/15 mm. and n, 1.6042; the p-compound, b. p.
188-189"/15 mm. and nD 1.6082.
The whole product of the reaction between styrene and pheiiyl
mercaptan was washed with alkali, dried, and ftactionated. The
distinction between unchanged styrene, sulphide, and a small
residue (probably metastyrene) was clearly marked. The middle
fraction (yield, go%), b. p. 187-190"/15 mm., heated in a stream of
carbon dioxide under reduced pressure a t 100" and refractionatetl,
gave the following refractive indices : first 6 drops, 1.6079; 9 c.c.,
1.6082; 2 drops, 1.6085; residue, 1.6088. Fractions 1 and 2 after
refractionation gave : first 2 drops, 1.6073; 10 drops, 1.6081 ;
thirteenth drop and the remainder, 1.6082, This indicates less
than 2% of ol-isomeride in the product.
Experiments on the velocity of the reaction and phutocatulysts.
During the reaction of styrene and phenyl mercnptan a lOy,
eontraotion takes place, and the course of the reaction was followed
by cooling a mixture of styrene (11 c.c.) and phenyl mercaptan
(10 c.c.) in a glass tube (0.2 to 0.5 cm. bore) in running water,
marking the level of the meniscus, sealing the tube, and marking
the level from time to time. As the velocity is influenced by light,
the two reagents were distilled in only the light of a Bunsen flame
(the mercaptan was distilled in hydrogen to prevent the formation of
disulphide), and two tubes were prepared as described above, marked
and sealed in a similar light, and left in a dark room. The part
of each tube containing the liquid was kept inside rubber tubing down
which water was running and the subsequent marks were made with
a minimum of light in the dark room. The two reactions proceeded
at the same rate. After 173 hours one tube was taken into sunlight!,
the water-cooling being continued. The contraction was nearly
doubled in the next 15 minutes and was practically complete after
1 hour. The other tube in the dark showed no signs of reaching
the same limit after 3 weeks.
Time (hrs.) ............... 0.1 1.5 2.6 14.0 17.5 17-75 18-0 18.95
Contraction (cm.) in
tube I in dark ...... 0.5 0.75 1-05 2.16 2.36 4-O* 4.4* 4.7*
Contraction (cm.) in
tube I1 in dark ...... 04 0.8 1.1 2.16 2.3
Time (days) ............... 1.6 2.5 3.5 4.5 6.5 8.6 22-5 26.5
Contraction (cm.) in
Contraction (cm.) in
tube I in light ...... 4.9 4.9 4.9
tube I1 in dark ...... 2.6 2.96 3.25 3.4 3.6 3-75 3-8 3.8
* In gunlight.
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EFFECTS INDUCED BY THE PHENYI, GROUP. PART I. 1801
The phenyl mercaptan remaining in the reaction carried out in
the dark was extracted in alkali and the sulphide which was distilled
out of the residue was shown to contain no detectable amount of
the a-isomeride by determinations of the refractive index and boiling
point.
A side tube was inserted about 70 em. from the lower end of the
reaction tube, the mixture was introduced until it overflowed from
the side tube, and a similar experiment was carried out entirely in
the dark. The speed was similar to that in the previous experi-
ments before taking into sunlight.
Cryoswpic evidence on the coume of the reaction [with CECIL
ROBINS]. The freezing points were taken of solutions of phenyl
mercaptan and of styrene in suitable amounts of benzene, and also
the freezing points of the mixed solutions, with a view to deter-
mining whether any considerable amount of association took place
quickly, followed by a photocatalytic depolarisation. No such
association was observed and the rate of combination appeared
to be in agreement with t'he velocity measured by contraction.
In this case, however, i t was clear that the reaction did not
slow down again at once when the tube was returned to diffused
light.
Action of Alcoholic Potassium Hydroxide on Styrene Dibromide.--
Styrene dibromide (Qlaser, Annulen, 1870, 154, 164) (26.4 g.) was
boiled in alcoholic solution with potassium hydroxide (1 mol.), the
alcohol removed by distillation, and the oily product distilled in a
vacuum. No fraction was obtained between or-bromostyrene and
styrene dibromide.
The product was almost instantly attacked by boiling formic acid,
the odour of acetophenone becoming perceptible. As p-bromo-
styrene was found to be very stable towards boiling formic acid,
undergoing but slight hydrolysis after 20 minutes, this observation,
which fhds a parallel in the ready hydrolysis of a-chlorostyrene by
hydrochloric acid, suggested a means of detecting p-bromostyrene
when mixed with the cc-isomeride.
0-774 G. of the lower-boiling portion of the product from styrene
dibromide was boiled for about 5 minutes with formic acid, and
sodium acetate and semicarbazide hydrochloride were then added ;
acetophenonesemicarbazone (0.653 g. ; 92 yo of the theoretical yield)
was deposited. It is evident that at least 92% of the oil consisted
of or-bromostyrene. No trace of p-bromostyrene (easily detected by
its powerful hyacinth-like odour) could be found in the liquor after
the acetophenone had been removed. A s a small quantity of
unchanged styrene dibromide was present, having escaped the
fractionation process, it is clear that removal of hydrogen
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1802 BRIERS AND CHAPMAN : THE INFLUENCE O F THE
bromide from styrene dibromide takes place almost solely in one
direction.
The authors gratefully acknowledge their indebtedness to
Professor Lapworth for suggesting this investigation and for his
advice and interest in it.
‘FEE UNIVERSITY, MAWCIFESTER. [Received, March 29th, 1928.1
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THE PEROXIDE EFFECT I N THE ADDITION O F REAGENTS
TO UNSATURATED COMPOUNDS AND I N
REARRANGEMENT REACTION3
FRANK R . MAY0 AND CHEVES WALLING'
George Herbert Jones Laboratory. Universi ty of Chicago. Chicago. I l l inois
Received J u l y 1. 1940
CONTENTS
I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
I1 . The addition of halogen acids to unsaturated compounds . . . . . . . . . . . . . . . . . 352
A . Normal and abnormal additions of halogen acids . . . . . . . . . . . . . . . . . . . . . . 352
. . . . . . . . . . . . . . . . 352
2 . The peroxide effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353
3 . The normal addition of halogen acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354
B . The peroxide effect in the addition of hydrogen bromide. . . . . . . . . . . . . . 359
1 . Products of addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
2 . Rates of addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
3 . Catalysts of abnormal addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
4 . Inhibitors of abnormal addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
5 . The bromine-atom mechanism of abnormal addition . . . . . . . . . . . . . . . . 372
6 . Other mechanisms proposed for abnormal addition . . . . . . . . . . . . . . . . . 377
7 . The influence of experiment. a1 conditions on additions of hydrogen
bromide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
8 . The addition of hydrogen bromide to cyclopropane . . . . . . . . . . . . . . . . . 386
I11 . The addition of mercaptans and thio acids to unsaturated compounds . . . . 387
A . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
B . The abnormal addition of mercaptans and thio acids . . . . . . . . . . . . . . . . . . 388
C . The normal addition of mercapt'ans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
1 . Status of the subject in 1930 . . . . . . . . . . . . . . . . . .
D . Scope of the mercaptan-alkene reaction . . . . . . . . . . . . .
A . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394
B . The addition of bisulfites to aliphatic unsaturated compounds . . . . . . . . . 394
C . The reaction of bisulfites with styrene., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397
V . The peroxide effect in rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399
A . Rearrangement of 1-bromo-2-butene and 3-bromo-1-butene . . . . . . . . . . . . 399
B . Rearrangement of a-bromoacetoacetic esters . . . . . . . . . . . . . . . . . . . . . . . . . . 401
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403
VI . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
I . INTRODUCTION
IV . The reaction of bisulfites with unsaturated compounds .
C . Cis-trans isomerizations . . . . . . .
The fact that oxygen and peroxides affect the direction of addition of
Subse-
' Present address: E . I . du Pont de Nemours and Company, Inc., Wilmington,
hydrogen bromide to allyl bromide was discovered in 1931 (53) .
Delaware .
351
352 FRANK R. MAY0 AND CHEVES WALLING
quent work has demonstrated that the same agents also determine the
direction of addition of various addenda to a large number of other ethylene
derivatives. Because of the many papers concerned with such phenomena
and the variety of interpretations offered, a critical review of the experi-
mental work and theoretical conclusions seems desirable. Recently, J. C.
Smith (139, 140, 141) has reviewed the field fully, but from a different
viewpoint.
This review is based on the premise that several types of substitution and
addition can proceed in solution by a t least two mechanisms: a chain reac-
tion involving free radicals and occasionally atoms, and an ionic (or molec-
ular) reaction. The chain hypothesis has shown that several apparently
different types of reaction are actually closely related, and has led to the
discovery of new syntheses. Although, in most instances, details of the
ionic or molecular mechanism are not yet clear, the data available permit
prediction of the products of many reactions.
This review will be concerned with the addition of hydrogen bromide,
mercaptans, and bisulfites to unsaturated compounds, and with certain
rearrangements. The discussion will refer almost entirely to liquid-phase
reactions. The important effects of oxygen and peroxides on the bromina-
tion and chlorination of hydrocarbons, acids, acid halides, and ketones,
on chlorinations with sulfuryl chloride, and on carboxylation reactions will
be omitted because rapid development of these fields has just begun.
11. THE ADDITION OF HALOGEN ACIDS TO UNSATURATED COMPOUNDS
A. NORMAL AND ABNORMAL ADDITIONS OF HALOGEN ACIDS
1. Status of the subject in 1930
Previous to 1930, many hypotheses were current regarding the factors
which controlled the direction of addition of unsymmetrical reagents t o
ethylene derivatives. Particular confusion existed with respect to some
addition reactions which could be used to help in deciding between con-
flicting hypotheses. It was with the hope of reconciling some of the dis-
cordant data that an intensive investigation of the addition of halogen
acids to unsymmetrically substituted ethylenes was undertaken in this
laboratory.
The addition of hydrogen bromide to allyl bromide seemed of particular
interest, as various workers had reported addition products ranging from
nearly pure l12-dibromopropane to nearly pure 1 ,3-dibromopropane, even
under supposedly identical experimental conditions. These investigators
ascribed the discrepancies to variations in t,he temperature, the reaction
time, or the concentration of hydrogen bromide, and to the presence of
water or light (for references, see 53). Other factors which have been
THE PEROXIDE EFFECT 353
thought to direct the addition of a halogen acid to an alkene are solvents
(46, 114, 117), their dielectric constants or internal pressures (44), magnetic
fields (17), and previous treatment of the alkene (for references on the elec-
tromer controversy, see 77). In the vapor-phase reaction, surfaces and
metal halides (15, 163, 164), oxidizing atmospheres (ll), and light (10)
have also been supposed to play a r61e.
It is noteworthy that most of the work on the addition of halogen acids
has been carried out with hydrogen bromide, since hydrogen chloride often
adds too slowly and hydrogen iodide frequently gives unstable addition
products. This choice is unfortunate, because hydrogen bromide is the
only halogen acid whose direction of addition to alkenes is affected by air
and peroxides, and therefore the only one which can give results of doubtful
significance. The following discussion will show the extent of this peroxide
effect with hydrogen bromide and the absence of this effect with hydrogen
chloride and hydrogen iodide.
2 . The peroxide eflect
The peroxide effect was discovered during an investigation of the addi-
tion of hydrogen bromide to allyl bromide (53). When these substances
are allowed to react a t room temperature in the dark, the reaction may
take either one of two courses:
CH3CHBrCH2Rr (1)
7
L
CHZ=CHCHzBr + HBr
CHzBr CHzCHzBr (2)
If the reactants are pure and freshly prepared, and if oxygen is excluded
from the reaction vessel, reaction 1 takes place exclusively2 and several
days are required for substantially complete reaction. If the reaction
occurs in the presence of small quantities of oxygen or if peroxides are in-
troduced either deliberately or by use of old allyl bromide, then reaction 2
takes place almost quantitatively in a few hours. As reaction 1 is that
which takes place with pure reagents in the absence of catalysts, i t has been
termed the normal reaction; reaction 2 is abnormal. Such a reversal of
addition has been called a “peroxide effect”, and it has been observed in
additions of hydrogen bromide to many ethylene derivatives. In order to
show that the normal and abnormal reactions take place by different
mechanisms, the characteristics of the normal addition reaction will be
discussed before the abnormal reaction is taken up in detail.
* Slight corrections (87) to the original work (53) justify this statement.
354 FRANK R. MAY0 AND CHEVES WALLING
3. The normal addition of halogen acids
The normal product of the addition of hydrogen bromide to an alkene
is uniquely and conveniently defined in terms of the addition of hydrogen
chloride and hydrogen iodide, since these halogen acids are not susceptible
to a peroxide effect and never add abnormally in the liquid phase. The
addition of hydrogen chloride or hydrogen iodide and the normal addition
of hydrogen bromide to an alkene or a halogenated alkene usually give a
single product, the one which would be predicted from Markovnikov's
rule (109). That is t o say, the halogen of the halogen acid always becomes
attached to the least hydrogenated of the.two doubly bound carbon atoms.
The product thus obtained is apparently the thermodynamically more
stable one of the two possibilities (2O).3 The hydrogen chloride and hydro-
gen iodide additions in tables 1 and 2 have been carried out since the dis-
covery of the peroxide effect. In many instances antioxidants or perox-
ides have been employed, or else the reaction has been carried out in a
solvent with the object of altering the composition of the addition product.
The addition of hydrogen chloride to propene has also been carried out in
quartz apparatus in the presence of ultraviolet light (94). With three ex-
ceptions, each of these additions yields only one addition product. One
exception is l-bromopropene, which with both hydrogen chloride and hy-
drogen iodide gives a mixture made up of one-third 1,l-dihalide and two-
thirds 1,2-dihalide, thus supplying convincing evidence that (within the
limits of experimental error) these two halogen acids always add in the
same way. Another mixture is that obtained by the action of hydrogen
chloride on 2-pentene. As will be shown in table 4, hydrogen bromide
behaves similarly to 172-dialkylethylenes. The third example of a mixture
is the addition of hydrogen chloride to 1,3-butadiene, resulting in 75 to
80 per cent of the 1,Saddition product and 20 to 25 per cent of the 1,4-
addition product. This proportion is not affected by temperature over the
range -78" to 25°C. or by the presence of acetic acid. An earlier com-
pilation (52) showed that in general the addition of hydrogen chloride or
hydrogen iodide to alkenes has been reported to give only one addition
product. Michael and Leighton (115) claim to have obtained a small
proportion of n-propyl iodide by the addition of hydrogen iodide to
Isopropyl and
propyl bromides isomerize in the liquid phase a t 250-275°C. to equilibrium mixtures
containing about 30 per cent of primary halide (15); isobutyl and tertiary butyl
bromides are known to behave similarly. The vapor-phase addition of hydrogen
chloride to isobutene (2-methylpropene) at 270°C. gives an addition product con-
taining mostly tert-butyl chloride but also about 8 per cent of isobutyl chloride (112).
Thus a portion of the product obtained by the addition of a halogen acid a t high
temperature disagrees with Markovnikov's rule. The nature of such reactions is
now under investigation in this laboratory.
3 These statements do not hold for temperatures above 200°C.
THE PEROXIDE EFFECT 355
CHsCHsCH=CHCHa <
propene, and Ingold and Ramsden (44) claim to have obtained up to
24 per cent of primary iodide by carrying out this reaction in certain sol-
vents. In this laboratory, repeated attempts (84) to confirm these claims
have failed. It has been found that peroxides accelerate the rate of addi-
tion of hydrogen iodide (62,84), but this effect is explained by the fact that
TABLE 1
The addi t ion of hydrogen chloride to unsaturated compounds
UNSATURATED
COMPOUND
Vinyl chloride.. . .
Trichloroethylene
Propene, . , , . , . , , .
1-Bromopropene..
2-Bromopropene..
1-Chloropropene..
2-Chloropropene..
Allyl chloride. . , .
Isobutylene.. . . . .
2-Pentene . . . . . . . .
Trimethylethyl-
ene. . . . . . . . , . . . .
A2-Pentenoic acid
A3-Pentenoic acid
A4-Pentenoic acid
Ala-Undecenoic
acid. . , . , , . . . . , .
1,3-Butadiene
1-Hexyne . , . . . . . .
2-Chloro-1-hexene
FORMULA
CHz=CHCl
C C12=CH C1
CH3CHxCHz
J
'i CH&H=CHBr
CH3CBr=CH2
CHaCH=CHCl
CH3CCI=CHz
CHzClCH=CHz
(CHa) zC=CIIz
CHz=CH(CHz) 8COOH
CHz=CHCH=CHz I
CH=C(CHz)3CHa
C H F C C l (CHz)sCH3
ADDITION PRODUCT
CH3CHCIz
CC13CHzCl
CHsCHCICHs
CH3CHzCHBrCl (35%)
CH3CHClCHzBr (65%)
C€13CBrClCH3
CHaCHzCHClz
CHaCClzCH3
CHzClCHClCH3
(CH3)aCCI
(50%)
(50%)
CHsCHzCHClCHzCH3
CH3CHzCHZCHClCH3
(CH3)zCClCHzCHS
CHsCHzCHClCHzCOOE
CH3CHClCH2CH&OOE
CHaCHCICHsCHzCOOE
CHsCHCI(CH2) 8COOH
CHaCHClCH-CHz
CHaCH=CHCHzCl
(75-80%)
(20-25%)
CHz=CCl (CIIt) aCH3
CH3CCls(CHz)aCH3
REPEREBCBIB
peroxides liberate iodine from hydrogen iodide and that iodine is a catalyst
for the normal addition of hydrogen iodide.
Because i t has sometimes been difficult to eliminate completely the
abnormal addition of hydrogen bromide, the use of hydrogen chloride or
hydrogen iodide as standards for the normal addition has been found con-
venient and reliable in doubtful cases (83). The normal addition products
of many alkenes are listed in table 3.
Present indications are that the addition of hydrogen fluoride t o alkenes
356 FRANK R. MAY0 AND CHEVES WALLING
also follows Markovnikov’s rule (32, 102, 143). It will be indicated later
that a peroxide effect with this reagent is very unlikely.
The available data justify only the following general statements as to
the relation between the rate of the normal addition and the structure of
the alkene. In the case of any one alkene, hydrogen chloride adds more
slowly than hydrogen bromide, and the latter usually adds more slowly
than hydrogen iodide. In the normal addition of hydrogen bromide (on
which the most information is available), substitution of the ethylene hy-
drogens by alkyl or phenyl groups increases the rate of addition, whereas
substitution by halogen retards the addition. Even substitution of hydro-
gen on an adjacent carbon atom by halogen retards addition. Some data
to illustrate these statements appear in table 5.
TABLE 2
The addit ion of hydrogen iodide to unsaturated compounds
UNSATURATED COMPOUND
~~
Vinyl chloride.. . . . . . . . .
Propene. . . . . . . . . . . . . . . .
l-Bromopropene . . . . . . . .
Allyl bromide. . . . . . . . . .
l-Butene . . . . . . . . . . . . . . .
Trimethylethylene . . . . .
4,4-Dimethyl-l-pentene
Undecenoic acid. . . . . . . .
Allyl chloride. . . . . . . . . .
8-Iodocrotonic acid. . . . .
Tetrolic acid. . . . . . . . . . .
FORMULA
C H F C H C l
CHICH=CHZ
CHICH=CHBr
CH2=CHCH2Br
CHz=CHCH*CI
CEIFCHCH~CH~
(CHI)zC=CHCHs
(CHI) sCCHZCH=CH:
CH?CH(CHz) 8-
CHsCI=CHCOOH
CHICECCOOH
COOH
ADDITION PBODUCT
CHICHIC1
CHICHICHI
CH3CHICHtBr (64%)
CHSCHzCHIBr (36%)
CH3CHICHzBr
CHaCHICHzCl
CHsCHICHzCHs
(CHI)ZCICH~CH~
(CHI) ICCHZCH ICHs
CHsCHI(CHz)&OOH
CHICI~CHZCOOH
CH,CI=CHCOOH
Since a large portion of this review will be concerned with the mechanism
of the abnormal addition of hydrogen bromide, it seems appropriate to
discuss first the mechanism of normal addition. Maass and his associates
have investigated the reaction of hydrogen bromide with propene and of
hydrogen chloride with propene and the butenes in the absence of solvents.
They find that those alkenes which (as indicated by the melting-point
curves of mixtures) form 1 : 1 complexes with halogen acids a t low tempera-
tures react around room temperature to give addition products more easily
than those which do not form complexes, that the addition iscomplicated
by a dimerization reaction so that rate equations could not be established,
and that excess halogen acid is more effective than excess alkene in acceler-
ating the addition (19, 106, 107). No electrical conductance could be ob-
TEE PEROXIDE EFFECT 357
served in these mixtures, and it was found that hydrogen chloride ac-
celerated the addition of hydrogen bromide (106). The rate of addition
of hydrogen chloride to propene increases with temperature from about
-80" to +45"C. Between 45°C. and the critical temperature of the mix-
ture (70"C.), the reaction has a negative temperature coefficient which
Holder and Maass (39) have ascribed to loss of orientation or "structure"
in the liquid. These liquid-phase reactions, as well as the normal addition
of hydrogen bromide to allyl bromide (53), are mostly or entirely homo-
geneous. Above the critical temperature, a t high pressure, the hydrogen
chloride-propene reaction is very slow, and has a positive temperature
coefficient (39). The vapor-phase additions of halogen acids to alkenes are
heterogeneous, bimolecular reactions (19, 39, 99), accelerated by metal
halides (15, 163). Equilibria and activation energies have been reported
in a few instances (48, 99).
Kinetic investigations of the addition of halogen acids to alkenes (113,
122) in inert solvents indicate that the reaction is largely, if not entirely,
of an order higher than the second. Consequently, inert solvents greatly
reduce the rate of addition. That the rates are lower in ether and in diox-
ane than in hydrocarbon solvents is apparently a consequence of combina-
tion of the halogen acid with the solvent (122). Among oxygen-free sol-
vents, the rate of addition increases in approximately the same order as the
dielectric constants of the solvents, but in the presence of water or car-
boxylic acids, the reaction may be faster than in the absence of a diluent
(53).
The existing data, inadequate as they are, nevertheless indicate that the
conventional mechanism of normal addition, as described by Robinson
(128) and Ingold (43), is an oversimplification. According to this mech-
anism, the proton of the halogen acid (with or without ionization) becomes
attached to one of the doubly bound carbon atoms, leaving the other as a
positive carbonium ion which subsequently reacts with the halide ion.
Ogg (123) has pointed out that such a positive ion would be configura-
tionally unstable and therefore inconsistent with known trans additions
of halogen acids to some ethylene bonds. He suggests that the negative
halide ion adds first to give a configurationally stable negative carbonium
ion which later adds a proton. Neither mechanism is satisfactory under
conditions where the reaction is of higher than the second order. Sher-
man, Quimby, and Sutherland (134) have discussed both a non-ionic
bimolecular mechanism and a chain mechanism for the normal addition in
the vapor-phase or in inert solvents. The chain mechanism is that to be
described in section 11, B, 5 for the abnormal reaction; reasons for rejecting
i t as a normal mechanism will subsequently become clear. No homo-
geneous bimolecular addition has yet been observed in the vapor phase,
358 FRANK R. MAY0 AND CHEVES WALLING
but the possibility of such a reaction cannot yet be wholly excluded for
the liquid phase.
Urushibara and Sinamura (151) have suggested a different chain mech-
anism for the normal addition of hydrogen bromide:
RCH=CH2 + H* 3 RCHCH3 (3)
RCHCK + HBr -+ RCHBrCH3 + H. (4)
Aside from the fact that this chain lacks any experimental support, i t must
be discarded on thermodynamic grounds. As has been mentioned else-
where (87), reaction 4 is endothermic by about 35 kg-cal., an impossible
condition for a step in a rapid chain reaction.
An explanation which seems to eliminate all of the above difficulties is
that the reaction occurs, not simply between halogen acid and alkene, but
between halogen acid and a halogen acid-alkene complex4 which may
contain a hydrogen bond. Such a mechanism explains why the rate de-
pends more upon the halogen acid than upon the alkene concentration
(19, 39, 106). Because the complex would be expected t o be less stable
at higher temperatures, the negative temperature coefficients observed
in some instances (39, 111) can be accounted for. The increased reaction
velocity in hydroxylic and carboxylic solvents may be due to the fact that
reaction is easier when either the complex or the halogen acid is ionized.
The effects of catalysts are consistent with this idea of the mechanism
of normal addition. Anhydrous ferric and aluminum chlorides are the
most powerful known accelerators for the addition of hydrogen chloride
and hydrogen bromide, and are particularly useful with those alkenes which
otherwise react very slowly. Tests have shown that these catalysts
affect only the rate and not the product of addition; in their presence only
the normal addition product is obtained (83) .5 Zinc, thallous, cobaltous,
and ferrous halides are moderate accelerators of the addition of hydrogen
bromide to allyl bromide; cadmium, lead, stannous, cuprous, and nickelous
4 Coffin, Sutherland, and Maass (18a) have previously considered this and other
possibilities, but have rejected them in favor of the hypothesis that reaction takes
place between two molecules of complex. They consider that the latter explanation
best interprets the retardation of some additions by excess alkene. This possibility
must still be admitted, but in view of the fact that the authors mentioned used no
solvents and could obtain no rate constants, the simpler mechanism seems preferable
for the present, since i t has some support from additions in solvents (113).
6 An anomalous case is reported by Schjhnberg (131), who considers that ferric
chloride is a negative catalyst for the addition of hydrogen chloride to the three
isomeric straight-chain pentenoic acids. That this is not a general rule for acids
is shoFn by the fact that normal addition to Ala-undecenoic acid is accelerated by
ferric chloride (139). Further work may clarify this question.
THE PEROXIDE EFFECT 359
bromides (87) and platinum black (152) are weak accelerators of the same
reaction. There has been little need to accelerate the addition of hydro-
gen iodide to alkenes, but mercuric iodide and free iodine have been found
effective (84). tert-Butyl isocyanide accelerates the addition of hydrogen
bromide to allyl bromide (53), vinyl bromide (54), and vinyl chloride (57).
This effect is probably due to the formation of some kind of an ammonium
salt by the reaction of the isocyanide with hydrogen bromide, since di-
methylammonium bromide and tetraethylammonium bromide have been
found t o accelerate the addition of hydrogen bromide to allyl bromide
(112). These various catalysts may alter the uncatalyzed mechanism
in several ways. The metal halide (or iodine) may replace one molecule
of halogen acid in the halogen acid-alkene complex; or by combining with
the halogen acid, it may activate the proton so that the latter more easily
forms complexes with alkenes. Whatever complex is formed, the activity
of substituted ammonium halides suggests that the alkene complex may
react with the halide ion of a salt more easily than with a halogen acid.
B. THE PEROXIDE EFFECT IN THE ADDITION OF HYDROGEN BROMIDE
I. Products of addition
Table 3 lists those ethylene and acetylene derivatives for which the
direction of addition of hydrogen bromide is known to be affected by
oxygen or peroxides. In many instances, both products indicated have
been obtained in a pure condition. In the others, more or less difficulty
has been encountered in completely suppressing one of the competing
additions, and mixtures have been obtained, their composition depending
on the experimental conditions. In the whole of table 3, there is only one
instance where it is well established that the normal addition product is a
mixture: l-bromopropene gives about one-third 1 , l-dihalide and two-
thirds 1 , 2-dihalide, a result already indicated for the additions of hydrogen
chloride and hydrogen iodide. In the presence of peroxides, hydrogen
bromide adds to give 1,2-dibromopropane exclusively.
Table 3 shows that peroxides alter the direction of addition of hydrogen
bromide to unsaturated hydrocarbons, halides, acids, and esters. Five
examples indicate that the effect apparently applies t o acetylene as well
as to ethylene derivatives. In most of the compounds cited, the double
bond is a t the end of the carbon chain, but recent work with trimethyl-
ethylene, 2-bromo-2-butene, and 2-methyl-2-nonadecene shows that the
effect is not confined to terminal double bonds.
Table 4 lists alkenes for whichit has not yet been possible to reverse the
addition of hydrogen bromide. This table refers only to studies carried
out since the discovery of the peroxide effect, where attempts have been
made to obtain more than one product. There is some uncertainty about
360 FRANK R . MAY0 AND CHEVES WALLING
. . . . . . . . , . . . . . . . . . . . . . . .
, . . . . , . . . .
THE PEROXIDE EFFECT 361
8
8
8
0
Y
2-A
II
x - q
Y
x
2
. . . . . . . . . . . . . . . . I : : : : : : I , . . . .
362 FRANK R. MAY0 AND CHEVES WALLING
N
9 x x"
E
9 9-
E
I x
h u-
. . . .
. . . .
. I . . . . . . . . . . . . . . . . . . . .
. .
. .
x"
u_
h
THE PEROXIDE EFFECT 363
364 FRANK R. MAY0 AND CHEVES WALLING
placing compounds in this table, because a past failure to obtain a second
addition product does not prove that future attempts will likewise be
failures. For example, early attempts to reverse the addition of hydrogen
bromide to styrene failed, but improved technique gave up to 80 per cent
of the abnormal product (162).
It seems well established that peroxides do not affect the direction of
addition of hydrogen bromide to the ethylene derivatives in section A of
table 4. Here the normal addition products are 50-50 mixtures, and com-
plete reversal of the addition gives the same result. Note that, as far as
directing tendency is concerned, methyl has the same effect as ethyl or a
long-chain alkyl with a substituent on the end.6 Thus there are few ex-
amples of a peroxide effect upon non-terminal double bonds. It should be
pointed out here that symmetrically substituted ethylenes, e.g., 2-butene,
can give only one structurally distinct addition product, but this fact does
not preclude the possibility that the product in question can be formed by
two mechanisms.
Alkenes containing a group which inhibits abnormal addition may fail
to give an abnormal product. The only known example of such a com-
pound is A1o-undecenol (in section B of table 4); primary alcohol groups
are weak inhibitors of abnormal addition. Until more data on such
compounds are available, i t should be concluded that the presence of an
inhibiting group in an alkene makes abnormal addition less likely but not
impossible.
In section C are listed some doubtful cases in which attempts to obtain
two addition products have been made. It is probably because the best
experimental procedures have not yet been employed that the second
products have not been detected. In the opinion of the reviewers, pro-
penylbenzene, safrole, and (if isomerization’ can be retarded) l-bromo-2-
butene should give two addition products. Because of the behavior of
l-bromopropene, no comment is made on additions to 1 -bromohexene.
Camphene is a special case, because its predicted abnormal addition
product is unknown and because acids rearrange its carbon skeleton.
Section D (table 4) consists of a, p-unsaturated acids and esters. Even
when an improved technique was used, crotonic acid and crotonic ester
gave only P-bromobutyric acid or ester, with no indication of a-brominated
derivatives. Attempts to obtain a second addition product from the other
acids listed have failed, but the improved technique has not yet been
e The case of A3-pentenoic acid, listed in table 3, is not an exception to this
statement, since the carboxyl group is separated from the double bond by only one
methylene group.
7 This rearrangement, together with the addition of hydrogen bromide to buta-
diene, will be considered in the section on rearrangements (V, A).
Kormal addition gives one product.
THE PEROXIDE EFFECT 365
employed. Explanations will be considered in the section on the mech-
anism of the abnormal addition. A fact pertinent t o bromomaleic and
bromofumaric acids is that 1 , l-dibromosuccinic acid is unknown and may
be incapable of existence. Although no instance is known of a peroxide
effect on an acid containing an a,@-double bond, there are indications of
such an effect on the a,P-triple bonds of tetrolic and phenylpropiolic
acids (table 3).
The present section has thus far dealt only with the effects of oxygen and
peroxides on the direction of addition of hydrogen bromide in liquid-phase
reactions. The data on vapor-phase reactions are contradictory but of
interest because, about ten years before the peroxide effect was reported
in the scientific literature, suggestions of such an effect in the vapor phase
appeared in Bauer’s patents. He claimed that the addition of hydrogen
chloride, bromide, or iodide to acetylene and vinyl bromide, in the presence
of an oxidizing atmosphere containing oxygen, ozone, chlorine, or nitrogen
oxides (ll), or in the presence of light (lo), gives ethylene halides rather
than ethylidene halides. Kharasch and Walker (97) have found that oxy-
gen accelerates the addition of hydrogen bromide to propene, 2-pentene,
and butadiene, and Ki’stiakowsky and Stauffer (99) mention a similar
effect in the addition of “halogen acids” to ‘%obutene.” Brouwer and
Wibaut (15) state that oxygen does not affect the direction of addition of
either hydrogen bromide or hydrogen chloride to propene. Vapor-phase
reactions need further investigation.
2. Rates of addition
A consideration of the rates of some normal and abnormal addition reac-
tions will be of assistance in showing that the two reactions are independent
and competing, and that the susceptibility of an alkene to the peroxide
effect is directly related to the rate of its normal addition reaction. No
quantitative data are available, but table 5 has been compiled from
experiments which have been carried out in the same laboratory under
comparable conditions: that is, a t room temperature, with about 1.5
moles of hydrogen bromide per mole of alkene, and (except as noted) in
the absence of solvents.
The alkenes are listed approximately in order of increasing rate of nor-
mal reaction with hydrogen bromide and of decreasing susceptibility t o
the peroxide effect. When the normal addition is extremely slow (section
A), traces of oxygen and peroxides can exert a maximum effect; unless the
normal addition is specifically catalyzed, abnormal addition predominates,
even in the presence of an antioxidant. When the normal addition is
somewhat faster (section B), it can outrun the abnormal addition only
when the latter is inhibited by an antioxidant. With allyl bromide (sec-
366 FRANK R. MAY0 AND CHEVES WALLING
UNSATURATED COMPOUND
Section A :
Trichloroethylene.. . . . . . . . . . .
1-Bromopropene. . . . . . . . . . . . . .
tion C), exclusion of air and ordinary purification of materials permit the
normal to exclude the abnormal addition, although the latter occurs in the
presence of air. With the alkenes in section D, air alone is ordinarily
insufficient to influence the course of the reaction and definite admixture of
peroxides is required to prevent fairly rapid formation of the normal
addition product. In section E, the normal addition is so fast that it must
Yield' Time
per cent hours
0 1440
--
Very little
TABLE 5
Rate of addition of hydrogen bromide to unsaturated compounds
1-Chloropropene . . . . . . . . . . . . . .
Vinyl bromide . . . . . . . . . . . . . . . .
Vinyl chloride.. . . . . . . . . . . . . . .
Section B:
Section C :
Section D :
Allyl bromide.. . . . . . . . . . . . . . .
2-Bromopropene. . . . . . . . . . . . . .
2-Chloropropene . . . . . . . . . . . . . .
Propylene . . . . . . . . . . . . . . . . . . . .
Isobutylene. . . . . . . . . . . . . . . . . .
Section E:
2-Bromo-2-butene . . . . . . . . . . . . .
Trimethylethylene . . . . . . . . . . . .
Styrene. . . . . . . . . . . . . . . . . . . . . .
1 NORMAL ADDITION
25
79
59
95
92
70
70
100
55
850
1632
336
240
27
40
3
1
2
:ondi-
tionst
ABNORMAL ADDITION
Yield'
er ceni
27
84
21
70
76
100
80
> 86
41
Time
hours
24
3
1
31
48
: 16
5
7
0.2i
%
Faster
%
4
__
2ondi-
tionst
* When a 90 to 100 per cent yield was obtained, the time given represents only an
upper limit to the period required for substantially complete reaction.
t Conditions: (1) air absent, antioxidant present; (2) air present; (3) peroxide
present; (4) dilute pentane solution with a peroxide present (otherwise no solvent
was employed). All reactions were carried out near room temperature, usually in
sealed tubes.
3 Reaction appeared to occur as rapidly as hydrogen bromide was added.
be retarded by the use of a solvent in order t o obtain the abnormal product.
Even so, about 20 per cent normal addition occurs.
These observations are all consistent with the view that the two types
of addition are independent but competing; the structure of the alkene and
the conditions of addition determine the product which predominates.
This view is supported by the fact that each alkene in sections B and C
THE PEROXIDE EFFECT 367
has yielded many different mixtures of the two possible products a t rates
intermediate between those of the independent reactions. Similar ex-
amples will be indicated in the section on the influence of experimental
conditions.8
Evidence showing how the rate of abnormal addition varies with the
structure of the alkene is unsatisfactory, but indications are that the al-
kenes in section A, particularly trichloroethylene, are also distinguished
by slow abnormal additions.
3. Catalysts of abnormal addition
Of the materials which catalyze the abnormal addition, the first ones to
be detected, and the most important on account of their wide prevalence,
are oxygen and peroxides. Most alkenes, on standing, react with air t o
form materials which give the familiar test for peroxides; that is, they give
an intense red color when shaken with an aqueous solution containing
ferrous and thiocyanate ions.Y The ability of these natural peroxides to
cause abnormal addition depends on the extent of their formation and on
the rate of the normal addition. For allyl bromide, where the most data
are available and where the rate of the normal addition is moderate, the
quantities of oxygen or peroxide required are small. Urushibara and
Takebayashi (154) state that 1.5 cc. of oxygen per 24.2 g. of allyl bromide
(0.03 mole per cent) is sufficient to give a product containing 96 per cent
of 1 , 3-dibromopropane (abnormal). Because about three times as much
8 Such observations disagree with the opinion of Urushibara and Takebayashi
(151) that the normal and abnormal additions so cancel each other that only one can
proceed a t a time. This view is based on a chain mechanism for normal addition
which the reviewers have questioned in section 11, A, 3. Assumptions about the
effect of one mechanism on the other seem unnecessary, because any rate of reaction
or any addition product thus far recorded may be accounted for on the following
basis: The abnormal addition may have an induction period (140), proceed very
rapidly for a time through the action of peroxides, and then more slowly through the
effect of oxygen.
9 Urushibara and Takebayashi (153) state that when oxygen was passed through
allyl bromide in the dark, no peroxides were formed in one month, whereas a strong
test could be obtained after the bromide was kept for a few hours in diffused light.
However, Urushibara and Sinamura (151) record that when oxygen and hydrogen
bromide together were passed through allyl bromide in the dark, considerable oxida-
tion took place, as indicated by formation of water, the liberation of heat, the
temporary formation of bromine, and the development of a peroxide test. A good
yield of 1,3-dibromopropane was obtained. Such experiments have never been
carried out in this laboratory, but i t has been repeatedly observed that, when allyl
bromide is stored in the dark in a bottle containing air, and only intermittently
exposed to light for the purpose of withdrawing samples, a weak peroxide test cau
be obtained after a few days and a very strong test after a few weeks. The 1- and
2-chloro- and bromo-propenes, as well as styrene, form peroxides more rapidly.
368 FRANK R. MAY0 AND CHEVES WALLING
peroxide (formed spontaneously and determined by its ability to oxidize
iodide ion) was required to produce a similar effect, they suggest that
molecular oxygen, rather than organic peroxides, is responsible for the
peroxide effect, and that peroxides function by liberating this gas. The
reviewers know of no observations that alkene peroxides liberate oxygen.
Added peroxides are quite as effective as those formed spontaneously
in the alkene. Among those which have been successfully used are ben-
zoyl peroxide, lauroyl peroxide, perbenzoic acid, and ascaridole (natural
menthene peroxide). The quantities employed have usually been of the
order of 1 mole per cent.
Smith has observed the rate of addition of hydrogen bromide to unde-
cenoic acid in various solvents and under various atmospheres. He found
that there is an induction period in the abnormal addition when air or
benzoyl peroxide is the catalyst, but not when perbenzoic acid, which liber-
ates bromine immediately, is used. In experiments with purified mate-
rials in fairly concentrated solution, air, but not perbenzoic acid, gave an
abnormal addition reaction. Smith concludes that oxygen is essential for
the abnormal reaction and that peroxides serve as subsidiary catalysts
(35, 140). Observations in this laboratory (53, 55, 61, 63, 70) show that
both benzoyl peroxide and ascaridole can serve as catalysts for the abnor-
mal reaction in the absence of air, but that the rate and extent of abnormal
addition are often more unpredictable with peroxides'*, particularly
perbenzoic acid, than with oxygen.
A simple explanation of these results, as well as those of Urushibara,
Takebayashi, and Smith, is as follows: Either oxygen or peroxides can
serve as catalyst for the abnormal addition reaction by reacting with
hydrogen bromide. If the peroxide reacts rapidly, as do ascaridole and
perbenzoic acid, the catalyst is quickly destroyed before much abnormal
addition has taken place. On the other hand, benzoyl peroxide" and oxy-
gen react very slowly with hydrogen bromide under the usual experimental
conditions, and thus exert a catalytic effect over a comparatively long
period. The observations of other workers that peroxides may be less
effective than oxygen in catalyzing abnormal addition are not questioned ;
nevertheless their conclusion that molecular oxygen is necessary seems to
the reviewers to be unjustified.
Other gases than oxygen, and other oxidizing agents than peroxides,
have been investigated for a possible effect on the abnormal addition.
Nitrogen, hydrogen, nitric oxide, and nitrogen dioxide were tried with
1 0 With trichloroethylene (74) benzoyl peroxide caused abnormal addition when
11 This statement applies particularly t o experiments without solvents, when the
air did not.
benzoyl peroxide dissolves slowly.
THE PEROXIDE EFFECT 369
allyl bromide and found to be without effect (53). Neither lead dioxide
nor bromine (in the dark and in the absence of air) had any effect on the
addition to allyl bromide (53), nor did N-bromobenzamide and iodine af-
fect addition to propene (111). Smith, however, working with diffused
light and in a hydrogen atmosphere, found that intermittent additions of
bromine caused an abnormal addition to undecenoic acid in carbon
tetrachloride solution (140). Harris and Smith (35) report that a-hep-
tenylheptaldehyde and 10,ll-epoxyundecanoic acid are catalysts for the
abnormal addition of hydrogen bromide to undecenoic acid in the presence
of air, but have only a small effect in its absence. In the faster addition of
hydrogen bromide to propene (without a solvent), both propylene oxide
and propionaldehyde, when peroxide-free, were ineffective even in the
presence of air (111). Therefore such catalysts probably function because
they, or impurities which they contain, are oxidized by air t o peroxides.12
Finely divided iron, cobalt, and nickel are strong catalysts for the ab-
normal addition of hydrogen bromide to allyl bromide and undecenoic
acid; these reactions will be considered later.
4. Inhibitors of abnormal uddi t ion
It has been found that small proportions, usually 1 to 5 mole per cent,
of certain substances prevent abnormal addition when this reaction would
otherwise predominate. These inhibitors are commonly called antioxi-
dants because they overcome the effect of oxygen, but many of them are
not inhibitors of autooxidation reactions. Table 6 summarizes some of the
available information on inhibitors of abnormal addition, and permits
rough evaluation of their effectiveness. The alkenes in section A are
those which put antioxidants t o the most severe test. For these com-
pounds no antioxidant has been found which completely prevents abnormal
addition, except in the presence of a catalyst for the normal reaction.
I n section B, the inhibitors are eflective for propene only when this com-
pound is carefully and freshly purified and when a clean vacuum line is
used. The other sections bring out further differences in the effectiveness
of antioxidants, and it can be concluded that compounds with sulfhydryl
groups are the best inhibitors; some phenols and aromatic amines are less
effective. It should be noted that the effectiveness of the inhibiting sub-
stances is related t o their solubility. This fact may account for the moder-
ate efficacy of diphenylamine (the hydrobromide of which is rather soluble
l2 Yote added August 8, 19.10: This conclusion is supported by the paper of M.
Talrebayashi (Bull. Chem. SOC. Japan 16, 116 (1940)). He found that the effect of
aldehydes on the addition of hydrogen bromide to undecenoic acid depended on the
peroxide contcnt of the aldehydes.
370 FRANK R. MAY0 AND CHEVES WALLING
I-
+
THE PEROXIDE EFFECT 37 1
3’72 FRANK R. MAY0 AND CHEVES WdLLING
in the reaction mixtures), for the fact that catechol is often better than
hydroquinone, and for the differences between some of the metal halides.
Ethanol has been found to inhibit abnormal addition to AI0-undecenoic
acid (35) and to trimethylethylene (162). In a later section i t will be
shown that this inhibiting effect decreases a t lower temperatures.
5 . The bromine-atom mechanism of abnormal addition
At this point i t seems desirable to correlate the data so far presented by
means of a mechanism for the abnormal addition. Since this reaction
can be caused by relatively small quantities of oxygen and peroxides and
inhibited by equally small quantities of antioxidants, it is established that
the abnormal addition is a chain reaction. In choosing a mechanism for
the reaction, i t must be explained how oxygen and peroxides start reaction
chains, how the chains give the abnormal addition product, and why hydro-
gen bromide is the only halogen acid capable of reacting through such a
chain. In order to meet the last requirement, it seems necessary that the
oxygen or peroxides function through the hydrogen bromide rather than
through the alkene; otherwise it would be difficult to explain why the
addition of other unsymmetrical reagents is not also reversed by these
reagents. If oxygen or peroxides attack hydrogen bromide, bromine
should result; but molecular bromine has no effect on the reaction in the
dark. It has therefore been suggested that the slow oxidation of hydrogen
bromide in dilute solution gives bromine atoms. In adding to a double
bond, these atoms need not attack the same doubly bound carbon atom
which would be attacked by the proton in the normal addition. Although
oxidation of hydrogen bromide might conceivably give positive bromine
ions, and although these ions should add abnormally to double bonds, for-
mulation of the mechanism of the abnormal addition in terms of bromine
atoms (’70)13 is preferred, because more energy would be required to sepa-
rate charged particles in non-polar solvents. According to this mechanism,
the reaction should proceed as follows:
HBr + O2 ( 5 )
(6)
(7)
(a1 kene) +H-O--O. + Bra
RCH=CH2 + Br* + RCHCH2Br
RCHCHzBr + HBr + RCH2CH2Br + Br*
The individual steps suggested will next be discussed.
13 The suggestion that the abnormal addition is a chain mechanism involving
bromine atoms was made simultaneously and independently, but without details,
by Hey and Waters (37). Previously, Burkhardt (16) had suggested that free
radicals might account for the abnormal addition of thiophenol to styrene; cf.
section 111, B.
THE PEROXIDE EFFECT 373
The oxidation of hydrogen bromide by oxygen is ordinarily slow, but
Urushibara and Sinamura have shown that in the presence of an alkene
such as allyl bromide, peroxides are very rapidly formed (151). These
organic peroxides react slowly with hydrogen bromide. The same authors
also showed that the interaction of hydrogen bromide, oxygen, and stilbene
gives stilbene dibromide, thus proving the oxidation of hydrogen bromide.
Similarly, the abnormal addition of hydrogen bromide to allyl bromide is
accompanied by the formation of small amounts of a high-boiling liquid
which is apparently 1 ,2,3-tribromopropane (94, 151). The essential
feature of reaction 5 is that a t least a small portion of the bromine formed
must be liberated in the atomic state. This requirement is consistent
with the fact that those peroxides which react more slowly with hydrogen
bromide are usually more effective; i t also agrees with the observation of
Smith (140) that bromine is a catalyst for the addition of hydrogen bromide
in diffused light. It may
react with hydrogen bromide to give another bromine atom and hydrogen
peroxide, or it may combine with alkene.
In the nor-
mal addition to the type of alkene chosen as an example, the proton be-
comes attached to the terminal carbon atom, showing that this atom is the
relatively negative carbon atom of the double bond. In abnormal addition
the bromine atom becomes attached to this terminal carbon atom. It
has already been suggested (70) that the oxidizing properties of the bromine
atom cause it to attack the carbon atom with the higher electron density.
This explanation is simple and plausible, but the following one has addi-
tional advantages: The point of attack by the bromine atom is little af-
fected by the polarity of the double bond, but depends upon the relative
stability of the two bromoalkyl radicals which may be formed. If the
directions of all additions by the chain mechanism are to be explained on
this basis, the following orders of decreasing stabilities of free radicals
(each formed by the addition of a bromine atom to a double bond) are
required :
The fate of the HOP radical is unimportant.
If reaction 5 is assumed, reaction 6 can follow very easily.
Radicals from hydrocarbons : tertiary > secondary > primary
H H H H
I I I I
Radicals from vinyl-type halides : R-C-C-X > R-C-C-X
* I I *
Radicals from acids, esters:
Br Br
H H H H
I I I I
. I I .
R-C-C-COOR’ > R-C-C-COOR’
I3r Br
374 FRANK R. MAY0 AND CHEVES WALLING
Radical stability in this discussion is intended in the sense of higher heat
of formation; no reference to the mean life of the radical is implied. If
these relative stabilities can be proved correct, then the hypothesis of
radical stability explains why addition of hydrogen bromide by the abnor-
mal rather than the normal mechanism gives complete reversal of addition
with hydrocarbons, only partial reversal with 1-bromopropene (to give
exclusively 1 , 2-dibromopropane), and no change when the ethylene bond
is conjugated with the carboxyl group.
This hypothesis suggests that the addition of hydrogen bromide to such
conjugated double bonds may take place through two mechanisms to give
only one product. Addition by the normal mechanism must be fast as
compared with addition by the chain mechanism, in order to agree with the
following qualitative rate indications. The rate of addition of hydrogen
bromide to crotonic acid and its ethyl ester is unaffected by peroxides,
even when the normal addition is retarded by the use of an inert solvent
(162). These conclusions as t o the relative rates of addition by the two
mechanisms are in agreement with experiments (to be described in section
V, C) on the relative rates of isomerization of maleic acid to fumaric acid
by corresponding mechanisms. Smith (141) has suggested that since
ethylene bonds conjugated with carboxyl groups should be represented by
the formula
and that since there is no “accumulation” of electrons on the a-carbon
atom, there is no tendency for the bromine atom to attack this position or
for abnormal addition to occur. This explanation is in good agreement
with the qualitative observations on the rate of addition of hydrogen
bromide, but not with the isomerization studies mentioned. In the opin-
ion of the writers, no explanation yet proposed for the absence of abnormal
addition to conjugated systems is wholly satisfactory.
In order to obtain the addition product from the free radical formed in
reaction 6, reaction 7 is necessary. This latter step regenerates a bromine
atom, so that the chain reactions (6 and 7) can continue indefinitely.
The inhibition of the abnormal reaction by small quantities of antioxidants
is excellent evidence that this reaction is of the chain type. The small
amounts of antioxidants usually needed to inhibit and the small amounts of
peroxides necessary to cause abnormal addition indicate that the chains
must be very long. If 0.03 mole per cent of oxygen causes nearly com-
plete abnormal addition to allyl bromide (154), and if each molecule of
THE PEROXIDE EFFECT 375
8TW IN CHAIN BDACTION
( 6 ) RCH=CH2 + F+ RCHCHzX., . . . . . . .
(7) RYHCH2X + H X -+ RCHzCHzX + X'.
oxygen generates four bromine atoms by oxidizing hydrogen bromide, then
the average chain length is a t least 1000. If the efficiency in generating
bromine atoms is low, as seems likely, then the chains must be much longer.
These chain lengths, together with the observed high velocity of the ab-
normal addition and the known instability of aliphatic free radicals, in-
dicate that both steps in the chain reaction must take place very rapidly
and with little or no activation energy. The inhibition of the reaction by
antioxidants suggests that these function by reacting with bromine atoms.
It is also possible, as originally suggested (53), that antioxidants to some
extent destroy organic peroxides. Other observations on the abnormal
addition are consistent with the chain mechanism suggested. The ab-
normal reaction is retarded by glass wool (139), although apparently not
by coarser packing (155). When non-polar solvents are used, the absence
AH (IN PILOQRAM-CALORIEE PEE MOLD)
X = F X - C 1 X = B r 1 X = I
-60 -15 0
TABLE 7
Heats of addit ion of halogen acids to double bonds*
* When two values are given for AH, the first is based on the bond-energy estimates
of Sherman and Ewe11 (31), the second on the estimates of Pauling (124). One
value for AH indicates that both sources give the same result.
of a large dilution effect (113) shows that the abnormal reaction cannot be
of second or third order.
It will now be indicated why hydrogen bromide is the only halogen acid
capable of giving an abnormal addition. The exposition assumes that a
rapid chain reaction is impossible when any step is appreciably endother-
mic. Table 7 gives estimates of the heats evolved in reactions 6 and 7
for various halogen acids; it is assumed that AH for each reaction is simply
the difference between the energies of the bonds formed and the energies of
the bonds broken. The heats indicated are only approximate, for the
bond energies are none too well established for the molecules to which they
are meant to apply and are still less reliable when applied to free radical^.'^
Table 7 shows that reaction 6 for hydrogen bromide is definitely exothermic
and that the heat of reaction 7 is very close to zero. However, the addition
l4 Actually, the activation energy rather than the AH will determine the prob-
Because long chains occur in some additions, i t is assumed here ability of reaction.
that the activation energy is small when AH is positive or zero.
376 FRANK R. MAY0 AND CHEVEB WALLINQ
of hydrogen chloride by the same mechanism encounters difficulty in two
places, reactions 5 and 7. Since the oxidation of hydrogen bromide is
ordinarily slow (reaction 5), the oxidation of hydrogen chloride is probably
slower, possibly negligible. In reaction 7 the carbon-chlorine bond formed
is weaker than the hydrogen-chlorine bond broken ; the reaction is endo-
thermic by about 15 kg-cal. per mole, and unlikely to occur rapidly enough
to support long chains a t ordinary temperatures. Both of these difficulties
apply also to hydrogen fluoride and to most other acids. Attempts to
reverse the direction of addition of sulfuric acid to propene, 1-pentene, and
2-pentene have failed (14).
In the addition of hydrogen iodide, reactions 5 and 7 should proceed
easily, but reaction 6 may not. Another difficulty with the addition of
hydrogen iodide is that the normal addition is ordinarily rapid and is
further catalyzed by molecular iodine. Any reagent which generates
iodine atoms must necessarily generate also the catalyst for the normal
addition. Still another possibility is that iodine, since it does not add
readily to alkenes, may accumulate in the reaction mixture and inhibit any
possible abnormal addition of hydrogen iodide by an iodine-atom mech-
anism. According to the chain mechanism proposed, the unsymmetrical
reagents, HX, which can add by a chain mechanism are restricted to
narrow limits. The requisites are as follows: the radical (or atom) X must
be able to break a carbon-carbon double bond and add to one carbon atom;
the free radical formed must be able to take a hydrogen atom away from
H X to regenerate another X radical. It will be shown later that this
mechanism can also be applied to additions of mercaptans and bisulfites.
The next abnormal additions to be considered are those occurring when
air and peroxides have been excluded from reaction mixtures (135,136) and
when antioxidants have been added (25, 70, 117). The discussion of anti-
oxidants showed that these substances exert effects ranging from complete
to barely perceptible repression of the abnormal addition. Since different
antioxidants have variable effects under the same experimental conditions,
the occasional failure of the best antioxidants and the more frequent failure
of inferior inhibitors indicate only that the proportions used or the inhibit-
ing qualities of the compounds were inadequate; they do not indicate a
difference in the nature of the abnormal reaction. It is thus proved for the
weaker antioxidants, and inferred for the rest, that only a small proportion
of the collisions of a bromine atom with an antioxidant molecule are effec-
tive in terminating chains. The smaller this proportion, the greater must
be the chain lengths and the fewer the number of bromine atoms necessary
to cause abnormal addition. Since there is no basis for estimating an
upper limit on chain lengths, i t can be argued that traces of peroxides and
oxygen which would defy elimination or detection by any known method
THE PEROXIDE EFFECT 377
may be wholly responsible for all abnormal addition reactions. On the
other hand, infinitesimal quantities of atoms or radicals may appear spon-
taneously in the solution with the result that chains are initiated. The
dissociation of hydrogen bromide into atoms would require 87 kg-cal. per
mole (31, 124), but the dissociation of a carbon-bromine bond in an alkyl
halide requires on the average only 50 t o 60 kg-cal. Such a dissociation
might initiate two chains if the free radical reacted with hydrogen bromide
to give an alkene and a bromine atom. Sherman, Quimby, and Sutherland
(134)16 have utilized such a chain-initiating step in calculating the activa-
tion energy of the addition of hydrogen bromide to vinyl bromide by the
chain mechanism just described, and have arrived a t a value of 29 kg-cal.,
one-half the estimated strength of the carbon-bromine bond. If this value
is approximately correct, the abnormal addition can be initiated without
the assistance of oxygen or peroxides. Such an activation energy, how-
ever, is probably higher than that for most normal addition reactions; hence
appreciable abnormal addition of spontaneous origin could be expected
only when the normal addition is very slow. Although oxygen and perox-
ides are usually responsible for the abnormal addition of hydrogen bromide
to alkenes, i t is futile a t present either to assert or to deny that they are
always entirely responsible. The significant aspects of the peroxide effect
in additions of halogen acids are that abnormal addition takes place only
with hydrogen bromide, and then by a chain mechanism, and that oxygen
and peroxides are largely, if not entirely, responsible for this reaction.
6. Other mechanisms proposed for abnormal addi t ion
The first mechanism to be proposed for abnormal addition was that of
Urushibara and Takebayashi (155). Because iron and certain other
metals also caused abnormal addition, they suggested that the effect of
oxygen was due to its paramagnetic properties. The oxygen or metal
was supposed to exert a physical influence on the surrounding alkene molec-
ules such that the polarity of the double bond was affected. The short-
comings of this hypothesis have been mentioned elsewhere (87), and are
admitted by a t least one of the above workers (151), who now favors the
bromine-atom chain mechanism.
Winstein and Lucas (165) have proposed an explanation for abnormal
addition t o a double bond the polarity of which (as indicated by the nor-
mal addition) is -C-C-. They suggested that oxygen forms with
16 These workers proposed the chain mechanism for both the normal and the
abnormal additions of all halogen acids to alkenes a year before investigators in this
laboratory suggested i t only for the abnormal addition of hydrogen bromide.
I /
+ -
378 FRANK R. MAY0 AND CHEVES WALLING
double bonds of this type a complex represented by the following resonance
forms:
I / -c-c-
- I
O+
I 1
+ I
-c-c-
0
0-
II
0
If the second form makes the larger contribution to the complex, the pro-
ton of the attacking halogen acid attaches itself to the carbon atom which
would have been relatively positive in the pure alkene. The oxygen is
then displaced by a bromide ion, and abnormal addition results. Conn,
Kistiakowsky, and Smith (20) concur essentially in this suggestion. How-
ever, such behavior of an oxygen molecule in forming successive loose
complexes with several hundred molecules of alkene is not wholly consist-
ent with the rapid irreversible reaction of oxygen on alkenes in the presence
of hydrogen bromide (151).
Michael (114, 117) has recently attacked all previous mechanisms for
abnormal addition and has suggested one of his own. On the basis that
oxygen in the atmosphere or in the molecules of the solvents is negative,
he proposes that oxygen from either source may become associated with
the more positive of the doubly bound carbon atoms, thus making this
atom relatively negative. If the effect of oxygen (or oxygen compound) is
sufficient to reverse the polarity of the double bond, then the addition of
halogen acid is also reversed.
All of these hypotheses may be said to account for an abnormal addition
of hydrogen bromide, but all fail t o indicate why hydrogen chloride and
hydrogen iodide do not give a similar reaction. Further, all of them except
that of Urushibara and Takebayashi fail t o explain why some finely divided
metals cause abnormal addition. This latter effect will soon be considered
in the light of the chain mechanism.
7. T h e in$uence of experimental conditions o n additions of hydrogen bromide
The following discussion of the effects of solvents, temperature, light,
and metals on the liquid-phase addition of hydrogen bromide to alkenes
will assume that all of the abnormal product results from addition of hydro-
gen bromide by the bromine-atom chain mechanism, whereas all of the
normal product results from addition by other mechanisms. The reaction
product obtained is the result of competition between the two mechanisms;
hence its composition may vary from one extreme to the other. Experi-
mental conditions may favor one mechanism and hinder the other, but
the fact that the direction of addition of hydrogen chloride and hydrogen
iodide has not yet been certainly (44, 84) altered by any agent indicates
THE PEROXIDE EFFECT 379
that these conditions affect the halogen acid or its mechanism of addition,
and not the unsaturated compound.
Solvents principally affect the rate of the normal addition reaction.
Because of the high order of this reaction, its rate is greatly decreased by
dilution with inert solvents, and the abnormal addition may then outrun
the normal reaction. This effect is strikingly demonstrated in the addition
of hydrogen bromide to propene. When no solvent is used, the normal
addition requires only a few minutes and abnormal addition is difficult t o
obtain, except in the presence of added peroxides (54). But when the
reaction mixture is diluted with about ten volumes of pentane, abnormal
addition is substantially completeI6 within half an hour a t O'C., even when
the reaction mixture is prepared from purified materials in the absence
of air (113). However, the addition of an antioxidant prevents abnormal
addition; then several weeks are required for the normal reaction. Smith
has shown that, a t high dilution, the addition to undecenoic acid is highly
sensitive to the presence of air (140). Both observations show clearly
how dilution affects the competition between the two mechanisms, why
past claims that a direct solvent effect is responsible for abnormal addition
are open to serious doubt, and how a peroxide effect may be obtained with
alkenes to which normal addition is rapid.
Small amounts of polar solvents such as water and acetic acid may
increase the rate of the normal addition, probably because of dielectric
effects and the change from a molecular t o an ionic mechanism. This
acceleration is partly responsible for the small amount of abnormal addi-
tion in polar solvents. Some solvents may, however, be weak inhibitors
of abnormal addition, as will be indicated in the discussions of the tem-
perature and light effects.
Table 8 summarizes work on the addition of hydrogen bromide to alkenes
in solvents. All the experiments listed fulfill the following conditions:
(a) they have been carried out since the discovery of the peroxide effect;
( b ) both products have been obtained in the same solvent by the same
workers; (c) either addition product can be made to predominate by the
suitable use of oxygen, peroxides, or antioxidants. The facts that a large
variety of alkenes and solvents have been employed, and that many dif-
ferent workers have participated in obtaining these results, are convincing
evidence that the mechanism of addition is the factor which determines
the composition of the addition product.
l6 Such a velocity would lead one to expect considerable abnormal addition in the
absence of a solvent and of air, but no such reaction has actually been observed.
If some association product of hydrogen bromide or propene or both is a weak in-
hibitor of abnormal addition, then this reaction, in agreement with qualitative in-
dications, would be accelerated by dilution.
380 FRANK R. MAY0 AND CHEVES WALLING
Some workers in the field disagree with the above conclusion, and main-
tain that solvents immediately affect the direction of addition, presumably
through their effect on the polarity of the double bond. Gaubert, Lin-
stead, and Rydon base their contention on the inability of diphenylamine
to prevent formation of primary bromides from terminally unsaturated
acids in hexane solution (25). Sherrill and coworkers (135, 136) insist
that their 1-alkenes were peroxide-free and that the formation of primary
Solvents an which
ALKENE
Allyl bromide. . . . . . . . . .
Vinyl bromide, . . . . . . . .
Vinyl chloride.. . . . . . . . .
Propene . . . . . . . . . . . . . . .
1-Butene . . . . . . . . . . . . . . .
I-Pentene . . . . . . . . . . . . . .
Isobutylene. . . . . . . . . . . .
Trimethylethylene
Styrene. . . . . . . . . . . . . . . .
2-Bromo-2-butene.. . . . .
2-Bromo-1-hexene.. . . . .
Allylacetic acid.. . . . . . .
AlO-Undecenoic acid. . .
Ethyl AIo-undecenoate .
Undecenyl acetate. . . . .
Undecynoic acid.. . . . . .
TABLE 8
drogen bromide yields either of two addi t ion products
80LVENTS E W L O Y E D
Water, 90% acetic acid, propionic acid,
ligroin, carbon disulfide, chloroform,
carboii tetrachloride, 1,2-dibromopropane,
1,3-dibromopropane, acetyl bromide*
Acetic acid, nitrobenzene
Acetic acid, nitrobenzene, mesitylene
Pentane, acetic acid
Ligroin
Pentane, propionic acid
Pentane, carbon disulfide, propionic acid,
ethyl bromide, benzonitrile, nitrobenzene,
water
Pentane, ethyl bromide, ether, methanol,
ethanol, acetic acid*
Pentane
Acetic acid
Benzene
Hexane
Ligroin, hexane, toluene, benzene, carbon
disulfide, carbon tetrachloride, chloroform
Benzene, ligroin
Benzene, ligroin
Benzene
REFERENCES
* In the experiments indicated in these two solvents, less than 50 per cent ab-
normal addition in the presence of air or peroxides was observed.
bromides in solvents must therefore be a solvent effect. They used no
antioxidants. Michael and coworkers claim to have demonstrated a
solvent effect in the addition of hydrogen bromide to tetrolic and phenyl-
propiolic acids (table 3) and to trimethylethylene (table 8). In the first
instance, they employed no antioxidants; in the second, the “solvent
effect” was diminished by antioxidants and vanished in experiments with
hydrogen chloride (table 1) or hydrogen iodide (table 2). The reviewers
question not the experimental results of these workers, but their conclusions.
THE PEROXIDE EFFECT 381
An effect of the solvent on the polarity of the double bond, and thus
on the direction of normal addition, would be most likely in instances
where the normal addition product is a mixture, and where the directing
effects of the substituents on the doubly bound carbon atoms are nearly
balanced. No such effect has yet been reported with this type of ethylene
derivative. The addition of hydrogen bromide to 2-pentene has been
carried out without a solvent and in acetic acid solution (77). With
A9-undecenoic acid the reaction has been run in ligroin, hexane, benzene,
and acetic acid (2,34). In all cases, the addition product consisted, within
experimental error, of equal proportions of the two possible halides. The
fact that no solvent effect on the direction of normal addition has yet been
established does not prove that none will ever be found, but the existence
of such an effect should be demonstrated, not with hydrogen bromide
(which is the only halogen acid capable of addition by an abnormal mecha-
nism) but with hydrogen chloride or hydrogen iodide.
Increasing the reaction temperature generally increases the difficulty
of eliminating abnormal addition. Air does not cause much abnormal
addition to pure allyl bromide a t O"C., but i t does a t room temperature (53).
Removal of air prevents abnormal addition a t room temperature, but this
procedure becomes ineffective a t 76°C. However, the addition of an anti-
oxidant reduces the proportion of abnormal addition product to about 10
per cent, even a t lOO"C., indicating that the effect of temperature is on the
relative rates of the normal and abnormal addition reactions, and not on
the allyl bromide (see footnote 3). Vinyl bromide (54) and vinyl chloride
(57), to which the normal additions are slower, present similar difficulties
beginning a t lower temperatures, for antioxidants are always required to
eliminate abnormal addition to these substances at room temperature,
and partial failure of antioxidants has been observed a t 46" and 76"C.,
respectively.
Abnormal addition to allyl bromide (53) and trimethylethylene (117)
has been observed a t -78°C. with small quantities of peroxides, showing
that long chains are formed and that their propagation requires a negligible
activation energy. Differences in the effectiveness of different anti-
oxidants indicate that a t least some, and probably all of them, require
a t least a small activation energy for reaction with'bromine atoms. It
follows that the proportion of effective collisions between a bromine atom
and an antioxidant must increase as the temperature rises.I7 Therefore,
the increasing predominance of abnormal addition a t high temperatures
in bhe presence of antioxidants cannot be ascribed to increased chain
lengths, but must be due t o the origin of many more chains. Although the
l7 This phenomenon has been observed when alcohols, acetic acid, or acet3-1
bromide have been used as solvents; it will be described shortly.
382 FRANK R. MAY0 AND CHEVES WALLING
decomposition of peroxides or their reaction with hydrogen bromide may
have a high temperature coefficient, either reaction should lead to rapid
exhaustion of traces of peroxides in experiments from which air has been
excluded. Here the spontaneous origin of chains (i.e., without the influ-
ence of oxygen and peroxides), a reaction of high activation energy and
high temperature coefficient, may become significant. The normal addi-
tion of hydrogen bromide to allyl bromide and vinyl bromide has a tem-
perature coefficient of about 2 for 10°C. That of the abnormal addition
is much larger, a t least over certain ranges of temperature.
The temperature effect in solvents fits into the scheme already outlined.
If the solvent (ligroin, chloroform, carbon tetrachloride with allyl bromide
(53), ether with trimethylethylene (117)) decreases the rate of normal
reaction, then the abnormal addition becomes more prominent and appears
a t a lower temperature. If the solvent (acetic acid, water, acetyl bromide
with allyl bromide) increases the rate of the normal reaction, then abnormal
addition becomes less prominent.
The fact that abnormal addition is easily obtained in many solvents
indicates that their inhibiting properties are usually negligible. Never-
theless, almost any of the aliphatic solvents in table 8 can react with
bromine atoms, as shown by the mechanism of aliphatic substitution.
In the solvents which inhibit, the effect increases with increasing tempera-
ture. In the presence of peroxides, no abnormal addition to trimethyl-
ethylene takes place in ethanol solution a t 20°C. or in methanol a t O"C.,
but the proportion of abnormal addition increases a t lower temperatures
(117). Acetic acid and acetyl bromide seem to have definite inhibiting
properties in additions to allyl bromide a t 76°C.; moreover, in the light,
acetic acid seems to be a better inhibitor of abnormal addition a t 25°C.
than a t 10°C. Ordinarily, little difficulty is encountered in obtaining
abnormal addition in glacial acetic acid (cf. table S), but experiments with
allyl bromide in this solvent show that small amounts of admixed water
favor abnormal addition.
Light accelerates both the normal and the abnormal additions of hydro-
gen bromide to allyl bromide (53) and vinyl bromide (54), but the effect
on the abnormal addition is usually far greater than on the normal reaction.
The rate of addition to allyl bromide in the light is greatest in the presence
of air, slower in the absence of air, and still slower in the presence of added
antioxidants. In the first two instances, the abnormal addition product
is formed exclusively; in the last, predominantly. These observations
are corroborated by others made on vinyl chloride (57) and trichloro-
ethylene (74).
In additions to allyl bromide in the light, the effects of various spectral
regions and of solvents have been investigated. In the absence of air
THE PEROXIDE EFFECT 383
and antioxidants, abnormal addition was substantially complete in 1 to
4 hr., regardless of whether the light supplied was near ultraviolet, red and
infrared together, or a combination of either with visible light.ls In the
presence of 1.4 mole per cent of diphenylamine, a combination of red and
infrared light was clearly shown to accelerate the normal addition by a
factor of 5 to 10 without causing much abnormal addition. Visible and
near ultraviolet radiation gave increasing proportions (up to 74 per cent)
of abnormal addition product a t about the same total reaction rate.
Thus the shorter wave lengths make repression of abnormal addition more
difficult. In the absence of air and antioxidants, abnormal addition occurs
exclusively when heptane, carbon disulfide, acetyl bromide, or benzoyl
chloride are used as solvents. In acetic acid solution, only very small
proportions of abnormal addition product are obtained; none of this
product is formed in the presence of hydroquinone. Comparison of these
experiments with those carried out in the absence of a solvent indicates
that acetic acid has some inhibiting properties for the abnormal addition,
and that the effect of light on the competition between the normal and
abnormal reactions is much like that of increased temperature.
Inasmuch as the individual effects of light and temperature on the rates
of the normal and abnormal additions are known only qualitatively, and
since the effects of temperature on the inhibiting properties of solvents
and antioxidants have not been isolated, the combined effects of light and
temperature cannot yet be completely resolved. The effect of lowering
the reaction temperature in the photochemical addition of hydrogen bro-
mide to vinyl bromide (54) is impressive. Although no normal addition
occurs a t room temperature in the presence of antioxidants, a 50 to 60
per cent yield of an 80 per cent normal product was obtained in 16 hr. a t
5°C. Thus not only was the proportion of abnormal addition greatly
reduced by lowering the temperature, but the normal addition product
was formed a t the rate of about 3 per cent per hour, as against 1 per cent
per day in the dark a t room temperature,-an increase by a factor of
about 70 in spite of a 20°C. temperature decrease. The case of allyl
bromide is less clear-cut. The effect of light in accelerating the normal
addition is smaller, and lowering the reaction temperature in the presence
of antioxidants does not significantly increase the proportion of normal
addition product.lg The available data do not permit estimation of the
1s Comparisons of rates are not possible, because the amount of light transmitted
by the filters is not known.
19 Kharasch and Mayo (53), using thiocresol as an antioxidant, observed 71 per
cent and 30 per cent normal addition a t 5°C. and a t room temperature, respectively,
but in view of the fact that the combined yields of both addition products were
54 per c-nt and 91 per cent, respectively, and that normal addition is fastest during
384 FRANK R. MAY0 AND CHEVES WALLING
effect of light on the rates of the two possible additions in glacial acetic
acid solution, but they show conclusively that in the light more abnormal
addition takes place a t 5°C. than at room temperature. This fact can
be explained by the assumption that acetic acid is less effective as an
inhibitor a t lower temperatures. Such a conclusion is consistent with
observations made a t higher temperatures or in the presence of alcohols.
It may be noted that a t 5°C. a reaction mixture containing diphenylamine
reacted more slowly than one containing oxygen and gave slightly less
abnormal addition product,-a fact which may be taken to indicate that
the effect of light in accelerating the abnormal addition is large compared
with that of oxygen.
The acceleration of both the normal and abnormal addition by light,
the increased light effect in the presence of oxygen, some relations between
the combined effects of light and temperature, and the partial and variable
effects of antioxidants in repressing abnormal addition in the light all
show the close analogy between the effect of light and that of increased
temperature. These observations are consistent with the concept that all
abnormal addition products result from a reaction by the chain mechanism.
Since the effect of light is greatest in the presence of oxygen, illumination
apparently accelerates peroxide formation and oxidation of hydrogen bro-
mide, but it apparently also serves to initiate chains without the assistance
of oxygen. The fact that light accelerates the normal (as well as the
abnormal) addition, and that the effect is much greater with vinyl bromide
than with allyl bromide, shows that either the alkene, its complex with
hydrogen bromide, or some impurity absorbs visible radiation. The
greater effectiveness of the shorter wave lengths in accelerating abnormal
addition to allyl bromide is consistent with the greater activation energy
of this reaction. Although in the absence of air and peroxides, reaction
mixtures usually remain colorless in the dark, the presence of oxygen,
peroxides, or light usually leads to the development of variable quantities
of dark-colored materials. These may assist in the transference of energy
to substances which otherwise absorb only weakly.
It was found by Urushibara and Takebayashi that, if the addition of
hydrogen bromide to allyl bromide takes place in the presence of finely
divided and freshly reduced iron, nickel, or cobalt, varying quantities of
abnormal product are formed, even in the presence of some antioxidants.
the first part of the reaction when the concentrations of reactants are highest,
the conclusion that temperature lowering increases the yield of normal addition
product is not justified. A recent experiment with catechol at 5-10°C. (112) gave
in 65 hr. a 92 per cent yield of a mixture containing only 26 per cent of the normal
addition product, again indicating that temperature lowering may have no marked
effect.
THE PEROXIDE EFFECT 385
They observed similar phenomena when hydrogen bromide was added
t o undecenoic acid in toluene solution. Their work has been reviewed
elsewhere (151) and that portion of i t concerned with the effect of reduced
iron on allyl bromide has been confirmed, extended, and explained in a
paper from this laboratory (87). The complete inhibition of the iron-
promoted abnormal addition by some antioxidants, and its partial inhibi-
tion by others (cf. table 6 ) , are strong indications that abnormal addition
is the result of the bromine-atom chain mechanism. The fact that some
hydrogen and metal bromide are formed suggests that interaction of the
metal with hydrogen bromide or allyl bromide yields some hydrogen atoms
or free radicals. Reaction of hydrogen atoms with hydrogen bromide or
allyl bromide would yield free radicals or bromine atoms, either of which
might initiate chains for the abnormal addition.
Fe + HBr -+ FeBr* + H*
H* + HBr ---f Hz + Br*
Fe + R’Br ---f FeBr* + R’*
Thus far, iron-promoted abnormal addition has not been found with
any alkene which gives a rapid normal addition with hydrogen bromide,
but i t has been observed with allyl chloride and possibly with vinyl bro-
mide. Other metals have failed to cause abnormal addition to allyl bro-
mide, because they do not react with anhydrous hydrogen bromide, because
the bromides formed are strong catalysts for the normal addition (sec-
tion I, A, 3), or because their bromides are strong inhibitors for the
abnormal reaction (table 6).
The success of the bromine-atom chain mechanism in correlating the
metal effect with the peroxide effect, an advantage not possessed by any
other mechanism yet proposed for either reaction, is a strong point in
favor of this interpretation. However, the value of a hypothesis lies in
its ability t o predict new reactions as well as t o explain known ones. At
the end of this paper, it will be shown how many new applications of this
concept have been developed since it was first formulated.
In concluding this section dealing with the influence of experimental
conditions on the direction of addition, the conditions which should be
chosen to promote either normal or abnormal addition of hydrogen bro-
mide are summarized. To favor normal addition, this reaction may be
accelerated by the use of high concentrations of reactants, particularly
hydrogen bromide, or by the use of fairly small proportions of polar solvents
(acetic acid), or of catalysts (ferric and aluminum bromides); and the
abnormal addition can be inhibited by the use of antioxidants. To favor
(8 )
(9)
(10)
(11)
RCH=CHZ + H* * RCHCH3 0
386 FRANK R. MAY0 AND CHEVES WALLING
abnormal addition, this reaction may be accelerated by the use of oxygen,
peroxides, certain metals (iron, nickel), light, or elevated temperatures;
the normal addition (unless it is naturally slow) should be retarded by
dilution with inert, non-polar solvents. It should be added that any atom
or free radical which can react with the hydrogen atom or the bromine
atom of hydrogen bromide or with an alkene may be capable of starting
a reaction chain.
8. The addition of hydrogen bromide to cyclopropane
Cyclopropane is closely related to propene, in that both react with some
acids and oxidizing agents to give propane derivatives. They differ in
that cyclopropane gives 1 , 3-derivatives of propane, whereas propene gives
1,2-derivatives. In an investigation of the addition of hydrogen bromide
to cyclopropane (78), although the product was always n-propyl bromide,
striking analogies with addition to alkenes were found in rates of reaction.
When equimolecular mixtures of cyclopropane and hydrogen bromide
were allowed to react in sealed tubes a t room temperature in the absence
of air and light, 50 to 60 per cent reaction took place in 4 hr. If 3 mole
per cent of water or acetic acid was added to the mixture, about 90 per
cent reaction took place in the same period, indicating that addition oc-
curred largely through a polar molecular or ionic mechanism like that for
the normal addition of halogen acids to alkenes. Similar proportions of
catechol or thiocresol acted like water or acetic acid, but oxygen or visible
light had only a small accelerating effect on the reaction. However, if
the reaction mixture was made up of 10 moles of cyclopropane to 1 mole
of hydrogen bromide, thus in effect diluting the previous reaction mixtures
with 9 moles of hydrocarbon, then characteristics of the abnormal addition
of hydrogen bromide to alkenes appeared. In the absence of oxygen and
light, only 8 per cent reaction took place in 2 hr. Light alone increased
this yield t o 11 to 12 per cent; oxygen alone, to 81 per cent; oxygen and
light together, t o 99 per cent; peroxides and light, to 74 per cent. Catechol
and diphenylamine had small t o moderate inhibiting effects on the accel-
erated reaction. That the effects were not larger is due to the fact that
the antioxidants (and also water) accelerated addition by favoring the
competing normal mechanism, as in concentrated solution.
These phenomena suggest that oxygen and peroxides react with hydrogen
bromide to give bromine atoms, or with cyclopropane to give free radicals,
and that addition of hydrogen bromide niay then take place through the
following chain mechanism:
H2C\
H2C/
1 CH2 + Br. +. CH2BrCH&H2* (12)
(13) CH*BrCH&Hz* + HBr +. CH2BrCHtCK3 + B r
THE PEROXIDE EFFECT 387
In additions of hydrogen bromide to pure alkenes, light is more powerful
than oxygen in promoting abnormal addition, but the reverse is true for
cyclopropane. Hence the analogy between alkenes and this compound
is not complete. Possibly the chain may require modification to include
oxygen (78).
111. THE ADDITION OF MERCAPTANS AND THIO ACIDS TO
UNSATURATED COMPOUNDS
A. INTRODUCTION
A mercaptan, like a halogen acid, when it is added t o an unsymmetrically
substituted ethylene bond, can yield either of two products:
RCH=CHz + R’SH + RCHZCH2SR’ (abnormal addition)
RCH=CHz + R’SH + RCH(CH3)SR’ (normal addition)
(14)
(15)
These two possible reactions are here designated “normal” and “abnor-
mal” additions on the assumption that mercaptans should add like halogen
acids. Actually, both additions have been recorded, but when the absorb-
ing compound is a hydrocarbon, the abnormal addition reaction is the one
commonly observed when no catalysts for the normal addition are em-
ployed. This fact accounts for the acceptance of the rule prematurely
proposed by Posner (125) for the addition of mercaptans to alkenes:
namely, that the sulfur becomes attached to the carbon atom holding the
most hydrogen atoms. The products and mechanism of the abnormal
addition of mercaptans to alkenes will be first discussed; then those of
the normal addition reaction. It will be shown that both have much in
common with the additions of hydrogen bromide. Finally, the range of
applicability of both reactions will be indicated.
Although mercaptans usually add abnormally, there is no evidence of
such an addition for hydrogen sulfide, which adds only a t fairly high tem-
peratures. Under pressure and below 200”C., all the mercaptans and
sulfides obtained are normal addition products, and their formation is
catalyzed by sulfur (45, 49). The vapor-phase reaction at atmospheric
pressure has been studied at 200-300°C. over a nickel-kieselguhr catalyst
(9) and a t 300°C. over silica gel (108). In the first instance, 5 to 25 per
cent of a mixture containing about 65 per cent of isopropyl mercaptan and
35 per cent of n-propyl mercaptan was obtained from propene. This reac-
tion product was considered to be an equilibrium mixture of the two
possible addition products (cf. footnote 3). In the second instance, mer-
captans, sulfides, and thiophene derivatives of unknown structure were
obtained.
388 FRANK R. MAY0 AND CHEVES WALLING
B. THE ABNORMAL ADDITION OF MERCAPTANS AND THIO ACIDS
It has been clearly shown (6, 16, 49, 76) that the abnormal addition of
mercaptans to alkenes is catalyzed by oxygen and peroxides, that it is
inhibited by hydroquinone and piperidine, and that it is accelerated by
light. The peroxides formed when an alkene is exposed to air are sufficient
to catalyze the abnormal addition; careful purification of the reactants
and exclusion of air prevents, or greatly retards, any addition. Table 9
summarizes the evidence that the abnormal addition of mercaptans to
alkenes can be inhibited by antioxidants, and initiated by oxygen or per-
oxides, particularly in the presence of light. It is therefore a chain
reaction. The structures of the products formed also supply excellent
evidence that the addition does not proceed through a polar or ionic
mechanism. Table 10 lists other reactions which show that abnormal
addition products are commonly formed when air is not excluded. Table 9
records two instances in which both normal and abnormal addition
products are formed, although the latter predominate. In every other
addition listed in either table, the abnormal addition product is formed
exclusively. The known examples include both aliphatic and aromatic
mercaptans, mercaptoacetic acid, and thioacetic acid; the alkenes contain
aromatic and aliphatic substituents, but all except trimethylethylene have
terminal double bonds.
In 1934, Burkhardt (16) mentioned that the abnormal addition of thio-
phenol to styrene might be due to the presence of “sulfur in a positive ion
or in an oxidizing form,” possibly as a free radical, and possibly through a
chain reaction. A more definite proposal that the abnormal addition takes
place through a chain reaction involving free radicals was subsequently
made from this laboratory (76) :
+ RS* + HO2. (1.6) (alkene) RSH + 0 2 (or peroxide)
RS* + R’CH=CH2 ---f R’CHCH2SR (AH = 13 kg-cal. per mole) (17)
R’CHCHZSR + RSH + R’CH2CHtSR + RS* (AH = 0) (18)
0
The effects of oxygen, peroxides, light, and antioxidants and the nature of
the addition product so resemble those observed in the abnormal addition
of hydrogen bromide that little further comment on these points is neces-
sary. In both instances, the heats of reaction of the corresponding steps,
as calculated from estimated bond energies (124), are almost identical
(cf. table 7). The well-known easy oxidation of mercaptans to disulfides
and the suggested dissociation of disulfides into free radicals (132) further
support the mechanism suggested.
Substantial addition of mercaptoacetic acid to styrene in the absence
THE PEROXIDE EFFECT 389
$ a
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II
390 FRANK R. MAY0 AND CHEVES WALLING
THE PEROXIDE EFFECT 391
f CsHs .
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m- It
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C-S-
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C6H4
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m- II
C-S- CaHd
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392 FRANK R. MAY0 AND CHEVE8 WALLING
c
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THE PEROXIDE EFFECT 393
but several analogies to the normal additions of halogen acids can be
pointed out: (a) The catalysis of mercaptan addition by sulfuric acid
resembles the participation of two molecules of halogen acid in the addi-
tion of one such molecule, and suggests that the alkene-acid complex
reacts with mercaptan. (b) The catalysis of mercaptan addition by bases
recalls the accelerating effect of ammonium salts on halogen acid additions,
and suggests that mercaptide ion may sometimes participate in the reac-
tion. (c ) Catalysis by sulfur resembles the catalysis of hydrogen iodide
addition by iodine. (d) Catalysis by metal sulfides is somewhat analogous
to catalysis of halogen acid additions by metal halides.
The fact that an alkene is symmetrically substituted does not prevent
the addition of a mercaptan from taking place through two distinct mecha-
nisms. The addition of ethyl mercaptan to cyclohexene is catalyzed by
either peroxides or sulfur, although cyclohexyl ethyl sulfide is the only
possible product. Peroxide catalysis is prevented by hydroquinone, with
the result that no addition takes place (49).
D. SCOPE OF THE MERCAPTAN-ALKENE REACTION
The following work is cited to indicate the applicability of the mer-
captan-alkene reaction, but, except in additions to ethylene, the structures
of the addition products have not been established. Posner (125) found
that thiophenol or benzyl mercaptan would add to a large number of solid
and liquid hydrocarbons, the only exceptions noted being stilbene and
1,4diphenylbutadiene. He obtained no significant addition to ethylene
or propene at ordinary temperatures, but Jones and Reid (49) added
ethyl mercaptan and trimethylene dimercaptan to ethylene a t high tem-
peratures and pressures in the presence of sulfur. Von Braun and
Plate (13) explained the ready polymerization of allyl, crotyl, furfuryl,
and cinnamyl mercaptans by the interaction of sulfhydryl groups with
double bonds. Holmberg (40) found that mercaptoacetic acid added
readily to cinnamyl alcohol, its acetate, and its benzoate. The ready
addition of mercaptoacetic acid to many unsaturated compounds and the
easy estimation of the sulfhydryl group have led to the use of this reaction
in the determination of the degree of unsaturation in oils, fats (7), and
gasoline (42). It is stated that peroxides in the latter instance retard
the reaction of 2-octene1 a claim not in accord with any other available
information.
Morgan and Friedman (118) have studied the rate and extent of addi-
tion of mercaptoacetic acid, cysteine, and glutathione to maleic acid.
They used evacuated reaction tubes and buffered aqueous solutions of
sodium salts a t a pH of 7.4 and a temperature of 37°C. Part of the maleic
acid which did not react was isomerized to fumaric acid, a point to be
394 FRANK R. M A Y 0 AND CHEVES WALLING
considered in a later section. However, none of these mercaptan deriva-
tives was found to add to fumaric, citraconic, mesaconic, or a-phenyl-
/3-styrylmaleic acids or t o cis- and trans-cinnamic acids.
IV. THE REACTION OF BISULFITES WITH UNSATURATED COMPOUNDS
A. INTRODUCTION
It has long been known that ammonium and alkali-metal bisulfites in
aqueous solution add to carbon double bonds, thus forming alkyl sulfonates:
R1R2C=CR3R4 + NaHS03 --+ R1R2CHCR3R4(SO&a) (21)
Until recently, the best-known of these additions were those to aldehydes,
ketones, and acids unsaturated in the a,P-positions (144). Here the
products were exclusively those which correspond to the ones obtained by
normal addition of halogen acids,-the proton going to the a-carbon atom,
the sulfonate group to the P-carbon atom. It has also been shown that
bisulfites sometimes add to double bonds in alcohols, in aldehydes, and
in liquid and gaseous hydrocarbons where these bonds are not conjugated
with carbonyl groups (for references see 75, 144), but the structures of
most of the products are unknown. Ethylene and cyclohexene give only
one addition product; allyl alcohol was concluded to yield 3-hydroxy-
propyl-1-sulfonate ; the structure assigned to the styrene addition product
(6 ) was in error (75).
Kolker and Lapworth (100) shook their reaction mixtures with kieselguhr
in order to maintain contact between hydrocarbon and water solution,
and thus obtained addition products from several hydrocarbons. They
noted that dilution of the bisulfite solution favored reaction. Other work-
ers found that refluxing the reactants was sometimes successful; still others
reported that reaction often failed to take place in sealed tubes, even at
higher temperatures. Some of these observations suggested that the addi-
tion of bisulfite t o unconjugated double bonds may take place through a
radical-chain mechanism, and subsequent work agrees with this hypothe-
sis. Since the reaction with aliphatic double bonds seems to consist
mostly, if not entirely, of simple addition, i t will be discussed first. The
more complicated reaction with styrene will be considered later.
B. THE ADDITION O F BISULFITES TO ALIPHATIC UNSATURATED COMPOUNDS
The work dealing with the effect of oxidizing agents on the addition of
bisulfites t o alkenes has been carried out largely in this laboratory; the
three different techniques employed will be indicated. Except with allyl
alcohol, all the additions of bisulfites were carried out a t room temperature.
Bisulfites were added to ethylene, propene, and isobutylene (75, 29)
by shaking an aqueous solution of the salts with the gas under pressures
THE PEROXIDE EFFECT 395
of 15 to 40 pounds per square inch. Under these conditions the gases in
question did not react with bisulfites in the absence of oxygen. Admission
of a little air permitted reaction to proceed only temporarily, and repeated
intermittent admissions were required to obtain substantially complete
reaction of the bisulfite. Based on the amount of bisulfite consumed,
propene and isobutylene gave as much as 55 per cent and 62 per cent,
respectively, of organic sulfonates, the remainder of the salt being oxidized
to bisulfate. Ethylene gave lower yields. The products isolated were
exclusively the primary sulfonates, corresponding to an abnormal addition
of hydrogen bromide or mercaptan. No normal addition of bisulfite is
known where the double bond is not conjugated with a carbonyl group.
When such conjugation occurs, there is no peroxide effect; experiments
with crotonic acid (96) show that, just as in hydrogen bromide additions,
oxygen here has no effect either on the rate or the direction of addition.
Liquid cyclohexene, 2-pentene, trimethylethylene, 2 , 4,4-trimethyl-
2-pentene, isoprene, and pinene were shaken with bisulfite solutions under
constant oxygen pressures (130). Oxygen was consumed slowly during
the course of the reaction, and, if the supply was interrupted, interaction
of the hydrocarbon with bisulfite also stopped. The first three of the
substances named gave as much as 90 per cent or more of sulfonates, the
yields being greatest a t 30 mm. of oxygen and decreasing progressively a t
152 or 760 mm. The last three alkenes reacted less easily and gave yields
of only 15 to 20 per cent. Cyclohexene gave a cyclohexylsulfonate; no
other addition products were identified. The product from isoprene still
contained one double bond; that from pinene was unstable. Trimethyl-
ethylene and 2-pentene seemed to give mixtures of sulfonates.
Additions to allyl alcohol were carried out in sealed tubes a t 100°C. (75).
Evacuation of these tubes did not prevent addition, but when 10 mole
per cent of hydroquinone was added to the reaction mixture before evacua-
tion, no reaction occurred. In the presence of oxygen, up to 65 per cent
of sodium 3-hydroxypropane-l-sulfonate mas obtained, as demonstrated
by conversion of the product t o 3-chloropropane-l-sulfonamide. On the
assumption that unsymmetrical reagents add to allyl alcohol in the same
way that they add to the allyl halides, the above addition to allyl alcohol
is abnormal.
In additions to both propene (75) and the liquid alkenes (130), sodium
or ammonium nitrites, which are also capable of oxidizing bisulfites, have
been found to exert an effect like that of oxygen. In experiments with
cychlohexene, 0.06 mole of nitrite per mole of bisulfite, introduced slowly
over a long period, gave 84 per cent sulfonate, whereas a larger proportion
of nitrite (0.1 mole), introduced all a t once, gave only 55 per cent yield.
In an experiment with trimethylethylene, 0.005 mole of sodium nitrite
396 FRANK R. MAY0 AND CHEVES WALLING
per mole of bisulfite gave 90 per cent yield of addition product, indicating
that about 180 molecules of organic sulfonate were formed per molecule
of nitrite reduced.
It has already been shown by Franck and Haber (23) and by Back-
strom (8) that the oxidation of sulfite and bisulfite are chain reactions
involving the *SO3- ion radical and the *HS03 radical. The necessity
for using oxygen or other oxidizing agents in the addition reaction, the
small proportion of agent required, and the advantage of introducing this
agent gradually, as well as the inhibition of the reaction by hydroquinone,
and the fact that the product corresponds to an abnormal addition, all
suggest a chain reaction involving free radicals (75):
SO3-- + oxidant + 4 0 3 - + oxidant-
*S03- + RCH=CHz + RCHCHzS03-
RCHCHzS03- + HSOa- + RCH2CHzS03- + *S03-
The extent to which the sulfite-ion radical and the sulfonate-ion radical
may be associated with a proton is not known. 'The range of pH over
which addition can take place suggests that either or both charged and
uncharged radicals may participate.
The oxidation of bisulfite to bisulfate during oxygen-catalyzed additions
causes an increase in the acidity of the solution and a retardation of the
addition. The increase in acidity can be overcome by substituting sulfite
for that part of the bisulfite oxidized, so that normal sulfate rather than
bisulfate is formed. If too large a proportion of sulfite is used, some of
this substance adds to the alkene, thus liberating an equivalent of alkali
which also retards the addition reaction. If the ratio of sulfite to bisulfite
in the initial solution is the same as the ratio of sulfate t o sulfonate in
the reaction products, then the pH of the sulfite-bisulfite buffer remains
practically constant and the addition of bisulfite proceeds a t a maximum
rate and to a maximum extent. This optimum proportion of sulfite t o
bisulfite varies with both the rate of oxidation and of addition, and there-
fore depends upon the concentration of sulfite, the alkene used, and the
other experimental conditions. The pH of the sulfite-bisulfite buffer
depends upon the sulfite-bisulfite ratio and on the cation.
Although the pH of such buffers is thus of little theoretical significance,
some representative data are cited here. In the addition to cyclohexene,
using oxygen a t 1 atm. pressure, the optimum pH of a sodium salt buffer
was about 6.4 and that of a similar ammonium salt buffer about 6.0 (130).
In additions to propene, oxygen was supplied intermittently; the optimum
pH of a sodium salt buffer was 5.8 (76.5 per cent bisulfite, 23.5 per cent
sulfite), and that of an ammonium salt buffer was 6.0 (55 per cent bisulfite,
45 per cent sulfite). With propene, the pH of these solutions remained
(22)
(23)
(24)
THE PEROXIDE EFFECT 397
constant while 95 per cent of the available sulfite was consumed, and the
proportions of sulfonate and sulfate formed corresponded closely to the
bisulfite-sulfite ratio (29). It follows that if an alkene reacts sluggishly
with bisulfite, the proportion of oxidation increases and a higher proportion
of sulfite should be used.
Although the only products isolated from the reaction of bisulfites with
simple alkenes are the addition products, there is evidence that some by-
products are formed. Kolker and Lapworth (100) observed the formation
of variable, but usually very small, proportions of what they thought t o
be sulfite esters. These by-products were easily oxidized by bromine and
permanganate. When hydrolyzed with acid, they yielded sulfur dioxide.
By hydrolysis with acid, the formation of small amounts of sulfur dioxide
has in some cases been confirmed qualitatively in this laboratory, but
since most crude addition products do not reduce iodine (29), the ability
of these products to reduce bromine and permanganate is more likely due
to unsaturation rather than to the presence of sulfite esters. Attempts
have been made, by bromide-bromate titration (105) and by permanganate
assay (81), to estimate the degree of unsaturation in the crude sulfonic
acids. Since the two methods do not agree and since the existence and
proportions of unsaturated sulfonate, hydroxysulfonate, and sulfite esters
in the addition products have not yet been demonstrated, the results
cannot be considered reliable. In additions to propene (29), about 15
per cent unsaturation in the products is claimed when the buffers are of
nearly ideal composition. With more acid or more basic buffers, the yields
of sulfonic acids are much lower, but the proportion of unsaturation
increases with the acid concentration. In additions to liquid alkenes
using oxygen a t 1 atm. pressure, 1.5 per cent to 18 per cent unsaturation
has been reported in the product. With nitrite instead of oxygen, i t was
shown conclusively that unsaturation was absent (130). The significance
of unsaturation will be considered in the case of styrene, where various
reaction products have been isolated and analytical methods have been
tested.
The yields of addition products from the less reactive alkenes are
decreased by the use of either alcohol or hydrocarbon solvents and slightly
increased by the use of ethylenediamine. Since ethylenediamine has
little effect in nitrite-promoted additions, i t probably serves mostly to
inhibit the oxidation of bisulfite (29, 130).
Attempts to add bisulfites t o acetylene yielded only traces of unidenti-
fied sulfonic acids (29).
C. THE REACTION O F BISULFITES WITH STYRENE
Since the reaction of styrene with bisulfites is fully described in a recent
The reactions paper (Sl), the work need be only briefly summarized here.
398 FRANK R. MAY0 AND CHEVES WALLING
were carried out under constant oxygen pressure. At 1 atm. of oxygen,
sodium and ammonium sulfite-bisulfite mixtures gave 30 per cent or less
of organic sulfonates (the remainder of the sulfite being oxidized to sulfate),
but yields up to 68 per cent were obtained a t lower oxygen pressures. The
reaction of styrene was about as sensitive to excess acid and base as the
reactions of those aliphatic olefins which are liquids at room temperature.
In the presence of oxygen, the organic sulfonates always consisted of
three types of salts. Where sodium salts were used, the final mixture
contained about 25 per cent of addition product (I) corresponding to an
abnormal addition reaction, 10 per cent of substitution product (11), and
65 per cent of hydroxysulfonate (111) :
CeHaCHzCHzSOaNa CeH&H=CHSO3Na CeH6CHOHCH2S0&a
I I1 I11
The proportions of these products varied slightly. Ammonium salts (from
ammonia or the methylamines) gave somewhat more substitution product
(11) and correspondingly less addition product (I) than sodium salts;
ethylene diammonium sulfite gave a higher proportion (82 per cent) of
hydroxysulfonate (111). Additions of aqueous sulfurous acid to styrene
in the presence of a large excess of dimethylaniline or pyridine gave about
60 per cent yields of total sulfonates. This increase is probably due to the
fact that these bases increase the miscibility of bisulfite and hydrocarbon,
and that they maintain the pH a t a favorable level. Sodium and ammo-
nium nitrites and ammonium persulfate can replace oxygen in promoting
the reaction of bisulfite with styrene but they are no more efficient, 30
mole per cent or more of these substances being required for a 15 to 25
per cent yield of total sulfonates. With these oxidizing agents, no unsatu-
rated sulfonate (11) whatever was found, the proportion of both addition
product and hydroxysulfonate being increased.
All three types of sulfonates were isolated in the pure state. No one
of them is converted into any other under the conditions of the styrene-
bisulfite reaction, and all are stable to hot dilute acids and bases. There-
fore all three are thought to be primary products.
Since the presence of an oxidizing agent is necessary for the formation
of all three sulfonates, and since in all of them the sulfur is attached to the
terminal carbon atom, the three reactions are easily correlated by the
assumption that the sulfonate-ion radical formed by reactions 22 and 23
is an intermediate common to all. If such is the fact, then the low yields
of addition product (I) prove that only a small proportion of the sulfonate-
ion radicals formed undergo reaction 24, whereas the large amount of
oxidizing agent consumed in the styrene reaction suggests that this agent
reacts with the sulfonate-ion radical. The unsaturated sulfonate may be
THE PEROXIDE EFFECT 399
formed through reaction 25, oxygen being the only oxidizing agent known
t o give this result:
RCHCR2S03- + 0 2 + RCH=CHS03- + HOz* (25)
The H02* radical may react with bisulfite.
is formed in the presence of oxygen, nitrite, or persulfate:
The hydroxysulfonate (111)
RCHCH2S03- + oxidant .--f RCHCH2S03- + oxidant- (26) +
RCHCHzS03- + HzO + RCHOHCHzSO3- + H+ (27) +
The difference between styrene and the alkenes apparently lies in the
ease with which the sulfonate-ion radical reacts with bisulfite (reaction 24).
The aliphatic free radicals apparently need little or no activation energy
in order to undergo this reaction, and since the concentration of bisulfite
is much higher than that of oxygen, the simple alkenes give long chains.
The substituted benzyl radical formed from styrene obviously reacts slug-
gishly with bisulfite, but much more easily with oxygen, considering the
low concentration of the latter in solution. If the slow reaction of bisulfite
with the substituted benzyl radical is due to the stabilization of the latter
by resonance, then this stabilization has little or no effect on the ability
of the radical t o react with Oxidizing agents.
Except for compounds containing carbonyl groups, cinnamyl alcohol
seems to be the only styrene derivative whose reaction with bisulfite has
been investigated. The reaction depends upon the presence of oxygen
(75), but no products have been identified.
V. THE PEROXIDE EFFECT IN REARRANGEMENTS
A. REARRANGEMENT O F l-BROMO-2-BUTENE AND 3-BROMO-l-BUTENE
The peroxide effect in the rearrangement of l-bromo-2-butene and 3-
bromo-l-butene was discovered (66) during a study of the addition of
hydrogen bromide to butadiene, of which the bromides in question are the
two addition products:
p XCH&H=CHCH3 (“crotyl halide”, IV)
CHFCHCH=CHZ + H X . (28) ~r ’ CH2=CHCHXCI& (“secondary
halide”, V) (29)
It was shown by Winstein and Young (166) that either bromide when
pure undergoes, on standing a t room temperature, an allylic rearrangement
to an equilibrium mixt’ure of 85 per cent crotyl bromide (IV) and 15 per
cent secondary bromide (V). Later, it was shown in this laboratory (66)
400 FRANK R. MAY0 AND CHEVES WALLING
that under the combined influence of hydrogen bromide and ascaridole,
rearrangement, even a t - 12"C., is rapid and complete. Under the same
conditions, ascaridole alone has no effect; hydrogen bromide alone causes
only slight rearrangement. Young and Nozaki (169) have since employed
hydrogen bromide and benzoyl peroxide to accelerate these rearrange-
ments, but the reviewers know of no other application of the early finding.
The rearrangements of some homologs of these bromides are reported to
be highly susceptible to the effects of traces of unspecified catalysts (170).
It is probable that the peroxide effect is widespread in allylic rearrange-
ments of bromides, and that therefore the additions of hydrogen bromide
and bromine to conjugated systems need reinvestigation.
That the allylic rearrangement sometimes has a molecular or ionic mech-
anism is shown by a study of the behavior of l-chloro-2-butene (IV) and
3-chloro-l-butene (V) (71). As expected, peroxides and air have no effect
on their rearrangements, which are very slow except in the presence of a
catalyst. Small amounts of anhydrous ferric chloride or of a mixture of
cuprous and hydrogen chlorides cause isomerization of either chloride to
an equilibrium mixture containing about equal proportions of each isomer.
This isomerization is rapid with the former reagent, somewhat slower with
the latter. A large proportion of hydrogen chloride alone causes slow
rearrangement to a different equilibrium mixture, indicating that this acid
forms a complex with one or both of the organic halides. These phe-
nomena show that the rearrangements of these chlorides have much in
common with the normal addition of halogen acids to alkenes.
The facts that small amounts of both hydrogen bromide and peroxides
are required for a very rapid rearrangement of bromides, and that allyl-
type chlorides are not susceptible to corresponding influences, suggest that
the peroxide-catalyzed rearrangement of bromides proceeds by a chain
mechanism involving bromine atoms or free radicals. On this meagre
basis, two such mechanisms are tentatively suggested :
CH2=CHCHBrCH3 + Br* F? CHZBrCHCHBrCH3 F?
CH2BrCH=CHCH3 + Br* (30)
CH&=HCHCHB (indistinguishable from CH&H=CHCHa) +
CHZBrCH=CHCH3 Ft CH2=CHCHBrC& + CH&H=CHCHa (3 1)
There are two difficulties with the first mechanism. One is that in the
addition of a bromine atom to l-bromo-Zbutene the bromine atom might
not attach itself t o the 3-carbon atom,20 as required for rearrangement.
1 0 This difficulty may not arise if i t is assumed that the bromine atom adds to the
ethylene bond to give the more stable free radical.
THE PEROXIDE EFFECT 401
The other difficulty is that, wherever a bromine atom adds to either isomer,
subsequent separation of a bromine atom from the free radical formed may
be too endothermic for a chain reaction. The second mechanism avoids
both of these objections. The energy change involved in reaction 31 is
very close to zero, and if the exchanges proceed through a chain reaction,
the activation energies must also be small. The free radical necessary to
start the chain may be formed as follows:
CH2BrCHCHBrCH3 (cf. reaction 30) + CH2BrCH=CHCH3
--$ CHzBrCHBrCHBrCHa + #CHzCH=CHCH3 (32)
The conditions for the rearrangement of the products having been es-
tablished, the addition of hydrogen chloride and hydrogen bromide to
butadiene may be considered. As mentioned in the discussion accom-
panying table 1, the addition of hydrogen chloride to butadiene gives, over
a wide range of temperatures (71), 75 to 80 per cent of secondary chloride
(V) and 20 to 25 per cent of crotyl chloride (IV). Isomerization of the
products under the conditions of addition is negligible. Addition of hydro-
gen bromide under conditions most favorable for the normal addition (pres-
ence of an antioxidant, absence of air, temperature -78°C.) gives within
experimental error the same proportion of isomers. As the temperature
of addition is raised to 25"C., the proportion of crotyl bromide formed in-
creases to 56 per cent. If the addition is carried out in the presence of a
peroxide, only 40 t o 45 per cent of crotyl bromide is formed a t -78"C., but
70 to 80 per cent of this bromide (approximately the equilibrium mixture)
is formed a t -12°C.
Since the addition of hydrogen chloride shows no temperature effect,
and since a t low temperatures in the presence of antioxidants essentially
the same results are obtained with hydrogen bromide, it is concluded that
the normal addition of a halogen acid t o 1,S-butadiene gives about 80
per cent 1 ,&addition and 20 per cent 1,4-addition. In additions of hy-
drogen bromide a t room temperature, or in the presence of peroxides, some
or all of the additional crotyl bromide found is due t o isomerization of
secondary bromide first formed by 1 ,Zaddition. The possibility that
addition by an abnormal mechanism increases the proportion of direct
1,4-addition has not been ruled out, but the failure to find any 4-bromo-1-
butene excludes the possibility that appreciable abnormal 1 ,2-addition has
occurred in any experiment made to date.
B. REARRANGEMENT OF a-BROMOACETOACETIC ESTERS
It has been shown by Hantzsch and coworkers (33) that a-bromo-
acetoacetic ester rearranges slowly a t room temperature to ybromoaceto-
acetic ester, that this change is accelerated by hydrogen bromide, and that
402 FRANK R . MAY0 AND CHEVES WALLING
i t is inhibited by water. However, the a-bromo ester, in spite of its
instability a t room temperature, can be distilled four or five times in vacuo
at 100°C. without change. The effect of hydrogen bromide explains the
differences between the methods for preparing the two esters directly (21).
When acetoacetic ester is brominated in the presence of ice and water, pure
a-bromo ester is immediately obtained. When slow bromination lasting
several hours takes place in carbon disulfide solution, the y-bromo ester
is formed. If, in the latter preparation, hydrogen bromide is removed by
intermittent washings with water, mixtures are obtained. This result
suggests that the y-bromo ester is formed by rearrangement of the a-bromo
ester. The a- and y-bromo derivatives of methyl a-methylacetoacetate
have been similarly prepared. Ethyl a-chloroacetoacetate has no ten-
dency to rearrange, even in the presence of hydrogen chloride (21). This
difference between the a-chloro and a-bromo esters suggested the possi-
bility of a peroxide effect in the rearrangement of the bromo esters.
By a series of experiments, each lasting only a few hours and carried out
in glacial acetic acid solution, it was found that air, a peroxide, or light
greatly accelerated the rate of rearrangement of the a-bromo ester by
hydrogen bromide (68). In the absence of hydrogen bromide, no re-
arrangement took place in the presence of a peroxide and light, either with
or without hydrogen chloride. In glacial acetic acid, the bromination of
ethyl acetoacetate in the absence of air, peroxides, and light gave more than
90 per cent a-bromo ester; in the presence of any one of these agents, 80
per cent or more of the y-isomer was formed. The bromination of ethyl
a-methylacetoacetate was very similar, except that rearrangement was
more rapid.
The effects of oxygen, peroxides, and light on the rearrangement of
a-bromoacetoacetic esters by hydrogen bromide and the stability of the
corresponding chloro esters suggest that the rearrangement has much in
common with the rearrangements of the butenyl halides and the addition
of hydrogen bromide to alkenes. Probably a part, if not all, of the rear-
rangement takes place through a chain mechanism in some stage of which
bromine atoms are involved. Stability of the bromo esters in thepresence
of a suitable inhibitor would indicate whether the isomerization is ex-
clusively a chain reaction and whether the chain carrier is the same as in
the abnormal addition of hydrogen bromide to alkenes. Such experiments
have not yet been performed; consequently any discussion of a mechanism
is admittedly speculative. Only in order to show that a simple chain
can be written is the following mechanism ventured:
*CHZCOCHZCOOCZH~ 4- CH3COCHBrCOOC2H6
CH2BrCOCH2COOC2H6 + CH3COCHCOOC2H6 (33)
CH~COCHCOOCZH~ $ *CHzCOCHzCOOC2Hs (34)
THE PEROXIDE EFFECT 403
The mode of formation of bromine atoms has been indicated in an earlier
section. These atoms may attack ester molecules to yield the free radicals
necessary to start chains. Since the bromination of acetoacetic ester is
reversible, the bromine atoms may attack a bromo ester molecule to give
either bromine or hydrogen bromide. Like the second mechanism pro-
posed for the rearrangement of the butenyl bromides, the chain here sug-
gested consists of the transfer of bromine atoms from a molecule to a radi-
cal; the chain carrier is not a bromine atom, but a radical which can iso-
merize. Whatever the mechanism of the rearrangement, the product
should be an equilibrium mixture of the y-bromo ester with a small pro-
portion of the a-isomer.
C. CIS-TRANS ISOMERIZATIONS
A thorough discussion of the rearrangement of geometrical isomers is
beyond the scope of this review on the peroxide effect. Except for recent
work, the subject has been covered by both R. Kuhn (24) and Dufraisse
(28). In order that isomerization may occur, the resistance of the double
bond to free rotation must be overcome. The assumption by some in-
vestigators that all such isomerizations take place by a single mechanism
has led to some confusion, for a t least four distinct mechanisms will be
cited for the conversion of a cis-ethylene derivative to its trans-form.
The first mechanism is associated with the simplest reaction, repre-
sented by the homogeneous, apparently unimolecular, rearrangement of
dimethyl maleate in the vapor phase (119, 147). This reaction has a
fairly high activation energy and requires a temperature of around 300°C.
The uncatalyzed liquid-phase isomerizations of isostilbene and its a-chloro
derivatives (149) are also unimolecular. They require a temperature of
a t least 200°C. and have activation energies of about 35 kg-cal. These
isomerizations apparently depend on violent collisions to effect rotation
about the double bond.
Ultraviolet radiation is known to cause isomerization of ethylene deriva-
tives in the absence of other catalysts. According to Mulliken (118a),
absorption of such radiation by an ethylene derivative causes a transition
to an excited electronic state in which the perpendicular configuration of
the groups placed about the double bond is more stable than the planar
configuration characteristic of the unexcited state.
A third type of mechanism is associated with a catalyst which can donate
a proton or accept a pair of electrons. Mineral acids (24, 28, 137, 138,
149) are known to be effective in many instances and ineffective in others;
primary and secondary, but not tertiary, amines rapidly isomerize di-
methyl maleate (18); aluminum, ferric, and zinc chlorides have been
effective with the same ester (26) ; boron trifluoride has been found to rear-
range isostilbene, but not dimethyl maleate (127). Various workers have
404 FRANK R . MAY0 AND CHEVES WALLING
suggested that one of the doubly bound carbon atoms shares a pair of
electrons with the catalyst, leaving the other previously doubly bound
carbon atom with a positive charge and free to rotate about the axis of the
former double bond. After such rotation, dissociation from the catalyst
permits reestablishment of the double bond and consequent formation of
the geometrical isomer. If a carbonyl group is conjugated with the double
bond, association of the catalyst may take place a t the carbonyl group,
but then the double bond is shifted and rotation between the carbon atoms
previously doubly bound becomes possible :
\ I \ I
/
C=C-C-+HX* C-CzC- .. /(+) * '
.o. :0:
..
WHX
Most of the remaining observations on the effects of catalysts suggest
that a fourth type of mechanism involves catalysts with two unpaired or
an odd number of valence electrons. For the sake of brevity, it will be
assumed that all such catalysts act through a single mechanism, although
future work may show that this class of substances should be subdivided.
The general scheme is represented by reaction 36:
VI VI1
As in the proton-catalyzed mechanism, rotation about the double bond in
the intermediate is possible, but the intermediate here is a free radical.
I t s stability is unknown. The catalysts include univalent atoms, free
radicals, molecules with odd electrons, and paramagnetic substances in
general. One class of catalysts which has been assumed to function in
this manner consists of the alkali metals (24); these cause isomerization
without much reaction on the part of the metal. That the isomerization
of isostilbene by stilbene disodium (in the absence of free metal) involves
free radicals or metal atoms is possible, but doubtful in view of the state-
ment by Ziegler and Wollschitt (171) that isomerization takes place on
regeneration of the ethylene compound (after exchange of metal), but not
on the appearance of a single free valence. Platinum and palladium
blacks have been found to isomerize maleic acid (148) and its methyl ester
(24). Paramagnetic metal ions are also said to isomerize the acid (148).
The weak effect of oxygen in accelerating both the vapor-phase (147) and
liquid-phase (146, 148) isomerizations may be partly due to the paramag-
netism of this substance, but in the isomerization of isostilbene (149) the
THE PEROXIDE EFFECT 405
effect of oxygen has been ascribed to the fact that i t causes the formation of
catalytically active acids. Since nitrogen oxides (24, 27, 146) are better
catalysts than oxygen for some isomerizations, the probability that they
function through a free radical addition product is somewhat greater. A
combination of hydrogen sulfide and sulfur dioxide causes isomerization
of maleic acid, although neither gas alone is effective. Heating with
aqueous bisulfite has caused rearrangement of erucic acid. In both cases
the effect has been attributed (24) to the colloidal sulfur formed. More
recent work has shown that a combination of aqueous sulfur dioxide and
manganese dioxide (120) rearranges maleic acid, its esters, and citraconic
acid. In additions of mercaptoacetic acid and glutathione to maleic acid
(118), part of the unreacted maleic acid was isomerized to fumaric acid.
Glutathione would neither add to, nor rearrange, cis-cinnamic acid; hence
it was concluded that the sulfhydryl group must be able to add to a double
bond in order t o cause isomerization. The close analogy between all of
of these observations and those on the addition of mercaptans and bisulfites
suggests that free radicals (formed as intermediates in the oxidation of
bisulfites or hydrogen sulfide by air, peroxides, or manganese dioxide) are
the active agents for the observed isomerizations.
This fourth type of mechanism is closely related to the peroxide effect,
because halogen atoms can cause the reactions in question. Wachholtz
(159) found that the photochemical isomerization of dimethyl maleate by
bromine in carbon tetrachloride solution depended only on the quanta
absorbed by the bromine, and that quantum yields as high as 600 were ob-
tained. The isomerization was thought to proceed according to reaction
36, but an exchange of bromine atoms between the free radicals (VII) and
the unsaturated molecules (VI) seems more likely than a dissociation.
Even higher conversions were obtained when bromine atoms, the pre-
sumably active agents, were generated by chemical means (160). In
water solution, the action of ferrous sulfate on bromine, hypobromous
acid, or bromic acid isomerized 10,000, 1000, and 500 molecules, respec-
tively, of maleic acid per atom of bromine formed. Iodine in the light
(24, 28) and a t elevated temperatures (167) has been found to catalyze
other isomerizations; here iodine atoms seem to be the active agent.
Since it is well established (24, 37) that bromine atoms can cause cis-
trans isomerizations proceeding through chain reactions, the fact that
hydrogen bromide can cause similar isomerizations under conditions favor-
able for previously described peroxide effects is strong support for the con-
tention that, where hydrogen bromide is thus effective, bromine atoms are
involved. The peroxide effect in the rearrangement of geometrical isomers
was observed in this laboratory in 1937, but only the first portion of this
work has yet appeared. Urushibara and Sinamura have subsequently
406 FRANK R. MAY0 AND CHEVES WALLING
published several papers in this field; their observations agree with, and
extend, those made here. The following description is intended to bring
out the relations indicated above and to show how the principal mech-
anism of isomerization changes with the structure of the unsaturated
compound.
In the absence of air and light, the isomerization of stilbene (93, 67,
150) by hydrogen bromide or hydrogen chloride in benzene solution is
very slow, requiring several days. The isomerization by hydrogen bro-
mide is accelerated by light, air, peroxides, or by reduced iron or nickel.
The accelerating effect of these agents can be overcome by catechol, less
effectively by diphenylamine. The isomerization by hydrogen chloride
is unaffected by light or peroxides. These results show that there is a
slow isomerization by acids through a molecular or ionic mechanism, but
that the isomerization by hydrogen bromide proceeds through a much
more rapid chain mechanism. That this chain mechanism involves bro-
mine atoms is suggested by the known ability of such atoms to cause iso-
merizations, together with the fact that small proportions of stilbene
dibromide are formed as a result of the oxidation of hydrogen bromide in
the presence of air and light (93, 150). a,a-Dichlorostilbene is isomerized
by a combination of hydrogen bromide and oxygen, but not by halogen
acids alone (149).
In the isomerization of maleic acid and some of its derivatives when the
halogen acids are present, rearrangement by the bromine-atom chain
mechanism is negligible compared with that by the ionic or molecular
mechanism. Sinamura (137) found that the isomerization of dimethyl
maleate by hydrogen bromide is unaffected by oxygen or antioxidants and
that hydrogen chloride is nearly as effective as hydrogen bromide in pro-
ducing the reaction. Kharasch, Mansfield, and Mayo (93) found that
isomerization of maleic acid, maleic ester, and bromomaleic acid in air was
catalyzed to about the same extent by both hydrogen bromide and hydro-
gen chloride.
Further work by Sinamura (138) shows that the behavior of methyl
allocinnamate is intermediate between that of maleic ester and stilbene
with respect to both mechanisms. With this compound, hydrogen chloride
is distinctly less effective than hydrogen bromide, and the isomerization
by hydrogen bromide is only moderately accelerated by oxygen or partially
inhibited by catechol. Work in this laboratory (93) shows that isomeriza-
tion of the labile form of a-phenylcinnamic acid to the stable form by
hydrogen bromide is accelerated by air and light, whereas the apparently
slower isomerization by hydrogen chloride is not.
From the foregoing discussion it seems probable that acids usually cause
isomerization of a cis- to a trans-ethylene derivative. This mechanism is
THE PEROXIDE EFFECT 407
most important when the ethylene bond is conjugated with one or more
carbonyl groups. Probably any type of ethylene derivative can also iso-
merize by the atom or radical type of mechanism. Examples of the
isomerization of maleic acid derivatives by the bromine-atom mechanism
have been cited, but two groups of workers have failed to observe any
evidence of this mechanism in the presence of hydrogen bromide. It may
therefore be concluded tentatively that, with maleic acid derivatives, the
action of hydrogen bromide or hydrogen chloride through the polar mech-
anism is large compared with the action of the former reagent through the
atom mechanism. The reverse is true for isostilbene, whereas the behavior
of allocinnamic ester is intermediate.
Such considerations show also that the varying effectiveness of different
reagents, which has often in the past been described as anomalous, may
easily be fitted into a general scheme. Some cis-derivatives should be
expected to rearrange easily by several mechanisms, whereas others may
rearrange with difficulty by some or all mechanisms. Accordingly, it is
not a t all surprising that one group of investigators found no correlation
between catalytic activity and magnetic susceptibility (26). In the large
amount of experimental work required to permit comparisons between
different alkenes or different catalysts, it will be necessary to establish
the mechanism of each isomerization.
VI. CONCLUSION
In all the studies discussed in the foregoing review the experimental
facts seem to be best explained by the hypothesis that the striking effects
of oxygen and peroxides arise out of their ability t o initiate chain reactions
in which atoms or free radicals act as chain carriers. So-called solvent
effects and inhibitory effects of traces of various materials are best inter-
preted as the result of their effects on chain reactions. Many discrepancies
in observations recorded in the earlier literature are explained ; many
hitherto isolated phenomena are correlated; and many supposed abnormal-
ities are reduced to parts of a logical pattern.
It is perhaps of even greater significance that a new and broader outlook
on organic reactions in general is opened up by this hypothesis. The con-
cept of chain reactions in solution, involving atoms or free radicals, will
doubtless in many cases supersede earlier limited and inadequate notions
which ascribed all reactions to simple unimolecular or bimolecular mech-
anisms. In any event, it now seems a necessary supplement t o such
ideas.
Even though future work may necessitate changes in the theoretical
concepts of chain reactions, the present ideas have served as a powerful
working hypothesis. Already they have been applied in this laboratory
408 FRANK R. MAY0 AND CHEVES WALLING
to several classes of reactions other than those here discussed. Such
applications are the following:
1. The bromination of phenanthrene (72), toluene (73), cyclopropane
(78), cyclohexane, methylcyclohexane, isobutane (go), aliphatic acids,
acid halides, and anhydrides (91).
2. The use of sulfuryl chloride (in the presence of a peroxide) as 8
chlorinating agent for aliphatic compounds (79, 82, 86).
3. The use of sulfuryl chloride (under illumination in the presence of
pyridine) as a sulfonating agent for aliphatic compounds (80, 89).
4. The introduction of the -COC1 group into aliphatic compounds by
the use of a peroxide and oxalyl chloride (92), or by the use of light and
either oxalyl chloride or phosgene (85).
5 . The use of peroxides in accelerating the chlorination of hydrocarbons
in the dark (88).
It is hoped that a review of these and related phenomena will be under-
taken when sufficient data become available.
The principles discussed in this paper have been developed in collabora-
tion with Prof. M. S. Kharasch over a period of several years. He has
assisted in determining the scope of this review and has offered many
helpful criticisms of its content. Dr. James K. Senior has greatly assisted
the authors in the revision of the manuscript.
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THE PEROXIDE EFFECT 41 1
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412 FRANK R. MAY0 AND CHEVES WALLING
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1x04 G. E. SERNIUK, F. W. BANES AND M. W. SWANEY Vol. 70
and on occasions the temperature was allowed to climb
to 160' in order to remove all of this fraction a t atmos-
pheric pressure.
The pressure was subsequently reduced to 50 to 60 mm.
and n-butanol (+OD 1.390 to 1.398) was removed. After
removing unreacted ethyl phosphate a t pressures varying
between 3 and 10 mm. of mercury (b. p. 66-69' at 3 to
4. mm.; n% 1.404-1.4055), diethyl n-butyl phosphate
(b. p. 82-87" a t 3 to 4 mm.) and ethyl di-n-butyl phos-
phate (b. p. 95-96' at 3 to 4 mm.) were fractionated out
of the reaction mixture.
Anal. Calcd.for C8HlnP04: P, 14.7; M ~ 5 1 . 1 . ~ Found:
P, 14.9; MR 51.5. Calcd. for C ~ O H ~ ~ P O ~ ; P, 13.0;
X ~ 6 0 . 5 . Found: P, 13.2; M ~ 5 9 . 8 . ~
The material balances in all runs including residues
amounted to 97 to 99%.
Isolation of Ethyl %-Butyl Ether.-Fifty grams of the
ethanol-ether fraction (b. p. 36-93') was added to 180
ml. of water. The upper layer, amounting t o 8.9 g., was
separated, dried over activated silica gel and distilled.
The main fraction boiled at 90-93 and was found to have
the following constants: d'4, 0.752; nMD 1.3818; MR
calcd., 31.6; found, 31.6.
Anal. Calcd. for C6H140: C, 70.5; H, 13.8. Found:
C, 70.6; H, 13.9.
Treatment of the ether with hydriodic acid yielded
ethyl and n-butyl iodides.
Alkylation of Morpholine .-An equimolar mixture of
morpholine and ethyl phosphate was charged to a round-
bottomed flask equipped with a water-cooled reflux con-
denser. The mixture was heated by means of a Glas-Col
mantle and brought to 150" in fifteen to twenty minutes.
At this temperature, vigorous refluxing took place due to
heat of reaction, and the mixture changed from a water
white to a reddish brown color. If to: well insulated,
the reaction temperature may rise to 190 . I t was found,
however, that by maintaining the reaction temperature
between I57 and 159" good results can be obtained. The
product was poured into 500 ml. of water and heated with
(8) The molecular refractivities were calculated from that of
ethyl phosphate by adding the proper value for the required number
of methylene groups to that molecule.
10% excess 144 g.) of sodium hydroxide. The basic
aqueous solution was charged to a still and the amine
distilled with water as an azeotrope over the range 95-
99.8'. The azeotropic distillate was saturated with po-
tassium carbonate whereupon the amine was salted out.
After separating from the aqueous layer, the amine was
dried over sodium sulfate, filtered and distilled; b. p.
137-138', azo4 0.919, n'% 1.4418; yield 70%.
Anal. Calcd. for CBH,,NO: C, 62.2; H, 11.4; N,
12.2.
All physical constants given for known compounds
agree satisfactorily with those previously published.
Acknowledgment.-The authors are indebted
to Messrs. N. Beitsch, S . Sass and B. Zeffert of
this Laboratory for having performed the neces-
sary analytical and physical determinations.
Summary
Sodium butylate behaves catalytically on a
mixture of n-butanol and ethyl phosphate yielding
diethyl n-butyl phosphate and ethyl di-n-butyl
phosphate. This alcoholysis is accompanied by a
side reaction which causes the alkylation of the
butylate ion to ethyl n-butyl ether.
This behavior indicates that ethyl phosphate
under the conditions employed behaves both as a
true ester undergoing alcoholysis and as an alky-
lating agent. The degree to which each of the
products of reaction is produced depends upon the
concentration of sodium butylate as well as upon
the mole ratio of n-butanol to ethyl phosphate.
The alkylation of morpholine to N-ethylmor-
pholine by means of ethyl phosphate is also de-
scribed.
ARMY CHEMICAL CENTER, MD.
Found: C,62.3; H, 11.5; N, 11.9.
RECEIVED DECEMBER 15, 1947
[CONTRIBUTION FROM THE ESSO LABORATORIES, CHEMICAL DIVISION, STANDARD OIL DEVELOPMENT COMPANY]
Study of the Reaction of Buna Rubbers with Aliphatic Mercaptans1
BY G. E. SERNIUK, F. w. B A N E S A N D M. w. SWANEY
Introduction
The relative proportion of 1,4- versus 1,2-addi-
tion of diene units and the elucidation of the par-
tial structure of polymers and copolymers of buta-
diene have been investigated by various chemical
and physical methods such as ozon~lysis ,~-~ per-
benzoic acid oxidati~n,~,' potassium permanga-
nate oxidation,6 and infrared absorption.* The
(1) This paper was presented before the Division of Rubber
Chemistry at the American Chemical Society Meeting in Chicago,
1946.
(2) Hill, Lewis and Simonsen, Trans. Forodny Soc., 86, 1067
(1939).
(3) Yakubchik, Vasiliev and Zhabina, RubEn Chcm. a*d Tech., 16,
780 (1945).
(4) Alekseeva nnd Belitzkaya, ibid., 16, 693 (1942).
(5) Rahjohn, Bryan, Inskeep, Johnson and Lawson, THIS JOUR-
(8) Weidlein, Jr., Chum. Eng. News, 24, 772 (1946).
(7) Kolthoff, Lee and Mairs, J . Polymer Science, 2.220 (1947).
(8) Rasmussen and Brattain, private communication.
NAL, 69, 314 (1947).
work presented in this paper was undertaken in an
attempt to obtain further information regarding
the structure of butadiene polymers and copoly-
mers by studying the reaction of these polymers
with aliphatic mercaptans.
The reaction of mercaptans with unsaturated
compounds including natural and synthetic rub-
bers is not new. Posner) Gunnar, Axberg and
Holmberg, lo Hoag and Eichwald,' Kharasch,
Read and Mayo,12 Jones and Reid,13 Cunneen,14
and others have treated mercaptans with various
types of unsaturated compounds. Holmberg's
treated natural pale crepe rubber with thiogly-
(9) Posner, Ber., 85, 646 (1905).
(10) Gunnar, Axberg and Holmberg, i b i d . , 66B, 1193 (1933).
(11) Hoag and Eichwald, Rec. trav. chim., 66, 481 (1939).
(12) Kharasch, Read and Mayo, Chem. and Ind., 67,752 (1938).
(13) Jones and Reid, THIS JOURN+ 60,2452 (1938).
(14) Cunneen, J . Chem. Soc., 38.134 (1947).
(15) Bolmberg, Ber., 66, 1349 (1932).
May, 1948 REACTION OF BUNA RUBBERS WITH ALIPHATIC MERCAPTANS 1805
colic acid, and more recently, Kolthoff and co-
workers] l6 and Marvel and co-workers1' studied
the reaction of aliphatic mercaptans with buta-
diene polymers and copolymers in latex form.
From a preliminary study of the reaction of mer-
captans with model compounds it was found that
ethylenic bonds in conjugated] vinyl, terminal
butenyl, and in closed ring structures added mer-
captans readily, while internal, non-conjugated
ethylenic bonds reacted a t a relatively slower rate.
Thioglycolic acid added to simple olefins more vig-
orously than n-aliphatic mercaptans. Since this
paper was originally submitted] Cunneen'4 re-
ported the reactions of unsaturated hydrocarbons
and various thiols. An apparent order of reactiv-
ity was found to be cyclohexene > dihydromyr-
cene > squalene > rubber; and for the thiols,
thioglycolic acid > thiophenol - isopentanethiol.
It is evident that the ease with which an ethylenic
bond can add thiols is dependent, in part, upon the
structural unit retaining the double bond.
It is quite probable that diene polymers contain
several types of ethylenic bonds, but from ozonoly-
sis data2-5 it must be concluded that a major por-
tion of these bonds results from either 1,2- or 1,4-
addition of butadiene units to the polymer chains.
These two types of ethylenic bonds should exhibit
different rates of mercaptan addition and the de-
termination of the proportion of mercaptan-re-
active units in the polymer chains should represent
a measure of the per cent. of ethylenic bonds pres-
ent as side vinyl groups. Ethylenic bonds in
structures formed by intramolecular cyclization
reactions should likewise be mercaptan reactive.
Obviously mercaptan addition reactions will not
show complete selectivity for side vinyl groups but
i t is quite probable that mercaptan-reactive struc-
tures other than the side vinyl groups will repre-
sent only a very minor portion of the total un-
saturation of the butadiene polymers.
This paper records the reaction of thioglycolic
acid and n-aliphatic mercaptans of CZ to Cle chain
length with diene polymers, and the reaction of
thioglycolic acid and a n-Clz mercaptan with model
compounds. Polymer-mercaptan reactions were
effected in solution, mass and in latex form a t
various temperatures, in the presence of air, or in
the presence of additives which were evaluated as
catalysts. The experimental data indicate a pro-
nounced difference in the rate and extent of mer-
captan addition by the various diene polymers,
and the difference in rate and extent of addition
has been utilized in estimating the relative pro-
portion of external and internal ethylenic bonds
in the polymer chains.
Experimental
Materials
Polymers.-The polymers used in studying the polymer-
mercaptan reactions were natural rubber, polyisoprene ]
polybutadiene, and copolymers of butadiene and acrylo-
(16) Kolthoff and co-workers, private communication.
(17) Marvel and co-workers, private communication.
nitrile, butadiene and styrene, and butadiene and alpha
methyl para-methylstyrene. Both emulsion and sodium
catalyzed polymers of butadiene were used, but in all
other cases, emulsion polymers and copolymers were em-
ployed. The emulsion polymers were prepared by the
standard technique. The polymerization reactions were
discontinued when 75% of the monomers were converted
to polymer. The latices were freed of unconverted
monomers by steam stripping under a pressure of 50-60
mm. Polymers required for solution and mass reactions
were obtained by coagulating the stripped latices with
997, isopropyl alcohol, followed by water washing and
drying at 175' F. No attempt was made to fractionate
the resulting polymers, or to free them of any developed
peroxide materials.
Mercaptans.-Mercaptans of G to CC chain length
were obtained from Eastman Kodak and were used after
distillation. Thioglycolic acid was first dried by removing
the water as a benzene azeotrope before distilling under
vacuum. Normal mercaptans of CC to CIS chain length
were of research grade from the Connecticut Hard Rubber
Company. These mercaptans were used directly without
further purification. Sharples 3B mercaptan was used
after distillation.
Procedure.-A modification of the procedure used by
HolmbergI6 was used in effecting the reaction of various
polymers with thioglycolic acid. A 5% solution of poly-
mer in benzene was placed in a flask and agitated while a
calculated amount of dry thioglycolic acid was added
slowly to the solution at room temperature. The re-
actants were allowed free access to air throughout the
course of the reaction. With butadiene polymers and co-
polymers the reaction was exothermic, and after a short
time the solution became cloudy and an insoluble layer
separated. The separated product was solubilized by the
addition of n-hexanol and the reaction continued.
Samples were withdrawn periodically for analysis. The
polymer-thioglycolic acid reaction products were purified
by water washing the benzene-n-hexanol solutions until
no further test for free thioglycolic acid could be obtained
by titration with 0.1 N iodine solution. The solvents
were then removed by heating the solutions on a steam-
bath under high vacuum. The reaction products were
further dried in a vacuum oven at 70 '. Sulfur analyses of
the products were obtained by combustion in a Parr
bomb.
Polymers in latex form were treated with mercaptans
in 2 oz. and one quart glass reactors which were charged
to varying levels and then agitated in a thermostated
bath at 50' for varying periods of time. The amount of
mercaptan employed corresponded to a 100% excess over
the amount theoretically required for complete double
bond saturation. Several conditions were employed
wherein the free space of the reactors was flushed either
with nitrogen, air, or pure oxygen; and the amount of
persulfate in the systems was varied. The polymer-
mercaptan reaction products were isolated from the
emulsions by coagulating in an excess of 99% isopropyl
alcohol. The products were thoroughly washed in fres,h
portions of alcohol and then dried in a vacuum oven at 70 .
Mass reactions of dry polymers and mercaptans were
carried out under essentially the same conditions em-
ployed by Jones and Reid18 in their study of the reaction
of mercaptans with unsaturated compounds. The dry
polymer was dissolved in the desired mercaptan, two mols
of mercaptan being used per mol of diene in the polymer.
The solutions were agitated in a glass reactor, sealed from
the atmosphere without displacing the air in the reactor,
at 180-200° for varying periods of time. Samples of the
reaction mixture were removed at intervals for purification
and analysis. The polymer-mercaptan reaction products
were isolated by coagulating the reaction mixture with a
large volume of 99% isopropyl alcohol followed by re-
peated dissolution of the mass in petroleum ether and co-
agulation until the mixed solvents showed no trace of free
mercaptan as determined by titration with 0.1 N iodine
solution. The purified reaction mass y s stripped of
solvents and moisture under vacuum at 80 .
1806 G. E. SERNIUK, F. W. BANES AND M. W. SWANEY \'#8



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Scheme 1. Laser-induced polymerization of a diacrylate monomer.
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molecule.
Figure 1. Influence of photoinitiator and of laser wavelength on the
polymerization of an acrylate photoresist: Ar
, 363.8nm; Kr
, 337.4nm;
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Figure 2. Time dependence upon UV
exposure of the IR band of the acrylate
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Figure 3. Laser-induced polymerization of an acrylate photoresist: Kr

laser, 337.4nm; light intensity, 250mWcm2; N2 atmosphere: -  -  -, Dark
polymerization after a 50ms exposure; ——, continuous exposure.


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Figure 4. Insolubilization profile of acrylic photoresists upon UV irradiation:
curve A, amino-polyester-tetraacrylate; curve B, polyphenoxy-diacrylate.
[Irgacure 369]=1wt%; light intensity, 50mWcm2.
Figure 5. Influence of the photoinitiator (0.5wt%) on the polymerization
profile of a wet acrylic photoresist exposed to visible radiation:
I =14mWcm2.
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PHOTOPOLYMERIZATION OF BINARY SYSTEMS
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Scheme 3. Polymer networks formed
by photoinitiated polymerization of a
bisphenol A diacrylate derivative (top)
and of a biscycloaliphatic diepoxide
(bottom).
Figure 6. Photoinitiated polymerization of an acrylate–epoxide blend in the
presence of ITX (0.5wt%) and a diaryliodonium salt (2wt%):I =60mW
cm2; air.


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Figure 7. Hardening of an acrylate–epoxide blend and of the neat
monomers upon UV irradiation in the presence of ITX (0.5wt%) and a
diaryliodonium salt (2wt%): I =500mWcm2; air.
Scheme 4. Crosslinked polymer formed upon UV irradiation of a mixture of
diacrylate and divinyl ether in the presence of both radical and cationic type
photoinitiators.
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Scheme 5. Photoinitiated copolymerization of acrylate and polybutadiene
double bonds.
Figure 8. Photocrosslinking of a 20/80 blend of a diacrylate (Ebecryl 150)
and a styrene–butadiene block copolymer: - - - -, neat SBS copolymer;
[Lucirin TPO]=3wt%; I =500mWcm2; air.


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Figure 9. Polymerization profiles recorded by RTIR spectroscopy upon UV
exposure of a photoinitiator-free maleimide–vinyl ether combination
(MI/VE)=1.
Scheme 6. Chemical structure of the crosslinked copolymer formed by
photopolymerization of a mixture of difunctional vinyl ether, maleate and
maleimide monomers.
Scheme 7. Thiol–ene polymerization reaction.
Figure 10. Photocrosslinking of the ABA–TRIS system upon UV exposure:
[TRIS]=2wt%; [Irgacure 819]=1wt%; light intensity, 600mWcm2.
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Figure 11. Influence of thiol and acrylate monomers on the
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[Irgacure 819]=1wt%.
Scheme 8. Crosslinked polymer
formed by photoinitiated thiol–ene
polymerization of an acrylonitrile–
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REFERENCES
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Kinetic Study and New Applications of UV
Radiation Curing
Christian Decker
Département de Photochimie Générale (UMR-CNRS N87525), Ecole Nationale Supérieure de Chimie de Mulhouse,
Université de Haute-Alsace, 3, rue Werner, 68200 Mulhouse, France
E-mail: c.decker@uha.fr
Keywords: crosslinking; infrared spectroscopy; interpenetrating polymer networks (IPN); nanocomposites; photopoly-
merization
1 Introduction
Light-induced polymerization of multifunctional mono-
mers or oligomers, also called UV radiation curing, has
become a well-accepted technology which has found a
large variety of industrial applications because of its unique
advantages.[1–4] It is considered to be the most effective
way to rapidly transform a solvent-free liquid resin into a
solid polymer, at ambient temperature. Under intense illu-
mination, the crosslinking polymerization of acrylate-
based resins proceeds extensively within a fraction of a
second to generate a dense three-dimensional polymer
network, showing excellent resistance toward organic
solvents, chemicals and heat.[5] Crosslinking can also be
readily achieved by UV irradiation of a solid polymer
bearing polymerizable functions, as shown in Scheme 1. In
both cases, a photoinitiator is needed to generate the active
species (free radicals or proton acid), which will initiate the
chain reaction.
The most important applications of UV radiation curing
are to be found in the coating industry for the surface
protection of various materials by fast-drying varnishes,
paints or printing inks. UV-curable systems are also increa-
singly utilized as quick-setting adhesives, sealants, release
coatings, as well as to produce composite materials. In
Feature Article: Highly crosslinked polymers can be rea-
dily synthesized by photoinitiated polymerization of multi-
functional monomers or functionalized polymers. The
reaction can be followed in situ by real-time infrared (RT-
IR) spectroscopy, a technique that records conversion versus
time curves in photosensitive resins undergoing ultrafast
polymerization upon UVexposure. For acrylate-based resins,
UV-curing proceeds with long kinetic chains (7700 mol/
radical) in spite of the high initiation rate. RT-IR spectro-
scopy proved very valuable in assessing the influence of
various parameters, such as initiation efficiency, chemical
structure of the telechelic oligomer, light intensity, inhibitory
effect of oxygen, on polymerization kinetics. Interpenetrat-
ing polymer networks can be rapidly synthesized by means of
UV irradiation of a mixture of difunctional acrylate and
epoxy monomers in the presence of both radical and cationic-
type photoinitiators. The same UV technology can be applied
to crosslink solid polymers at ambient temperature, which
bear different types of reactive groups (acrylate and vinyl
double bonds, epoxy ring). UV radiation curing has been
successfully used to produce within seconds weathering
resistant protective coatings, high-resolution relief images,
glass laminates and nanocomposites materials.
Photoinitiated crosslinking polymerization.
Macromol. Rapid Commun. 2002, 23, 1067–1093 1067
Macromol. Rapid Commun. 2002, 23, No. 18
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2002 1022-1336/2002/1812–1067$17.50þ.50/0
Scheme 1. Photoinitiated crosslinking polymerization.
photolithography, light-induced insolubilization of photo-
resists is being used to produce high-definition images
needed for the manufacture of printing plates, optical disks
and microcircuits. Besides its great speed and spatial
resolution, light-induced polymerization presents a number
of other advantages, such as solvent-free formulations, low
energy consumption, ambient temperature operations, and
tailor-made properties of the photocured polymers.
There are two main classes of UV-curable resins, depen-
ding whether the chain reaction proceeds by a radical-type
or cationic-type mechanism (Table 1). In the first case,
aromatic ketones are used to generate free radicals that will
initiate the polymerization of acrylate double bonds or
copolymerization of styrene with unsaturated polyesters or
of the thiol-polyene system by a step-growth addition
mechanism.[6] In photoinitiated cationic polymerization, a
proton acid is produced by photolysis of triarylsulfonium
(TAS) or diaryliodonium salts to initiate the polymerization
of epoxides or vinyl ethers.[7] Radical-type systems are by
far the most widely used in today’s UV-curing applications,
mainly because of their higher reactivity. The large choice
of acrylate monomers and telechelic oligomers also allows
to adjust more precisely the final properties of the UV-cured
polymer for the considered application.
During the last decade, a considerable amount of acade-
mic work has been devoted to UV radiation curing, with
most of the research efforts being focused both on the
kinetic and mechanistic aspects of such crosslinking reac-
tions and on the design of new photoinitiators, monomers
and telechelic oligomers, best suited to producing high-
performance polymer networks. A number of textbooks and
review articles have reported the progress made in this area
over the past few years.[1–19] In this paper, the recent work
done in our laboratory, regarding kinetics of ultrafast
photoinitiated polymerization of various monomers, func-
tionalized oligomers or polymers and monomer blends, is
reviewed before presenting some promising novel applica-
tions of this environmentally benign technology.
Christian Decker has been Research Director at the CNRS (Centre National de la
Recherche Scientifique) since 1980 at the Ecole Nationale Supérieure de Chimie de
Mulhouse, and head of the Polymer Photochemistry Laboratory at the Department of
Photochemistry (University of Haute Alsace). He received his doctoral degree from the
University of Strasbourg in 1967 at the Centre de Recherches sur les Macromolécules
(CRM). In 1971 he spent a year and a half at Stanford Research Institute in Menlo Park
(California) as a post-doc in Frank Mayo’s group, working on the effect of gamma rays on
polymers, a topic he continued to investigate at the CRM. In 1975, he joined the
Department of Photochemistry to study the reactions induced by light in polymer systems,
both photodegradation and photopolymerization. His main research interests include
kinetic studies into ultrafast light-induced polymerizations, photocrosslinking of
functionalized polymers, characterization of UV-cured polymers and nanocomposites,
photostabilization of polymer materials, and laser-assisted chemical processing of
polymers.
Table 1. Different types of UV-curable resins.
Mechanism RADICAL CATIONIC
Photoinitiator
Monomers and functionalized polymers
1068 C. Decker
2 Experimental Part
2.1 Materials
UV-curable formulations are usually made of three basic
components: (i) photoinitiator that effectively absorbs the
incident light and readily generates reactive radicals or ions,
(ii) functionalized oligomer that, upon polymerizing, will
constitute the backbone of the three-dimensional polymer
network to be formed, and (iii) mono- or multifunctional
monomer that acts as a reactive diluent to adjust formulation
viscosity and that will be incorporated into the polymer
network.
The photoinitiator plays a key role in that it governs both
the rate of initiation and the penetration of incident light
into the sample, and therefore controls the depth of cure.
The rate of polymerization depends initially on the reactivity
of the functional group, its concentration and the viscosity
of the resin. The chemical structure and functionality of
both monomer and oligomer are also important, for they will
determine the final degree of polymerization, as well as
the physical and chemical characteristics of the UV-cured
polymer. Chart 1 gives the chemical formulae and trade names
of some of the compounds used in our kinetic studies into the
polymerization of both radical and cationic-type UV-curable
resins.
2.2 Irradiation
The liquid resin was applied onto a KBr crystal, a trans-
parent poly(propylene) film or a siliconwafer bymeans of a
calibrated wire-wound applicator. In some experiments, a
second poly(propylene) film was laminated on top of the
sample to prevent oxygen diffusion. The thickness of the
UV-curable film was set typically between 5 and 25 mm, as
determined by IR spectroscopy by means of a calibration
curve. For the kinetic analysis, the sample was placed in the
compartment of an IR spectrophotometer where it was exposed
to UV radiation of a medium pressure mercury lamp (HOYA-
SCHOTT 200 U or EFOS Novacure) for a few seconds. The
light intensity at the sample position could be set between 5 and
200 mW
cm2, as measured by radiometry (International
Light IL-390). Some curing experiments were performed on an
industrial-type UV-line (IST Minicure-80 W
cm1), was
operated at belt speeds between 5 and 60 m/min, i.e. UV doses
of 600 and 50 mJ
cm2 per pass, respectively.
2.3 Analysis
Polymerization reactions were followed in situ by RT-IR
spectroscopy,[21] bymonitoring the decrease of the IR band
characteristic of the reactive functional group upon UV
exposure. The sample was exposed simultaneously to the
UV beam that induces polymerization and to the IR beam
that analyzes its extent, as shown in Figure 1, depicting the
Kinetic Study and New Applications of UV Radiation Curing 1069
instrumental set-up. With the FT-IR spectrophotometer
used (Bruker IFS-66), up to 57 spectra can be taken per
second at a spectral resolution of 4 cm1, thus allowing high
speed polymerizations to be accurately followed in real time.
The disappearance of the functional groups (acrylate, vinyl
ether double bonds, epoxy ring) was monitored continuously
by selecting the IR wavenumber where these functional groups
exhibit their characteristic absorption bands: 812 cm1 for the
acrylate double bond, 1622 cm1 for the vinyl ether double
bond, and 795 cm1 for the epoxy ring. The degree of con-
version (x) was calculated from the decrease in IR absorbance
(A) after a given exposure: x¼ 1 (At/A0). By setting the IR
spectrophotometer in the absorbance mode, conversion versus
time curves were recorded directly. From the slope of this
curve, the actual rate of polymerization (Rp) can be evaluated at
any stage of the reaction: Rp¼ [M0] (dx/dt), where M0 is the
initial concentration in reactive groups. In most systems Rp
reaches its maximum value in the 10 to 30% conversion range.
This analytical method also permits to determine the final
conversion reached at the end of UV exposure, and thus the
amount of unreacted functionalities in the UV-cured polymer, a
crucial factor with respect to long-term properties of the
materials.
The disappearance of the photoinitiator during photopoly-
merization was followed by real-time ultraviolet (RT-UV)
spectroscopy, by setting the detection wavelength at its maxi-
mum absorbance and monitoring continuously its decrease
upon UV exposure.[22] The time resolution of the instrument
used (Beckman DU-7400) was 0.1 s.
Chloroform was used as a solvent to measure the insoluble
fraction of the crosslinked polymer. The hardness of the UV-
cured polymer was evaluated by monitoring the damping of the
oscillations of a pendulum placed onto a glass plate coated with
a 30 mm thick film (Persoz hardness). Polymer hardness was
shown to be strongly dependent on the glass transition tempe-
rature.[23] Persoz values, expressed in seconds, are typically
ranging from 30 s for soft elastomeric materials up to 400 s for
very hard and glassy polymers.
3 Kinetics of Photoinitiated Polymerization
The polymerization of liquid resins, as well as the cross-
linking of functionalized polymers, can be readily achieved
by UV-irradiation in the presence of a photoinitiator. The
various analytical methods commonly used to study the
kinetics of photopolymerizations in situ (differential
scanning calorimetry,[18,24,25] dilatometry,[26] fluorescence
spectroscopy,[27,28] laser interferometry[29,30]) are not cap-
able of both following ultrafast reactions in real time and
providing quantitative information about the actual degree
of polymerization. By these techniques, one monitors
the physical effects resulting from polymerization, such as
the heat evolved, volume shrinkage, and increase in
viscosity or refractive index. A calibration curve is therefore
required to correlate these quantities with the actual mono-
mer conversion, a curve which is specific for each type of
resin formulation.
This is not the case for RT-IR spectroscopy, a technique
which monitors in situ the chemical changes occurring
upon UV exposure, i.e., the disappearance of the reactive
group of the monomer.[31] Conversion versus time curves
have thus been recorded for polymerizations occurring
within a fraction of a second, by radical-type[32] or cationic-
type mechanisms.[33] RT-IR spectroscopy proved to be
particularly well-suited to study the photopolymerization
of monomer mixtures, which leads to the formation of
copolymers or interpenetrating polymer networks, as it
allows the disappearance of each monomer to be followed
in real time.[34] This chapter is focused on the kinetic aspects
of both UV radiation curing of different types of liquid
resins and photocrosslinking of functionalized polymers,
by working under conditions similar to those found in
industrial applications, i.e., thin films of solvent-free resins
exposed to intense polychromatic radiation in the presence
of air.
3.1 Photoinitiated Radical Polymerization
RT-IR spectroscopy has been successfully used to evaluate
the kinetic chain length and initiation efficiency in UV-
curing of acrylate-based resins, as well as to quantify the
influence of oligomer structure, light intensity, and atmo-
spheric oxygen on the polymerization kinetics.
3.1.1 Kinetic Chain Length
Most of the resins employed in UV-curing applications
consist of acrylate-based monomers and telechelic oligo-
mers, because of the high reactivity of the acrylate double
bond. They are associated to aromatic ketone photoinitiators,
which undergo fast homolytic cleavage upon UV exposure,
according to a Norrish-type I mechanism, to generate free
radicals that will initiate the polymerization (Scheme 2).
The high value of the propagation rate constant (kp
104 l
mol1
s1)[35] together with a relatively slow termi-
nation process (kt 105 l
mol1
s1)[36] are the main
reasons why the polymerization of acrylate resins proceeds
so fast upon intense illumination. Crosslinking polymer-
ization was found to develop with long kinetic chains in
Figure 1. Instrumental set-up for real-time infrared spectro-
scopy analysis of ultrafast photopolymerizations.
1070 C. Decker
spite of the very high rate of initiation.[37] The kinetic chain
length (KCL) can be evaluated by monitoring the disap-
pearance of both monomer and photoinitiator after short
UV exposure.
Figure 2 shows the acrylate conversion and photoinitiator
decay curves recorded by RT-IR and RT-UV spectro-
scopies, respectively, upon continuous UV irradiation of
a polyester acrylate resin (Ebecryl 830þHDDA, [M0]¼
6 mol
l1) containing Lucirin TPO (1 wt.-%) as a
photoinitiator, at a light intensity of 20 mW
cm2 under
O2-free conditions. After 0.1 s of exposure (dashed lines),
polymerization continues to proceed in the dark up to a
conversion of 50%, while photolysis of the initiator stops
immediately, as expected. This post-polymerization is
attributed to the polymer radicals which continue to grow
until bimolecular termination.
The KCL was evaluated by calculating the ratio of the
amount of acrylate double bonds polymerized (3 mol
kg1)
to the amount of photoinitiator destroyed (3 104 mol

kg1). For this specific resin, 10 000 acrylate double bonds
were found to be polymerized for each photoinitiator
molecule destroyed. If each Lucirin TPO molecule gene-
rates 1.3 initiating radicals (see below), the value of KCL is
in the order of 7 700 mol per radical. This value is surpri-
singly high for a polymerization initiated at a very high rate
(ri 4 103 mol
l1
s1). Indeed, for radical-induced
polymerizations where termination is expected to occur by
bimolecular reactions of polymer radicals, KCL should
decrease as ri (or light intensity) is increased, in considera-
tion of the following equations
Rate of polymerization Rp ¼
kp
ð2ktÞ0:5
½Mr0:5i ð1Þ
Rate of initiation ri ¼ Fi
Ia ð2Þ
Kinetic chain length KCL ¼ Rp
ri
¼ kp½MF
0:5
i
ð2ktÞ0:5I0:5a
ð3Þ
where Fi is the initiation quantum yield and Ia the light
intensity absorbed.
The long kinetic chains observed in UV-curable acrylate
resins are attributed, on the one hand, to the high value
of ratio kp/(2kt)
0.5 (20 l0.5
mol0.5
s0.5)[36] and, on the
other hand, to the fact that each initiating radical is
surrounded by a great number of acrylate double bonds. In a
typical UV-curable formulation ([M0]¼ 3 mol
l1), each
photoinitiator molecule is initially surrounded by 100
acrylate double bonds. After a 100 ms UVexposure, 1% of
the photoinitiator (3 104 mol
l1) has been destroyed to
generate initiating radicals. Consequently, each initiating
radical will be surrounded by more than 5 000 acrylate
double bonds (the photoinitiator splits into 2 radical frag-
ments), some of these radicals having already disappeared
by reaction with the acrylate double bonds. This means that,
once the polymerization has been initiated under intense
illumination, there is no other choice for the polymer radi-
cal than to continue to grow extensively, until it encounters
ultimately another polymer radical. The situation remains
the same upon further UV exposure, the additional amount
of initiating radicals formed being compensated by the
amount of polymer radicals getting lost upon bimolecular
termination reactions (steady state conditions).
The overall effectiveness of photoinitiated polymeriza-
tion can be evaluated from the polymerization quantum
yield, Fp, i.e. the number of acrylate double bonds
polymerized per photon absorbed, by using the following
equation[37]
Fp ¼
½Mpmol l1  ‘cm
f  1000  ts  I0 einstein cm2s1
ð4Þ
where [Mp] is the concentration of acrylate double bonds
polymerized after UV exposure during time t at a light
intensity I0, with f being the fraction of incident light
absorbed by the sample of thickness l. For the most reactive
acrylate formulations, Fp values up to 10 000 mol

einstein1 were obtained under O2-free conditions.
[37,38]
Precisely this large amplification factor is responsible for
Scheme 2. Photoinitiation of a radical-type polymerization.
Figure 2. Real-time monitoring of the polymerization of the
acrylate double bond and photolysis of the initiator upon UV
irradiation (20 mW
cm2) under O2-free conditions: continuous
UVexposure (solid line), 0.1 s UVexposure (dashed line); [Lucirin
TPO]¼ 1 wt.-%.
Kinetic Study and New Applications of UV Radiation Curing 1071
the outstanding performance of UV-curable acrylate resins
and accounts for the success of this technology in a large
variety of applications.
3.1.2 Evaluation of Initiation Efficiency
The photoinitiator (PI) plays a key role in UV-curable
systems by controlling both the initiation rate and the
cure depth. It is essential to select a photoinitiator showing
the highest initiation efficiency and undergoing a fast
photobleaching upon UV exposure in order to achieve a
deep-through cure by frontal polymerization.[13,39] Figure 3
shows the UVabsorption spectra of an acylphosphine oxide
photoinitiator (Lucirin TPO) in an acrylate resin,[40] before
and after exposure to radiation from a medium pressure
mercury lamp for various durations. Initiation efficiency
can be evaluated from monomer conversion and PI decay
curves recorded by means of RT-IR and RT-UV spectro-
scopy, respectively.
The various processes occurring upon UVexposure of the
liquid resin can be represented schematically by Scheme 3.
Key parameters are fraction f1 of the PI excited states that
generate free radicals and fraction f2 of these radicals that
react with monomer and initiate polymerization. Initiation
efficiency is given by the product f1
f2 and corresponds to
the number of initiating radicals produced per PI molecule
destroyed. The rate of initiation can therefore be expressed as
ri ¼
d½PI
dt

f1
f2 ð5Þ
where d[PI]/dt is the initial loss rate of photoinitiator. From
the Equation (1), ri can also be expressed as a function of the
rate of polymerization:
ri ¼
2kt
R2p
k2p
½M
2
ð6Þ
Introducing this expression in Equation (5) leads to the
following equation for the initiation efficiency:
f1
f2 ¼
2kt
R2p
k2p
½M
2
d½PI
dt
ð7Þ
By measuring initial polymerization rate and PI loss rate,
one can evaluate the product f1
f2 from Equation (7).
Figure 4 shows polymerization profile and photoinitiator
decay curve recorded upon UV exposure of a polyurethane
acrylate resin ([M]0¼ 3.2 mol
l1) containing 1 wt.-%
Lucirin TPO. By taking kt¼ 105 l
mol1
s1 and kp¼
104 l
mol1
s1,[36] the initiation efficiency of this
acylphosphine oxide was found to be 1.3 radical per TPO
molecule, which is not so different from the maximum
value of 2.
Figure 3. UV absorption spectrum of Lucirin TPO (1 wt.-%) in
acrylate resin before and after UV exposures of up to 5 s at a light
intensity of 30 mW
cm2.
Scheme 3. Photoinitiated radical polymerization.
Figure 4. Decay profile of the photoinitiator and polymerization
profile of an acrylate resin upon UV irradiation under O2-free
conditions (30 mW
cm2).
1072 C. Decker
The initial quantum yield of the photolysis of Lucirin
TPO, FTPO, was determined by calculating the ratio of the
initial PI loss rate to the absorbed light intensity. The
fraction of incident light absorbed by the 24 mm thick
sample was evaluated by differential radiometry and
found to be 15%. From the FTPO value thus obtained
(0.62 mol
einstein1) initiation quantum yield Fi of Luci-
rin TPO could finally be evaluated to Fi¼FPI
f1
f2¼
0.8 radical einstein1. This high value of Fi, together with
fast photobleaching, are typical of the behavior of acylpho-
sphine oxides. Similar results were obtained with a
bis(acylphosphine) oxide (Irgacure 819) which generates
4 free radicals per molecule after absorption of 2 photons
(Scheme 4).[41]
The great reactivity of phosphinoyl radicals toward the
acrylate double bond was considered to be mainly res-
ponsible for the higher initiation efficiency of this type of
compounds.[42,43] These photoinitiators have also the ad-
vantage of absorbing in the near UV region (350–400 nm),
where the mercury lamps have their strongest emission, and
are thus particularly well suited for the UV-curing of
formulations containing pigments or light stabilizers.[41,44]
While fast photolysis of the initiator is beneficial for
increasing the cure speed and lowering the residual PI
content, it may yet lead to a premature ending of the poly-
merization due to complete consumption of the photo-
initiator, if its initial concentration is not high enough. Such
an effect is illustrated in Figure 5 for an aromatic polyether-
diacrylate (Ebecryl 600 from UCB) containing Lucirin
TPO (0.1 and 1 wt.-%) as the photoinitiator.
3.1.3 Influence of the Chemical Structure
of the Oligomer
Different types of structures can be used for the telechelic
oligomer which will constitute the framework of the UV-
cured polymer: polyurethane, polyester, polyether and
polysiloxane. The final properties of UV-cured acrylate
polymers depend primarily on the chemical structure of
the oligomer backbone, on its molecular weight (typi-
cally between 500 and 2 000), as well as on the cure extent.
Low-modulus elastomers are generally obtained with aliph-
atic telechelic acrylate oligomers associated to a mono-
acrylate reactive diluent, whereas hard and glassy materials
are formed when aromatic structures are introduced
into the polymer network, together with multifunctional
monomers.
The glass transition temperature (Tg) of the fully cured
polymer has a strong influence on the final conversion
reached upon UVirradiation at room temperature. TheTg of
UV-cured dimethacrylate polymers was shown to increase
steadily with monomer conversion, as well as with irra-
diation temperature.[45,46] Figure 6 shows the polymeriza-
tion profiles recorded for two types of acrylate end-capped
oligomers, an aliphatic polyurethane-acrylate and an aro-
matic polyether-acrylate.
The chemical structure has a great effect on the final cure
extent, while it does not affect resin reactivity significantly.
Scheme 4. Photolysis of a bis(acylphosphine oxide).
Scheme 5. Thermally induced polymerization upon heating of
a polyphenoxy-acrylate UV-cured in the presence of air.
Figure 5. Influence of photoinitiator concentration on the
photopolymerization of a polyphenoxy-diacrylate. Light inten-
sity: 100 mW
cm2, 12 mm thick laminated film.
Kinetic Study and New Applications of UV Radiation Curing 1073
High degrees of conversion (close to 100%) are reached
with the aliphatic polyurethane-acrylate that gives an elas-
tomeric material, well suited for adhesives applications. By
contrast, the aromatic polyether-acrylate yields a glassy
polymer (Tg 80 8C), hard but brittle. Here polymerization
stops prematurely because of vitrification and related
molecular mobility restrictions.
A more complete polymerization can be achieved by
rising the sample temperature to 80 8C, either before or after
UV exposure (Figure 7). In the latter case, the additional
polymerization and hardening observed upon thermal
treatment was attributed to the restored mobility of trapped
polymer radicals,[47] as well as to the decomposition of
previously formed hydroperoxides, which yields initiating
hydroxyl and alkoxy radicals (Scheme 5).[48] Evidence in
favor of this process was obtained by performing UV curing
under O2-free conditions. The conversion increase obser-
ved upon heating at 80 8C in the dark, due to the un-trapping
of the occluded polymer radicals, was found to be half as
large as the one observed in the presence of air. As expected,
the more complete polymerization achieved by operating at
elevated temperatures leads to an increase in Tg of the
samples (up to 100 8C) and, concomitantly, to an increase in
polymer hardness (Figure 8). The lower amount of remai-
ning acrylate unsaturations is also beneficial with respect to
long-term properties of UV-cured polymers.
Another way to achieve more complete polymerization
in photopolymer materials is by performing UV irradiation
at ambient temperature but with higher light intensity. As
the heat of the exothermal polymerization is released in a
shorter period of time (less than 1 s), the temperature of the
sample will rapidly rise and molecular mobility will be
increased. Figure 9 shows the marked influence of light
intensity on the final conversion of a UV-cured polyur-
ethane-acrylate (Actilane 20).
To confirm that the observed increase in conversion was
indeed resulting from a rise of the sample temperature, a
novel method, based on RT-IR spectroscopy, has been
developed to monitor the temperature in thin films under-
going high-speed photopolymerization.[36,49] The tempera-
ture probe is the poly(propylene) film serving as a support,
which exhibits a temperature-sensitive IR band at 842 cm1.
Figure 6. Influence of the chemical structure of the diacrylate
oligomer (aliphatic polyurethane or aromatic polyether) on poly-
merization kinetics. Light intensity: 50 mW
cm2, [Irgacure
651]¼ 2 wt.-%, laminated film.
Figure 7. Influence of temperature on the UV-curing of an
aromatic polyether-diacrylate (Ebecryl 600) in the presence of air
(I¼ 500 mW
cm2).
Figure 8. Influence of temperature on the hardening of an
aromatic polyether-diacrylate (Ebecryl 600) upon UVexposure in
the presence of air (I¼ 500 mW
cm2).
1074 C. Decker
Figure 10 shows some typical temperature profiles recorded
at various light intensities for a UV-curable polyurethane-
acrylate (PUA). Upon intense illumination, the sample
temperature reaches values up to 90 8C within a fraction of a
second. A linear relationship was found to exist between the
rate of polymerization and the rate at which the temperature
rises,[49] thus demonstrating that the increase in tempera-
ture is indeed caused by the exothermal polymerization. It
leads to a more complete polymerization, as shown in
Figure 11 where the final acrylate conversion was plotted as
a function of maximum temperature of the sample under-
going polymerization.
The maximum temperature reached upon UV exposure
depends on the film thickness, as well as on the substrate on
which the sample is coated. Indeed, no significant rise in
temperature was observed upon UV-curing of a sample
coated onto a silicon wafer which acts as a heat trap. By
contrast, for a 300 mm thick PUA sample coated on a glass
plate and cured by a short passage under a powerful UV
lamp (600 mW
cm2), a thermocouple revealed that the
temperature rose to 165 8C, whereas it hardly changed in the
absence of photoinitiator (no curing), as shown in Figure 12.
An important consequence of this behavior is that
irradiation conditions (light intensity, initial temperature,
film thickness, support) affect the cure extent, and therefore
the physico-chemical properties (Tg) of the photopolymer
material (Scheme 6).
3.1.4 Influence of Light Intensity
A distinct feature of photoinitiated polymerization is that
the initiation rate can be varied in a very large range by
changing the light intensity. It should yet be remembered
that a 100-fold increase in light intensity will lead to only a
10-fold increase in polymerization rate, because of the
square-root dependence of Rp on Ia (Equation (1) and (2)).
This relationship holds only in the early stages of polymeri-
zation when termination occurs by bimolecular reactions
between polymer radicals. As mobility restrictions increase,
the crosslinked polymer radicals are less likely to meet by
sole segmental diffusion. The radical site will move mainly
by reacting with neighboring functional groups until it
combines with another such radical. When termination is
controlled by reactive diffusion,[36,50,51] the termination
rate constant becomes proportional to the propagation rate
Figure 9. Influence of light intensity on the final conversion
of a polyurethane-diacrylate (Actilane 20) after a UV dose of
1 J
cm2.
Figure 10. Temperature profiles recorded by means of RT-IR
spectroscopy upon UV-curing of a polyurethane-diacrylate
(Actilane 20) at different light intensities.
Figure 11. Dependence of the final conversion on the maximum
temperature reached for a UV-cured PUA sample.
Kinetic Study and New Applications of UV Radiation Curing 1075
constant. For acrylate-based resins, it was found to occur at
conversions of 20% and above.[36] Later on, as the concen-
tration of acrylate double bonds in the vicinity of the radical
site has decreased and gelation has occurred, the polymer
chain stops growing and leaves long-lived radicals trapped
within the polymer network.[45,52,53] This corresponds to a
monomolecular termination process and will lead to a linear
dependence of Rp on light intensity. Actually, exponent x of
the kinetic lawRp¼KI x was found to increase steadily with
conversion as the liquid resin is transformed into a solid
polymer network (Figure 13). It shows that, as expected,
radical trapping takes an increasingly important part in the
termination step as UV-curing proceeds.[54] The overall rate
of polymerization can be expressed, in a first approximate,
as the sum of two terms which depend on the first power
and on the square root of the absorbed light intensity,
respectively[55]
Rp ¼ a
kp
k0t
½MFiIa  ð1  aÞ
kp
ð2ktÞ0:5
½MF0:5i I 0:5a
ð8Þ
where kt
0 is the rate constant of the unimolecular termi-
nation process, i.e., the reciprocal of the polymer radical
lifetime. Coefficient a is ranging between 0 and 1, and
reflects the relative contribution of the unimolecular path-
way, i.e., the probability for a given polymer radical to
become occluded in the polymer matrix. Value a is strongly
dependent on the structure and crosslink density of the
polymer network. It will be lower for elastomeric materials
than for glassy UV-cured polymers where reaches values up
to 0.8.[55,56]
A direct consequence of the first order variation in Rp
with light intensity is that the polymerization quantum yield
(Fp¼Rp/Ia) remains constant, instead of dropping with
increasing Ia (reciprocity law). When those conditions are
reached, i.e., toward the end of the crosslinking polymer-
ization, one makes a more efficient use of the energy delive-
red by the light beam, specially at high light intensities.
3.1.5 Influence of Atmospheric Oxygen
Another advantage of performing the polymerization under
intense illumination is to reduce UV exposure time during
which atmospheric oxygen diffuses into the film and sca-
venges both the initiating radicals and the polymer radicals
(Scheme 7).
The peroxy radicals are inactive toward the acrylate
double bond and disappear by hydrogen abstraction to
generate hydroperoxides and alkyl radicals, which are also
scavenged by oxygen. The free radicals, produced by
photolysis of the initiator during the induction period, serve
Figure 12. Temperature profiles recorded with a thermocouple
upon a 1.6 s UVexposure in air of a 300 mm thick PUA film, with
and without photoinitiator ([Irgacure 651]¼ 2 wt.-%). Light
intensity: 600 mW
cm2.
Scheme 6. Various factors affecting the properties of UV-cured
polymers.
Figure 13. Variation of rate exponent xwith exposure time, upon
UV-curing of a polyurethane-diacrylate resin at a light intensity of
10 and 100 mW
cm2 under O2-free conditions. [Lucirin
TPO]¼ 2 wt.-%.
1076 C. Decker
to consume the oxygen dissolved in the resin, which will
become chemically bonded to the polymer in the form of
hydroperoxides. The higher the light intensity, the faster
will oxygen be removed by the high flux of free radicals.
Polymerization of the acrylate double bond will only start
once the concentration of the dissolved oxygen has de-
creased sufficiently (by at least two orders of magnitude) to
allow the monomer to compete successfully with oxygen
for the scavenging of the initiating radicals.[57]
There is a number of factors controlling the importance
of the inhibitory effect of oxygen in UV-curable acrylate
resins: (i) photoinitiator efficiency in producing free radi-
cals, (ii) formulation viscosity, which controls the diffusion
rate of oxygen, (iii) sample thickness, thin films being more
difficult to cure in the presence of air (Figure 14), and (iv)
light intensity, which determines the exposure time during
which atmospheric O2 diffuses into the sample.
For thick coatings, a deep-through cure can be achieved
by low intensity UV radiation, but the surface in contact
with air will remain tacky. It may happen in some limiting
case, i.e., a few micron thick films of a fluid resin exposed to
dim light (e.g., sunlight), that polymerization does not
occur at all because the oxygen consumed by the free
radicals formed is steadily replaced by the air diffusing into
the sample. It is then necessary to perform UV irradiation
under O2-free conditions in order to avoid wasting the few
initiating radicals formed. To achieve such an inert environ-
ment, nitrogen can be advantageously replaced by carbon
dioxide (Figure 14), especially for the UV-curing of acrylic
resins coated onto three-dimensional objects.[58] As CO2 is
heavier than air, gas losses during the process are minimized
when conducted in a pool-type photoreactor.
3.1.6 Photopolymerization of the Thiol/ene System
There is another type of photoinitiated radical polymeriza-
tion which, unlike the acrylate-based resins, is not inhibited
by molecular oxygen, and which was successfully used to
photopolymerize thin films in the presence of air. The UV-
curable resin consists of a mixture of polyene and multi-
functional thiol that undergoes copolymerization upon UV
irradiation by a step-growth addition mechanism.[6] The
polymerization process, which is propagated by chain-
transfer reaction involving thiyl radical RS., can be repre-
sented by the following set of reactions:
Photoinitiator!hn ln
ln þ RSH ! ln H þ RS
RS þ CH2 CHR0 ! RSCH2C

HR0
RSCH2C

HR0 þ RSH ! RSCH2CH2
R0 þ RS ð9Þ
A linear polymer is formed in the case of a diene/dithiol
couple. For crosslinking to occur, a polyene must be asso-
ciated to a multifunctional thiol. In the presence of air, the
alkyl radical will be scavenged by oxygen to yield a peroxy
radical, which is also able to propagate the chain reaction by
hydrogen abstraction to yield a hydroperoxide:
ð10Þ
Figure 15 shows some typical conversion versus time
profiles recorded by means of RT-IR spectroscopy (vinyl
band at 1638 cm1) for a stoichiometric mixture of a
tetraene and a tetrathiol exposed to UV radiation in the
presence of a hydroxyphenyl ketone ([Irgacure 184]¼
2 wt.-%). In such thin films (20 mm), similar polymerization
profiles were obtained in the presence of air and under O2-
free conditions. The low sensitivity to oxygen of the
polyene/thiol system can be made even more apparent by
stopping UV exposure, and hence the production of free
radical, at an early stage of the reaction, e.g., at 20%
conversion. After an 0.1 s exposure, polymerization was
Scheme 7. Inhibitory effect of oxygen on photoinitiated radical
polymerization.
Figure 14. Inhibitory effect of oxygen on the photopolymeriza-
tion of a polyurethane-acrylate resin UV-irradiated at a light
intensity of 30 mW
cm2. [Irgacure 2959]¼ 3 wt.-%.
Kinetic Study and New Applications of UV Radiation Curing 1077
found to continue to proceed in the dark up to 50%
conversion, even in the presence of air (Figure 15). A quite
different behavior was observed with an acrylate resin
showing a similar reactivity, where dark polymerization
was found to hardly occur in the presence of air, because of
an efficient scavenging of the growing polymer radicals by
oxygen diffusing into the thin film.
In spite of its outstanding reactivity performance, the
polyene/thiol system is not used in UV radiation curing as
widely as acrylate-based resins. This is partly due to higher
costs and unpleasant odor of the formulation, the limited
choice of monomers, and the presence of hydroperoxides in
the UV-cured polymer, which may affect its long-term
stability. This is not the case for acrylate coatings UV-cured
in the presence of air, because polymerization starts to
proceed only when the sample has been essentially depleted
of the dissolved oxygen.[57] Indeed, UV-cured aliphatic
PUA coatings proved to be quite resistant to accelerated
weathering and were successfully used to increase the light
stability of polymeric materials.[59]
It should be noted that monomers that polymerize by a
cationic mechanism, such as epoxides or vinyl ethers, are
not sensitive at all to the presence of atmospheric oxygen.
Indeed, the photogenerated proton acid and the propagating
polymer carbocations are not reacting with molecular
oxygen. Tack-free coatings have thus been obtained upon
UV irradiation in the presence of air, even by operating at
very low light intensity, in particular with sunlight.[60]
3.2 Photoinitiated Cationic Polymerization
While most of the UV-curable resins currently used are
based on free-radical polymerization, the less utilized
cationic polymerization of multifunctional monomers and
oligomers bearing vinyl ether or epoxy groups still present a
number of advantages: (i) lack of inhibition by atmospheric
oxygen, (ii) post-polymerization in the dark, (iii) low shrin-
kage, (iv) high mechanical performance of the UV-cured
material, and (v) good adhesion onto various substrates, in
particular metals. One of the main limitations to a greater
industrial development of UV-curable epoxy resins lies in
their relatively low reactivity and the limited choice of com-
mercially available monomers and telechelic oligomers.
The photoinitiated cationic polymerization of epoxi-
des has been thoroughly investigated over the past 20
years,[7,15,33,61–70] in particular by Crivello and co-workers,
who pioneered this technology by developing triarylsulfo-
nium and diaryliodonium photoinitiators, as well as appro-
priate photosensitizers.[71] Most of these studies were
focused on cycloaliphatic epoxides which are the most
utilized monomers because of their superior reactivity. The
addition of a comonomer was shown to speed up the UV-
curing process and to lead to a more complete polymeriza-
tion, especially for aromatic diepoxides.[72] Promising
results have been obtained with an epoxidized soyabean oil
(ESO; see below),[73] a cheap compound which acts as
reactive diluent, while imparting some flexibility to the UV-
cured epoxy coating at the same time.[74] We have studied
the photoinitiated cationic polymerization of aromatic and
cycloaliphatic diepoxides, neat and in blends with ESO,
vinyl ethers or acrylate monomers, by monitoring both the
epoxy group consumption and the insolubilization and
hardening of the polymer upon UV irradiation.
3.2.1 UV Radiation Curing of Aromatic
Diepoxides (ADE)
Diglycidyl ethers are poorly reactive monomers which
undergo cationic polymerization at a much lower rate than
vinyl ethers. Under intense illumination in the presence of
an aryliodonium salt, the liquid resin can still be transfor-
med within seconds into a hard and insoluble polymer. The
crosslinking polymerization process of aromatic diepox-
ides (ADE) can be formally written as shown in Scheme 8.
Ring-opening polymerization can be quantitatively follo-
wed by infrared spectroscopy through either the decrease of
the characteristic band of the oxirane at 865 cm1, or the
increase of the ether band at 1080 cm1. Figure 16 shows
some typical kinetic curves obtained upon intense UV
irradiation of a diglycidyl ether derivative of bisphenol A
Figure 15. Lack of oxygen inhibition on the photopolymeriza-
tion of the thiol/ene system ([Irgacure 184]¼ 2 wt.-%): O2-free
conditions (solid line), in the presence of air (dashed line). Light
intensity: 50 mW
cm2, film thickness: 20 mm.
1078 C. Decker
(Araldite GY-250 from Ciba SC) in the presence of a
triarylsulfonium salt (Cyracure UVI-6990). As polymer-
ization proceeds, the polymer formed becomes increasingly
insoluble in organic solvents (e.g. chloroform); the gel
fraction reaches 80% after 3 s of UV exposure (Figure 17).
At the same time, the degree of swelling decreases steadily
with exposure time, down to a value of about two (the
amount of solvent absorbed is twice the amount of dry
polymer), which indicates the formation of a rather loose
three-dimensional polymer network.
A faster and more complete polymerization was achieved
by the addition of epoxidized soyabean oil ([ESO]¼ 20 wt.-
%), a comonomer which reduces the resin viscosity and will
be incorporated into the polymer network, as shown by the
decrease of its epoxy IR band at 825 cm1 (Figure 16).
Because it contains three epoxy groups, ESO is working as
an efficient crosslinker and accelerates the insolubilization
of the irradiated sample, with formation of a tighter polymer
network, as shown by the lower swelling ratio (Figure 17).
An additional advantage of using epoxidized soyabean oil is
to reduce the brittleness of the UV-cured epoxy polymer. By
combining hardness and flexibility, this material proved to
be quite resistant to both scratching and shocks.
A distinct characteristic of cationically initiated poly-
merization is that the propagating carbocations or oxonium
ions are not reacting among themselves. Consequently,
once initiated by a short UV exposure, polymerization will
continue to proceed in the dark for some time. After 2 h of
storage in the dark, the epoxy conversion of an ADE sample
UV-irradiated for 1 s was found to increase from 68% to
95%, with a concomitant increase in insoluble fraction and
polymer hardness. Therefore, by contrast to acrylate-based
polymers, the properties of cured epoxy polymer emerging
from the UV oven continue to change steadily until they
stabilize, usually after a 1 d storage.
3.2.2 UV Radiation Curing of Bis(cycloaliphatic)
Diepoxides
Cycloaliphatic diepoxides (BCDE) are the most widely
used monomers in photoinitiated cationic polymerization
(Scheme 9) because of their high reactivity, attributed to the
strain of the cyclohexane ring adjacent to the oxirane
ring.[7,15] Their photopolymerization can be easily followed
through the decrease of the IR band at 795 cm1 of the
epoxy ring.[33]
The photopolymerization of a typical BCDE (Araldite
CY-179 from Ciba SC) was shown to proceed more rapidly
Scheme 8. Photoinitiated cationicpolymerizationofa diepoxide.
Figure 16. Decay of the epoxy ring and build-up of the
ether group upon UV-curing of an aromatic diepoxide (ADE)
and an epoxidized soyabean oil (ESO). Triarylsulfonium (TAS)
PF6
 photoinitiator: [TAS]¼ 3 wt.-%, light intensity: I¼
600 mW
cm2.
Figure 17. Influence of ESO (20 wt.-%) on the insolubilization
of an aromatic diepoxide (ADE) upon UV exposure. [TAS]¼
3 wt.-%; I¼ 600 mW
cm2.
Kinetic Study and New Applications of UV Radiation Curing 1079
than that of ADE upon intense illumination in the presence
of a triarylsulfonium salt (Figure 18). A 0.1 s UV exposure
proved to be sufficient to polymerize 60% of the epoxy
groups. Upon further exposure, the degree of conversion
increased slowly to finally level off at a value around 80%,
because of vitrification and related mobility restrictions. As
a result of the high epoxy content of this BCDE ([epoxy]¼
8 mol
kg1), insolubilization occurs rapidly upon UV
exposure, with formation of a tight polymer network
(swelling ratio of 0.3), as shown in Figure 19. The addition
of the less reactive epoxidized soyabean oil (20 wt.-%) has
hardly any effect on the cure kinetics (Figure 18), while it is
slowing down the insolubilization process and generates a
looser polymer network, as expected (Figure 19).
The photoinitiated polymerization of BCDE was shown
to proceed more extensively under high-humidity condi-
tions, to yield a polymer containing no residual epoxy
groups.[72] While the addition of water to the formulation
will stop the polymerization by nucleophilic attack on the
oxonium cation, small amounts of water may enhance the
epoxide consumption through a chain transfer reaction
leading to re-initiation (Scheme 10).
The beneficial effect of moisture can also be attributed to
its role as plasticizer that allows the trapped polymeric
oxonium ions to react further. While insolubilization occurs
faster under high-humidity conditions, the cured polymer is
not as hard as when it is cured under normal conditions
(35% relative humidity): Persoz hardness of 160 s instead of
290 s.[72] The formation of a more elastomeric polymer
material under humid conditions was attributed to the
moisture-driven chain-transfer reaction.
The addition of a vinyl ether (VE) comonomer, such as
the divinyl ether of triethylene glycol (DVE-3), known for
its high reactivity, has a marked accelerating effect on the
polymerization of the epoxide, as shown in Figure 20 for an
equimolar mixture of BCDE and DVE-3. As much as 90%
of the epoxy and vinyl ether groups were polymerized after
0.1 s of UV exposure, complete polymerization being
achieved within 10 min upon storage in the dark of the
irradiated sample.[75]
A kinetic analysis by means of RT-IR spectroscopy of the
UV-curing of a BCDE/DVE-3 blend reveals that the two
monomers exhibit very similar polymerization profiles
(Figure 21), which is in favor of a copolymerization mech-
anism. Polymerization could proceed via a cross-propaga-
tion mechanism (Scheme 11), with the oxonium ion
Scheme 9. Photopolymerization of a dicycloepoxide.
Figure 18. Influence of ESO (20 wt.-%) on the photoinitia-
ted cationic polymerization of a bis(cycloaliphatic) diepoxide
(Araldite CY-179). [TAS]¼ 3 wt.-%, I¼ 600 mW
cm2.
Figure 19. Influence of ESO (20 wt.-%) on the insolubilization
of a BCDE resin (Araldite CY-179). [TAS]¼ 3 wt.-%, I¼
600 mW
cm2.
Scheme 10. Moisture-driven chain-transfer reaction.
1080 C. Decker
reacting with the VE double bond and the VE carbocation
with the epoxy ring.
Experimental evidence in favor of such a copolymeriza-
tion mechanism has been recently obtained by Kostanski
et al.,[76] who studied the photoinitiated copolymerization
of this BCDE with a monovinyl ether. The UV-cured
polymer was found to exhibit a single Tg, the value of which
being located between the Tg values of the two homopoly-
mers. Moreover, it was completely insoluble, thus showing
that the soluble and extractable homopolymer was not
formed. The addition of BCDE to DVE-3 causes a drastic
slowing down of the photopolymerization (Figure 21). This
effect is attributed to the conversion of the alkoxy-
carbenium ion into the less reactive oxonium ion.[77]
3.2.3 Ultrafast Synthesis of Interpenetrating Polymer
Networks (IPN) by UV Radiation Curing
Photoinitiated polymerization of a mixture of difunctional
monomers reacting by different mechanisms is one of the
most efficient ways to rapidly generate IPNs. This method
was successfully used with mixtures of multifunctional
acrylate and epoxy monomers that were exposed to UV
radiation in the presence of both radical- and cationic-type
photoinitiators.[72,78] By combining the physico-chemical
characteristics of the two types of polymer networks, UV-
curing of IPNs appears to be a powerful tool to achieve
tailor-made properties, in particular to produce hard and
flexible polymeric materials, well suited for coating ap-
plications. Figure 22 shows the polymerization profiles of
the two types of monomers for a 50:50 mixture by weight of
hexanediol diacrylate (HDDA) and ESO under intense
illumination (600 mW
cm2) in the presence of air. The
two polymer networks are produced simultaneously, as
photolysis of the two initiators (hydroxyphenyl ketone,
Figure 20. Influence of a divinyl ether ([DVE-3]¼ 41 wt.-%) on
the UV-curing of a bis(cycloaliphatic) diepoxide (Araldite CY-
179). [TAS]¼ 4 wt.-%, I¼ 600 mW
cm2.
Figure 21. Photoinitiated cationic polymerization of an equi-
molar mixture consisting of divinyl ether ([DVE-3]¼ 41 wt.-%)
and bis(cycloaliphatic) diepoxide ([Araldite CY-179]¼ 55 wt.-
%), [TAS]¼ 4 wt.-%, I¼ 600 mW
cm2.
Scheme 11. Copolymerization of a vinyl ether and a cycloep-
oxide.
Kinetic Study and New Applications of UV Radiation Curing 1081
triarylsulfonium salt) generates both free radicals and pro-
ton species, respectively. The diacrylate monomer yet
polymerizes more rapidly than the triepoxide due to its
higher reactivity. The polymerization is also more complete
than in neat HDDA (98% final conversion instead of 85%),
because of the plasticizing effect of the less reactive ESO
comonomer. Similar results have been obtained by UV
irradiation of blends of HDDA with an aromatic diepoxide
or a bisphenol-A-based diacrylate with BCDE,[78] by using
a newly developed diaryliodonium hexafluorophosphate
salt as a cationic photoinitiator that undergoes photolysis
without the formation of benzene.[20] It should be ment-
ioned that the use of such hybrid formulation is an easy way
to overcome oxygen inhibition in the UV curing of thin
films or under dim-light conditions. While the radical poly-
merization of the acrylate monomer is initially inhibited by
atmospheric oxygen, the cationic polymerization of the
epoxide proceeds effectively upon UVexposure. The build-
up of the epoxy network leads to a strong increase in
viscosity, which slows down the diffusion of atmospheric
O2 into the sample, thus allowing acrylate polymerization
to start. An acrylate/epoxide hybrid system has thus been
thoroughly cured within minutes by simple exposure to
sunlight.
A unique advantage of photoinitiation for the synthesis of
IPNs is to allow a sequential build-up of the two polymer
networks, one after the other, by a proper selection of both
photoinitiators and radiation wavelengths. The acrylate/
epoxide blend is first exposed to filtered UV radiation
(> 350 nm) which is absorbed only by the acylphosphine
oxide photoinitiator. The free radicals thus generated will
initiate solely the polymerization of the acrylate monomer,
while the photoinitiator is bleached out (Figure 23a). Here
again, the presence of the unreacted liquid epoxy monomer
allows a more complete and rapid polymerization of the
diacrylate monomer. In a second step, the sample is exposed
to unfiltered UV radiation which is absorbed by the triaryl-
sulfonium salt, thus promoting the cationic polymerization
of the epoxide (Figure 23b). As the polymerization of the
epoxy monomer occurs within the already formed acrylate
polymer network, it proceeds slower and less extensively
than in the neat epoxide. This drawback will be partly
alleviated by the post-polymerization upon storage in the
dark, which can be further enhanced by warming the UV-
cured sample.
Owing to their performance regarding both processing
and properties, UV-cured acrylate/epoxide IPNs are expec-
ted to find their main applications as fast-drying protective
coatings and for the rapid manufacturing of composite
materials and optical components.
3.3 Photocrosslinking of Functionalized Polymers
Besides transforming a liquid resin into a solid polymer, UV
radiation can also be used to induce the polymerization of
Figure 22. Photoinitiated polymerization of a 50:50 blend of
ESO with hexanediol diacrylate. [TAS]¼ 1 wt.-%, [Darocur
1173]¼ 1 wt.-%; I¼ 600 mW
cm2.
Figure 23. Two-step UV curing of an epoxide/acrylate hybrid
upon exposure to filtered (a) and unfiltered (b) light. Light
intensity: 30 mW
cm2.
1082 C. Decker
reactive groups located on a polymeric chain and thus
achieve a rapid insolubilization, as well as modify the
physical and mechanical properties of functionalized poly-
mers. The high initiation rate provided by intense illumina-
tion is beneficial in speeding up the reaction, which still
proceeds much slower in the solid state than in liquid resins.
UV technology could be successfully applied to crosslink
waterborne urethane-acrylate coatings, functionalized
polyisoprene rubbers, and polybutadiene-type thermoplas-
tic elastomers within seconds.
3.3.1 UV Curing of Water-Based Urethane/Acrylate
Coatings
The potential of water-based systems and their performance
in UV radiation curing have been thoroughly investigated in
the past decade.[23,79–84] Most of them consist of a disper-
sion or emulsion of acrylate functionalized oligomers that
can be photocrosslinked in the solid state after the removal
of water. The advantages offered by these environmental
friendly systems are partly offset by the required drying step
that increase the overall processing time.
The water-dispersible polyurethane-acrylate (PUA) oli-
gomer was synthesized by reaction of a diisocyanate with a
mixture of a telechelic diol oligomer, dimethylol propionic
acid, and a hydroxy-acrylate monomer.[84] The carboxylic
acid groups were neutralized by an amine, and water was
added to form an aqueous dispersion with a 35 wt.-% solid
content. The formulation containing a soluble photoinitia-
tor ([Irgacure 2959]¼ 0.3 wt.-%) was cast onto a BaF2
crystal so as to obtain a 20 mm thick dry film after water
removal. The photopolymerization of the acrylate end
groups, monitored by IR spectroscopy, proceeds relatively
slowly at ambient temperature and is leveling off at 50%
conversion because of severe mobility restrictions of the
reactive species (Figure 24). Both polymerization rate and
final cure extent were substantially increased by rising the
sample temperature to 80 8C, and even more so by adding a
diacrylate monomer ([HDDA]¼ 10 wt.-%) to act as a
reactive plasticizer (Figure 24). For maximum efficiency,
the UVexposure has therefore to be performed immediately
after the drying step on the hot sample emerging from the
infrared oven. It should be mentioned that the UV-curing of
waterborne PUA coatings is not inhibited significantly by
atmospheric oxygen, because of the very slow diffusion of
air into the solid film.[85]
The polymer obtained after UV curing is highly cross-
linked, as shown by the schematic 3D representation of
Figure 25 and therefore totally insoluble in organic solvents.
The crosslink density, typically on the order of 1 mol
kg1,
can be easily controlled through the molecular weight of the
telechelic PUA oligomer. The build-up of numerous chemi-
cal links between the polymer chains makes the coating
harder and more resistant to scratching. Figure 26 shows
how the pendulum Persoz hardness of the PUA coating
increases upon intense illumination to reach values up to
350 s, which are almost as high as for mineral glass. One of
the distinct advantages of water-based UV-cured polyur-
ethane acrylate coatings is that they combine hardness and
flexibility,[23] thus making such materials quite resistant to
abrasion and shock (Figure 27).
3.3.2 Photocrosslinking of Functionalized
Polyisoprene
It is difficult to crosslink polyisoprene by UV irradiation
because of the poor reactivity of the amylene double bond.
By transforming this function into an epoxy group or acry-
late double bond, a great improvement in reactivity was
achieved, thus allowing such functionalized polyisoprene
Figure 24. Influence of the temperature and of HDDA (10 wt.-
%) on the UV-curing of a waterborne PUA coating. [Irgacure
2959]¼ 1 wt.-%, I¼ 500 mW
cm2.
Figure 25. Three-dimensional polymer network formed by
photopolymerization of a polyurethane-diacrylate.
Kinetic Study and New Applications of UV Radiation Curing 1083
to be crosslinked at ambient temperature by a short UV
exposure in the presence of an adequate photoinitiator.
Epoxidized polyisoprene (EPI) was obtained by treat-
ment of natural rubber by peracetic acid for 6 h at 5 8C
(Scheme 12).[86] The ring-opening polymerization of the
epoxy groups proceeds effectively upon UV irradiation of
EPI in the presence of a triarylsulfonium salt, with the
formation of ether crosslinks.[87,88] As expected, the photo-
initiated cationic polymerization continues to proceed in
the dark, so that all epoxy groups had reacted after a 0.5 s
UV exposure and a 1 d storage in the dark, as shown in
Figure 28. At the same time, the UV-irradiated rubber be-
comes harder and insoluble in organic solvents (Figure 29).
The high value of the swelling ratio (SR¼ 8) indicates that a
very loose polymer network has been formed, most prob-
ably because a majority of the epoxy groups tend to
polymerize with neighboring groups located on the same
polymer chain. The fact that complete insolubilization was
achieved only when 60% of the epoxy groups had polyme-
rized (73 epoxy groups per chain) argues in favor of such
intramolecular process. Its importance was found to rise
with the epoxidation extent of EPI, as expected.[87] It is
therefore possible to achieve photocrosslinking with a func-
tionalized polyisoprene containing only a few epoxy groups,
since the intermolecular process is then predominant.
Polyisoprene can also be readily photocrosslinked by
introducing pendant acrylate double bonds on the polymer
backbone, simply by treating epoxidized polyisoprene by
acrylic acid (Scheme 13).
Acrylated polysioprene (API) was transformed into a
hard and insoluble material within a few seconds upon
intense illumination in the presence of a hydroxyphenyl
ketone photoinitiator.[89] The in-chain amylene double
bonds of polyisoprene were shown to copolymerize with the
Figure 26. Insolubilization and hardening of a waterborne
PUA coating upon UV exposure at ambient temperature. I¼
500 mW
cm2.
Figure 27. Correlation between hardness and flexibility in
polymeric materials. Performance of a UV-cured waterborne
PUA coating.
Scheme 12. Epoxidation of polyisoprene.
Figure 28. Photocrosslinking of epoxidized polyisoprene. Influ-
ence of storage in the dark on cationic post-polymerization of the
epoxy group. I¼ 500 mW
cm2; [TAS]¼ 3 wt.-%.
1084 C. Decker
acrylate double bonds, thus leading to the formation of a
relatively tight polymer network (SR¼ 1 for [acrylate]0¼
4.8 mol
kg1).[90] The crosslinking reaction can be
markedly accelerated by introducing in the solid API film
a di- or triacrylate monomer that acts as a reactive plas-
ticizer. UV exposure of 0.2 s proved to be sufficient to
transform the low-modulus elastomer into a high-modulus
thermoset polymer which is very hard (Persoz hard-
ness¼ 330 s), but still flexible (Figure 30). Such perfor-
mance was achieved immediately after UV irradiation, as
there is no dark post-cure in radical-type polymerizations.
By using monomers like epoxides that polymerize by a
cationic mechanism as reactive plasticizers, two IPNs have
been formed simultaneously upon UVirradiation in the pre-
sence of both radical and cationic photoinitiators.
A similar IPN has been obtained by photocrosslinking of
an epoxidized polyisoprene plasticized with a diacrylate
monomer (HDDA).[91] By changing the relative concentra-
tion of the monomer and its chemical nature, as well as the
degree of functionalization of polyisoprene, it is possible to
modify the properties of the photocured elastomer to make
it well-suited for given applications (adhesives, protective
coatings, composite materials). Owing to their high reac-
tivity, such monomer-plasticized functionalized polyiso-
prenes were readily crosslinked by a few minute exposure
to sunlight,[88,90] which makes this environmentally benign
technology (solvent-free, energy-cost-free) particularly
attractive for the curing of large-dimension rubber-made
materials.
3.3.3 Photocrosslinking of Thermoplastic Elastomers
Thermoplastic polymer networks combine the properties of
thermoplastic materials and elastomers. They are typically
made of block copolymers containing glassy domains (e.g.
polystyrene) and elastomeric domains (e.g. polybutadiene)
which tend to segregate to form a two-phase morphol-
ogy.[92] Such polymers exhibit a high extensibility and
resilience, but are still soluble in hydrocarbons and start to
flow upon heating above the Tg of the glassy polymer
moiety (80 8C). The physical network can be strength-
ened by creation of a chemical network through permanent
covalent bonds joining together the polymeric chains of the
elastomeric phase, thus making the material insoluble and
more resistant to heating. Such a crosslinking process was
effectively performed in polybutadiene-based thermo-
plastic elastomers (styrene/butadiene,[93,94] acrylonitrile/
butadiene (ABA)[95]) through a short UV exposure in the
presence of a radical-type photoinitiator. The shear adhe-
sion failure temperature was found to increase from 80 8C to
over 160 8C upon UV-curing, while the polymer became
insoluble in organic solvents, as a result of the intermolecular
reaction between the polybutadiene vinyl double bonds.[95]
Again, photocrosslinking was greatly accelerated by the
addition of a triacrylate monomer (TMPTA) that increases
the molecular mobility in the solid polymer and copoly-
merizes with the polybutadiene unsaturations. Figure 31
Figure 29. Insolubilization and hardening of an epoxidized
polyisoprene exposed to UV radiation (measurements were made
after 20 min of storage). I¼ 500 mW
cm2, [TAS]¼ 3 wt.-%.
Scheme 13. Acrylation of epoxidized polyisoprene.
Figure 30. Influence of a triacrylate monomer ([TMPTA]¼
50 wt.-%) on the photocrosslinking and hardening of an acrylated
polyisoprene. [Lucirin TPO]¼ 5 wt.-%, I¼ 500 mW
cm2.
Kinetic Study and New Applications of UV Radiation Curing 1085
shows some typical insolubilization and acrylate conver-
sion profiles obtained upon UVexposure of an ABA triblock
copolymer containing 20 wt.-% of TMPTA. The UV-irra-
diated rubber becomes totally insoluble in toluene within
less than 1 s, with formation of a tight polymer network
(swelling ratio of 0.5). Similar results were obtained with
styrene/butadiene copolymers.[93]
An equally important accelerating effect was observed
by using small amounts (2 wt.-%) of a trifunctional thiol
crosslinker (TRIS) which undergoes a step-growth addition
polymerization with the alkene double bonds of poly-
butadiene (Scheme 14). After a 0.1 s UVexposure of ABA in
the presence of a phosphine oxide photoinitiator ([Irgacure
819]¼ 1 wt.-%), 6% of the butene double bonds had already
disappeared (Figure 32), compared to less than 1% in the
absence of TRIS. After this initial sharp drop, the remaining
unsaturations are hardly reacting anymore because essen-
tially all thiol molecules have been consumed.[94] As expec-
ted, such an efficient crosslinking process leads to rapid
insolubilization of the ABA copolymer, with a moderate
hardening of the rubber, as shown in Figure 32. The fact that
the elastomeric character was retained is a distinct advan-
tage of the ABA/TRIS system, compared to the ABA/
TMPTA or SBS/TRIS systems. The soft and highly flexible
polymer thus obtained is particularly well suited for the
manufacturing of hot-melt adhesives and sealants, safety
glasses and flexible printing plates.
To illustrate the superior performance of the thiol/ene
system for achieving efficient crosslinking of polybuta-
diene-based rubbers, Figure 33 shows the insolubilization
profiles of ABA, ABAþHDDA (20 wt.-%) and ABAþTRIS
(2 wt.-%) samples, which were exposed to UV radiation
under the same conditions (I¼ 500 mW
cm2, [Irgacure
819]¼ 1 wt.-%). After 0.8 s of exposure, gel fraction values
were measured to be 22%, 83% and 98%, respectively. The
greater efficiency of the thiol/ene system can be accounted
for by molecular mobility and mechanistic considerations.
In a conventional polymerization where polymer radicals
Figure 31. Photocrosslinking of an 80:20 mixture by weight of
ABA and TMPTA. [Irgacure 819]¼ 1 wt.-%, I¼ 500 mW
cm2. Scheme 14. Reaction scheme of thiol/ene polymerization of the
ABA/TRIS system.
Figure 32. Photocrosslinking of an acrylonitrile/butadiene rub-
ber in the presence of a trifunctional thiol ([TRIS]¼ 2 wt.-%).
[Irgacure 819]¼ 1 wt.-%, I¼ 500 mW
cm2.
1086 C. Decker
are reacting with a close-by double bond, mobility restric-
tions affect drastically the growing of the polymer chain in a
solid medium. By contrast, in a step-growth addition poly-
merization, the coupling reaction between polymer radical
and thiol is regenerating a mobile thiyl radical that propa-
gates chain reaction more easily (Scheme 14).
Because of its process facility and efficiency, UV techno-
logy has been revealed as a powerful tool to crosslink
rapidly thermoplastic elastomers and modify, selectively in
the illuminated areas, their physico-chemical characteris-
tics. The main applications of these photocurable rubbers
are expected to appear in sectors where cure speed and
spatial control are a major concern, such as in flexography
and photolithography.
4 Applications of UV-Cured Polymers
UV radiation curing has found its major openings in various
industrial applications where its distinct advantages (fast
cure, solvent-free formulation, selective cure) have allowed
this environmentally friendly technology to outclass more
conventional processing techniques. The most important
applications of UV-cured polymers are to be found in the
following areas:[11] (i) graphic arts, with the manufacturing
of printing plates and ultrafast drying of printing inks and
overprint varnishes, (ii) coating industry, to protect all kinds
of materials (wood, paper, plastics, metals, optical fibers),
(iii) microelectronics, where UV-curable resins serve as
photoresists in the imaging step, but also as fast-drying
adhesives and conformal coatings, and (iv) bonding indus-
try, where solvent-free UV glues allow precise control of
the setting time and faster manufacturing of pressure-
sensitive adhesives and release coatings.
In all of these applications, the physico-chemical pro-
perties of the UV-cured material have to meet precise
specifications imposed for the considered end-use. This is
usually achieved by designing properly both resin formula-
tion and processing conditions (light intensity, exposure
time, temperature, atmosphere). The mechanical properties
of such photopolymers depend primarily on the chemical
structure, functionality and concentration of the various
constituents of the resin, as well as on the cure extent. Cross-
linked polymers containing aliphatic polyurethane chains
show usually an elastomeric character and are therefore
well suited for adhesives applications and surface protec-
tion of flexible supports. By contrast, UV-cured coatings
containing polyphenoxy chains are hard and glassy ma-
terials that are more appropriate for the treatment of rigid
substrates requiring a good resistance to scratching. In this
last section, some potential new applications of the UV-
curing technology are reviewed, in particular to produce
weathering-resistant protective coatings, laser-cured mate-
rials, glass laminates and nanocomposite materials.
4.1 UV-Cured Protective Coatings
A common characteristic of UV-cured polymers is that they
are strictly insoluble in organic solvents and quite resistant
to most chemicals. Because of their high crosslink density
that can reach up to 5 mol/kg, they undergo hardly any
swelling in good solvents like chloroform or tetrahydro-
furan and are therefore well suited to protect polymer
materials, such as polycarbonates or poly(vinyl chloride)
(PVC). By covering both faces of a 2 mm thick PVC plate
with a UV-cured coating made of a polyester hexaacrylate
(Ebecryl 830), the thermoplastic polymer was rendered
perfectly resistant to tetrahydrofuran, as shown in Figure 34.
This type of coating proved to be also resistant to strong
acids like concentrated HCl, H2SO4 and HNO3. It was
successfully used to protect a glass/epoxy composite
against hydrofluoroboric acid, which otherwise destroys
rapidly the uncoated material. Acrylate-based UV-cured
coatings can be made more resistant to staining by using
multifunctional monomers like trimethylolpropane triacry-
late or pentaerythritol tetraacrylate as reactive diluents.[96]
Specially designed UV-cured polyurethane-acrylate
polymers (Actilane 20, Ebecryl 284) have been shown to
be very resistant to photodegradation when exposed to
accelerated weathering.[97,98] The high crosslink density of
the photopolymer is favoring cage recombination of the
primary radicals formed by UV-irradiation, while it reduces
at the same time the extent of chain peroxidation because of
the restricted molecular mobility. The residual photoini-
tiator is destroyed within minutes by solar radiation and has
therefore no detrimental effect on the long-term light
stability of the photocrosslinked polymer.
Figure 33. Influence of thiol and acrylate monomers on the pho-
tocrosslinking of an ABA rubber. [TRIS]¼ 2 wt.-, [HDDA]¼
20 wt.-%, [Irgacure 819]¼ 1 wt.-%, I¼ 500 mW
cm2.
Kinetic Study and New Applications of UV Radiation Curing 1087
The weathering resistance of various organic materials
used in outdoor applications (plastics, wood, painted metal)
has been greatly improved by means of UV-cured polyur-
ethane-acrylate coatings containing light stabilizers.[98–101]
The main role of UV absorbers (hydroxyphenylbenzotria-
zoles or hydroxyphenyltriazines) is to screen the harmful
UV-A and UV-B radiation of sunlight to avoid photodegrada-
tionof thecoatedorganic substrate.HALSradical scavengers
are also needed to increase the light stability of the coating, as
well as the lifetime of the UV absorber.[102,103] The photo-
bleaching of a red painted metal exposed to accelerated
QUV-A weathering has been effectively prevented by
covering its surface with a 50 mm thick UV-cured PUA film
(Ebecryl 284þHDDA) containing both types of light stabi-
lizers (1 wt.-% each), as shown in Figure 35. After a 5000 h
exposure, the coating remained clear and glossy, while one
could hardly detect any change in the paint color. This
photostabilization process was also successfully applied to
prevent the discoloration of transparent PVC plates and
wood panels.[98,99] When properly stabilized, photocros-
slinked polyurethane-acrylate coatings were found to out-
perform the thermosetting melamine-acrylate clearcoats
currently used as automotive finishes, with respect to their
resistance to weathering, abrasion and scratching.[44,101]
4.2 Laser-Induced Curing
In the continuous search for faster photoprocessing and
higher image definition, lasers appear to represent the ulti-
mate light source to achieve ultrafast polymerization and
micronic resolution, because of the great power, concen-
trated in the collimated laser beam, which can be focused
sharply. Laser-induced polymerization has been applied to
different types of multifunctional monomers and oligomers
(acrylates, vinyl ethers, epoxides) to produce highly
crosslinked polymer materials[32,55,104–106] (for a review
cf. to the literature[107]). This advanced technology has
found new applications in various industrial sectors, where
the improved performance has outweighed the higher cost
of laser processing, in particular in photolithography,[108]
stereolithography,[109] and holography.[110]
In the manufacturing of printing plates or printed circuit
boards, the required high resolution patterns can be gene-
rated onto a photosensitive resin by laser direct imaging
(LDI) without the use of a costly mask.[105,108] With nega-
tive photoresists, the illuminated area becomes insoluble
and a relief image is obtained after development. By using
the highly reactive photocurable acrylate resins described
previously, writing speeds up to 100 m/s were reached by
focusing a 100 mW Krypton ion UV laser beam down to a
10 mm wide spot, corresponding to an exposure time of
0.1 ms.[106] The LDI technology offers a number of advan-
tages when compared to conventional photo-tool imaging
technology, such as sharper and well-defined lines, fewer
defects, rapid prototyping and process flexibility.
Figure 36 shows the insolubilization profiles obtained
with a polyester amino-tetraacrylate resin (Ebecryl 80 from
UCB) by plotting the thickness of the insoluble polymer
films formed as a function of the fluence (or dose) expressed
in mJ
cm2. When the photoresist was exposed to UV
emission at 337.4 nm of a Krþ laser beam, a UV dose as low
as 0.4 mJ
cm2 proved to be already sufficient to achieve
insolubilization in the presence of a morpholino ketone
photoinitiator ([Irgacure 369]¼ 1 wt.-%). Such a high
Figure 34. Influence of a 50 mm thick UV-cured PUA coating
on the dissolution profile of a PVC plate in tetrahydrofuran.
Figure 35. Photobleaching of a red paint upon exposure to
accelerated QUV-A weathering. Performance of a 50 mm thick
UV-cured coating containing light stabilizers (1 wt.-%Tinuvin
292 and 1 wt.-% Tinuvin 400)
1088 C. Decker
sensitivity is due to the large chemical amplification factor
resulting from the chain reaction (KCL 104 mol
radical1).
Since most of the continuous wave lasers have their
strongest emission in the visible range, there has been a
growing demand for photocurable polymers which would
be sensitive beyond 400 nm radiation. The sensitivity of
visible-type photoresists is not as great as that of UV-
photoresists because of the lower energy of visible photons
and of less efficiency of the suited photoinitiators. For the
blue diode laser emitting at 407 nm, bis(acylphosphine)
oxide photoinitiators can still be used. With the emission at
488 nm of the argon ion laser, the best results were obtained
with a combination of three photoinitiators: xanthone
dye (eosin), chloromethyltriazine and aryltitanocene
(Figure 36). The photosensitivity of this complex system
was still lagging behind that of the morpholino-ketone-
based UV photoresist.[105] In a different type of application,
laser-curable resins have been successfully used to connect
optical fibers. The tips of each fiber, coated by a PUA resin,
were put in contact. Bonding was achieved instantly by
simply firing the laser through either one of the optical
fibers.
The steady interest in laser-induced polymerization pro-
cesses is expected to grow further, as the potential of this
technology becomes more apparent with the development
of better performing photosensitive systems and the
manufacturing of reliable low-cost lasers and light-emitting
diodes.
4.3 UV-Cured Glass Laminates
Radiation-curable adhesives are now well-established pro-
ducts because of their distinct characteristics: a quick setting
of solvent-free glue in well-defined areas, with a perfect
control of setting time. The latter factor has resulted in the
development of light-curable resins to achieve an accurate
assembly of optical components. Other applications of UV-
curable adhesives have appeared in various sectors, includ-
ing release coatings, sealants, pressure-sensitive adhesives,
conformal coatings and hot-melt adhesives.[111]
An important limitation of the use of UV-curable glues is
that at least one of the two parts of the assembly must be
transparent to UV radiation. The two elements to be bonded
can be identical or of a different nature, and consist of either
flexible films or rigid transparent plates. Safety glasses have
been readily produced by photocuring at ambient tempera-
ture of an acrylic intercalate consisting either of a liquid
resin or a solid thermoplastic functionalized polymer.[112]
As the UV radiations reaching the resin are filtered by the
glass plate, it is recommended to use a photoinitiator which
absorbs above 330 nm, like acylphosphine oxides. Figure 37
shows the polymerization profiles recorded with a lami-
nated PUA sample exposed to glass-filtered light of a
mercury lamp, using Lucirin TPO or Irgacure 184 as the
photoinitiator (2 wt.-%). The concentration of photoinitia-
tor has to be kept as low as possible (0.1 wt.-% for 2 mm
thick intercalates) to achieve a deep-through cure and mini-
mize yellowing, which is more pronounced with Lucirin
TPO than with Irgacure 184. It should be emphasized that
Figure 36. Insolubilization profiles of an acrylate photoresist
(Ebecryl 80) upon UV exposure to a UV (337.4 nm) or visible
(488 nm) laser beam under O2-free conditions. Photoinitiator
1: Irgacure 369 (1 wt.-%), and 2: eosine (1 wt.-%)þ triazine
(0.5 wt.-%)þ Irgacure 784 (0.5 wt.-%).
Figure 37. Influence of photoinitiator (2 wt.-%) on the poly-
merization of a liquid PUA resin exposed to glass-filtered
UV radiation under laminate conditions. Film thickness: 24 mm,
I¼ 50 mW
cm2.
Kinetic Study and New Applications of UV Radiation Curing 1089
oxygen inhibition is not a critical factor anymore in this type
of application, because the UV-curable resin is not in direct
contact with air. After the oxygen dissolved in the resin has
been consumed during the induction period, polymeriza-
tion proceeds as rapidly as under an inert atmosphere. A
liquid formulation containing an aliphatic polyurethane-
acrylate oligomer (65 wt.-%), butyl acrylate (25 wt.-%),
and acrylic acid (10 wt.-%) was found to give the best
performance in terms of reactivity, adhesion, impact resis-
tance and transparency.[112] An organosilane (1 wt.-% of
g-methacryloxypropyl trimethoxysilane) had to be includ-
ed in the UV glue formulation to improve adhesion and
ensure a cohesive fracture in the impact resistance test. As
required for safety glass applications, all of the broken glass
pieces remained stuck on the elastomeric 2 mm thick
intercalate. Impact resistance was further increased by
introducing a polycarbonate core sandwiched between the
two glass plates and bonded on each side by a UV glue.
Similar results have been obtained by using a solid
aliphatic polyurethane (Mn¼ 125 000 g/mol, Tg¼30 8C),
plasticized with an aliphatic PUA oligomer, as an adhesive
agent. The 0.8 mm thick photosensitive polymer film was
laminated between two glass plates and maintained at 80 8C
under 1 MPa pressure. After UV exposure, the softening
temperature was increased from 75 8C to above 160 8C,
with formation of a tight assembly. The photocrosslinked
polymer exhibits the strong elastomeric character required
to ensure high-impact resistance, as shown by its tensile
properties: breaking occurred under a 22 MPa stress at
300% elongation.[112] Advantages of the UV technology
over conventional thermal processing for the manufactur-
ing of safety glasses are clearly apparent from Table 2.
In addition to their high transparency, these colorless
safety glasses appear to be very resistant to photodegrada-
tion, specially in the presence of HALS radical scavenger
(1 wt.-% of Tinuvin 292). After 2000 h of continuous
exposure at 40 8C in a QUV-A accelerated weatherometer,
the laminate stayed perfectly transparent and non-colored,
while the adhesive strength remained essentially unaf-
fected. Similar safety glasses have been recently produced
by solar processing, simply by exposing the laminated
sample to sunlight for a few minutes.[113] One of the distinct
advantages of this technology free of energy costs is to
allow large dimension items up to several square meters to
be uniformly insolated. Solar curing was also successfully
used to assemble mineral glass to a metal or to an organic
glass (PMMA, PVC, polycarbonate).[113] The adhesive
strength has been greatly enhanced by a styrene/butadiene
rubber using as polymer matrix, which copolymerizes with
the acrylate monomers and by adding to the formulation
nano-sized silica particles grafted with an acrylate function.
4.4 Synthesis of Nanocomposite Materials
by UV Curing
Mineral charges such as silica particles or glass fibers can be
introduced in the formulation of photocurable resins to
produce hard and abrasion resistant composite materials by
a short UV exposure at ambient temperature. This tech-
nology has found its major opening in dentistry, where
photocured dental polymeric materials are increasingly
used as fast-hardening cements, sealants, adhesives, and
dental prosthesis.[114] The inert filler, which can represent
up to 80% by weight of the total composition, is usually
made of micron-sized particles of quartz or hydroxyapa-
tite.[115] Light scattering by the filler particles may cause a
rise of the absorbed light through an increase in optical path
length, thus allowing up to a few millimeter thick composite
materials to be deep-through cured. This also requires to
use photobleachable photoinitiators in order to allow the
incident UV radiation to penetrate progressively deeper
into the sample and thus promote a frontal polymeriza-
tion.[13,39] The abrasion and scratching resistance of UV-
cured coatings was found to be greatly improved by
incorporation into the polymer matrix of some acrylate
functionalized colloid silica.[116–118] The mineral particles
can be chemically bonded to the polymer matrix by using a
silica gel functionalized with various groups (thiol, epoxy,
isocyanate) as a filler. UV radiation curing was also shown
to be a rapid and environmental friendly process to produce
composite membranes and fiber-reinforced plastics.[119,120]
In all of these applications, the spatial and temporal control
afforded by photopolymerization, together with the short
setting time, make this technology attractive for rapid and
inexpensive processing of polymeric composites.
In conventional composite materials, filler and polymer
are combined on a micron scale, which often leads to insuf-
ficient adhesion between matrix and reinforcing filler.
Composites that exhibit a change in composition and struc-
ture on a nanometer length scale have been shown to afford
remarkable property enhancement with respect to stiffness
and strength, heat resistance and gas barrier properties.[121]
Most of the nanocomposite materials are based on linear
polymers (polyamides, polyolefins, polystyrene) and there-
fore show poor chemical resistance. The mineral filler,
usually a layered silicate such as clay, is typically incor-
porated into the melted polymer. Thermodegradation may
occur during this treatment, depending on the required
temperature and swelling time, thus affecting the composite
heat stability.
Highly resistant nanocomposite materials have been
recently produced by UV curing at ambient temperature of
Table 2. Manufacturing of safety glasses.
Process Thermal UV
Temperature (8C) 150 25
Time (min) 60 < 0.1
Pressure (bar) 100 1
1090 C. Decker
an acrylate-based resin containing a nano-scale filler (exfo-
liated silicate platelets).[122,123] To obtain a true nanocom-
posite, the 1 nm thick platelets need to be uniformly
dispersed within the polymer matrix. This was achieved by
making the clay (bentonite or montmorillonite) organo-
philic through treatment with an alkylammonium salt,
which leads to a widening of the interlayer and allows the
liquid resin to penetrate into the clay galleries.[123] Exfoli-
ation of the silicate platelets then occurs, as demonstrated
by the disappearance of the characteristic X-ray diffraction
peak. The increase in transparency, compared to the micro-
composite, as well as the markedly reduced sedimentation
rate, further argue in favor of an effective exfoliation of the
clay layers.
Upon UV exposure in the presence of a photoinitiator
(Darocur 1173 and Irgacure 819), the crosslinking poly-
merization of a PUA resin containing the nano-filler was
found to proceed as fast and as extensively as that of the neat
PUA formulation. Figure 38 shows the conversion versus
time profiles recorded by RT-IR spectroscopy for 2 mm
thick samples, by monitoring the disappearance of the near
infrared band of the acrylate double bond at 6160 cm1.
Under more intense illumination (600 mW
cm2), 1 s of
exposure proved to be sufficient to obtain a hard and
insoluble polymer material containing essentially no resi-
dual unsaturation. In these UV-cured nanocomposites, 1 nm
thick silicate layers are randomly dispersed within the
polymer matrix to form a skeleton-like superstructure. This
explains why a significant improvement in the polymer
properties can be achieved already at low filler content.
Such performance, together with cost-effective high-speed
processing of a solvent-free resin, are key factors for the
success of these newly developed photopolymer nanocom-
posites. With respect to the potential hazard of materials
containing nanoparticles, an issue recently raised at a
symposium on ‘‘Nanotechnology: Environment Friend or
Foe’’ (Washington, 11 March 2002), it should be empha-
sized that these nanocomposite photopolymers are per-
fectly safe, because the mineral filler is a natural, nontoxic
product (clay). Moreover, the nanoparticles, present in
small amounts, are tightly trapped into the polymer network
and are therefore not able to ‘‘penetrate living cells and
accumulate in animal organs’’, as was observed for other
nano-sized particles.
Conclusions
Photoinitiated polymerization of multifunctional mono-
mers and oligomers can be considered as the most effective
method to generate quasi-instantly three-dimensional
polymers. This environmentaly benign technology has
found its major openings in the coating industry and in
graphic arts, because of its many advantages regarding
process facility and product performance.
Such ultrafast polymerizations are best followed by real-
time infrared spectroscopy, a technique that allows the
formation of the polymer to be monitored in situ on a milli-
second timescale. The influence of chemical and physical
factors on the polymerization kinetics has thus been
assessed for different types of UV-curable systems, as well
as for monomer blends, under the same conditions as those
used in most industrial applications, i.e., thin films exposed
to intense polychromatic radiation in the presence of air.
Significant progress has recently been achieved in UV
radiation curing with the development of very efficient
photoinitiators and highly reactive monomers and functio-
nalized oligomers allowing to produce macromolecular
materials with tailor-made properties. It has opened the way
to novel potential applications of such photopolymers, in
particular for the outdoor protection of organic materials by
UV-cured coatings, the production of micron-sized relief
images by direct laser writing, the rapid manufacturing of
safety glasses and the synthesis of quick-setting nanocom-
posite materials. In consideration of its unique features, one
can reasonably expect this advanced technology to continue
to expand and attract attention in an ever growing number of
industrial sectors.
Further research on photocrosslinking polymerization is
still needed to improve both resin performance and final
properties of UV-cured polymers. It should also concentrate
on the mechanistic aspects to gain a better understanding
and control of the manifold and complex processes invol-
ved in ultrafast photocuring reactions.
Figure 38. UV curing in air of a 2 mm thick PUA nanocomposite
material (solid line) and neat PUA resin (dashed line) monitored in
real time by means of near IR spectroscopy (6160 cm1). [Ebecryl
284]¼ 67 wt.-%, [HDDA]¼ 27 wt.-%, [organoclay]¼ 3 wt.-%,
[Irgacure 819]¼ 1 wt.-%, I¼ 110 mW
cm2.
Kinetic Study and New Applications of UV Radiation Curing 1091
Acknowledgement: The author wishes to thank his coworkers
at the Polymer Photochemistry Laboratory,C. Bianchi,D.Decker,
L. Keller, F. Masson, T. Nguyen Thi Viet, K. Studer, E. Weber-
Koehl, K. Zahouily.
Received: October 4, 2002
Revised: November 18, 2002
Accepted: November 19, 2002
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Kinetic Study and New Applications of UV Radiation Curing 1093
High-Speed Photocrosslinking of Thermoplastic Styrene–
Butadiene Elastomers
C. DECKER, T. NGUYEN THI VIET
Laboratoire de Photochimie Générale (UMR-CNRS No. 7525), Ecole Nationale Supérieure de Chimie de Mulhouse,
Université de Haute-Alsace, 3, rue Alfred Werner, 68200 Mulhouse, France
Received 2 June 1999; accepted 5 November 1999
ABSTRACT: Photoinitiated thiol/ene polymerization was used to crosslink a triblock
styrene/butadiene/styrene (SBS) polymer of low vinyl content (8%). The crosslinking
process was followed by infrared spectroscopy (loss of unsaturation), insolubilization,
swelling, and hardness measurements. The photogenerated thiyl radicals react with
both the vinyl and the 2-butene double bonds of the copolymer. Concentrations of less
than 1 wt % in the trifunctional thiol crosslinker and in the acylphosphine oxide
photoinitiator proved to be sufficient to create, within 0.5 s, a permanent chemical
network in the elastomeric phase. This UV-curing technology was successfully applied
to crosslink rapidly commercial SBS–Kratont thermoplastic elastomers. It proved also
effective in the case of the much less reactive triblock styrene/isoprene/styrene (SIS)
polymer which contains no vinyl double bonds. The thiol/ene polymerization was shown
to be a much more efficient process to crosslink SBS and SIS thermoplastic elastomers
than was the copolymerization of the rubber double bonds with a diacrylate monomer.
© 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1902–1912, 2000
Key words: photochemistry; crosslinking; UV curing; thermoplastic elastomers; po-
lybutadiene; thiol
INTRODUCTION
Thermoplastic interpenetrating polymer net-
works (IPNs) combine the properties of thermo-
plastic materials and of elastomers. They have
found various industrial applications, in particu-
lar, for the manufacturing of molded articles, pro-
tective coatings, and pressure-sensitive adhe-
sives. Thermoplastic elastomers are usually made
of block copolymers containing glassy domains
and elastomeric domains which tend to segregate
and form a two-phase morphology.1 A typical ex-
ample is the polystyrene-block-polybutadiene-
block-polystyrene (SBS)2 commercialized by Shell
under the trademark Kratont. Typical SBS
triblock thermoplastic elastomers exhibit high ex-
tensibility, elastic recovery, and resilience; they
are resistant to water and alcohol, but remain
soluble in ketones, esters, and hydrocarbons.
Because the network is made of physical
crosslinks rather than chemical crosslinks, the
glassy polystyrene (PS) domains lose their co-
hesion upon heating above the Tg (around 80°C)
and the polymer starts to flow. Such IPNs are
therefore ill-suited for producing hot-melt adhe-
sives or flexographic printing plates which must
exhibit a high resistance to elevated tempera-
tures (.150°C) and to aromatic solvents as
well. The physical network can be strengthened
by creating a chemical network through perma-
nent covalent bonds joining together the chains
of the elastomeric phase to give an insoluble
material.
Correspondence to: C. Decker.
Contract grant sponsor: Shell Research, Louvain la Neuve,
Belgium.
Journal of Applied Polymer Science, Vol. 77, 1902–1912 (2000)
© 2000 John Wiley & Sons, Inc.
1902
It was recently shown that such a crosslinking
process is easily induced in SBS thermoplastic
elastomers by UV irradiation in the presence of a
radical-type photoinitiator.3,4 The shear adhesion
failure temperature (SAFT) was found to increase
from 80 to over 160°C upon UV curing, while the
tack remained essentially unchanged.5,6 The net-
work was considered to be formed by polymeriza-
tion of the pendent vinyl double bonds, which are
known to be more reactive than are the 2-butene
double bonds of the polybutadiene chain. The
crosslinking process can be markedly accelerated
by the addition of multifunctional acrylate or
thiol monomers that copolymerize with the poly-
butadiene unsaturations.4,7 The use of light to
induce the chain reaction has several advantages,
such as high cure speed, low-energy consumption,
ambient temperature operations, dry processing
without VOC emission, and selective curing in the
illuminated areas. By performing the UV expo-
sure through a mask, one can generate, after sol-
vent development, the high-relief images which
are needed for the manufacture of flexographic
printing plates.
In a previous work, we showed that photoiniti-
ated thiol-ene polymerization is an effective way
to crosslink thermoplastic SBS rubbers, total in-
solubilization being achieved within less than
1 s.8 As thiyl radicals were found to be 16 times
more reactive toward vinyl than were 2-butene
double bonds, the first experiments were con-
ducted on a specially designed SBS sample hav-
ing a high vinyl content (59%). Crosslinking in
the elastomeric phase was shown to still proceed
when the polybutadiene vinyl content was de-
creased to 8%, as long as the thiol concentration
was kept above 5 wt %. To reduce the bad smell
associated with the use of mercaptans, an at-
tempt was made to further lower the concentra-
tion of the thiol crosslinker. Another objective of
this study was to reduce the photoinitiator con-
centration below 1 wt %, the lowest value used so
far, in order to increase the penetration of UV
radiation into the sample and be able to cure
thick samples. Most of the experiments were car-
ried out with a typical SBS sample, which, like
most commercial SBS rubbers, contains a low
amount of pendent vinyl groups. The crosslinking
process was quantitatively studied by following
the disappearance of the vinyl groups, as well as
the insolubilization and the hardening of the SBS
sample upon UV irradiation.
EXPERIMENTAL
Materials
The SBS from Shell used in this study was spe-
cially designed by Shell (Louvain-la Neuve, Bel-
gium). It contained 8% pendent vinyl groups, the
2-butenylene units (—CH2—CHACH—CH2) lo-
cated on the polybutadiene backbone, represent-
ing 92% of the unsaturation content. Because of
its relatively low styrene content (;20%), the SBS
behaves similarly to a cured conventional vulca-
nized rubber. Some experiments were carried out
with commercial SBS samples (Kratont D-1102
and Kratont D-1186 from Shell) that differ
mainly by the molecular weight and the structure
of the polybutadiene chain, which was linear or
branched, respectively. The UV-curing perfor-
mance of a polystyrene-block-polyisoprene-block-
polystyrene (SIS) was also evaluated on a com-
mercial sample (Kratont D-1107 from Shell).
A trifunctional thiol was selected as the
crosslinking agent, trimethylolpropane mercapto-
propionate (TRIS), from Evans Chemetics (Lex-
ington, MA). It was added to a toluene solution of
SBS at a concentration between 0.2 and 3% by
weight of SBS. Among the various radical-type
photoinitiators tested, acylphosphine oxides
proved to be the most efficient ones to pho-
tocrosslink the SBS samples. All the experiments
were carried out using 2,4, 6-trimethylbenzoyl
diphenylphosphine oxide (Lucirin TPO from
BASF, Ludwigshafen, Germany) as the photoini-
tiator, at a typical concentration of 1 wt %. The
chemical formulas of the three components used
in this study are given in Figure 1.
Figure 1 Chemical formulas of the SBS rubber, thiol,
and photoinitiator used.
HIGH-SPEED PHOTOCROSSLINKING OF SBS ELASTOMERS 1903
Irradiation
In a typical UV-curing experiment, 20-mm thick
films were cast from a toluene solution containing
the SBS rubber, the thiol monomer, and the pho-
toinitiator onto either a KBr crystal for infrared
analysis or a glass plate for insolubilization and
hardness measurements. Samples were exposed
at ambient temperature to the radiation of a 80
W/cm medium-pressure mercury lamp (IST) in
the presence of air, at a passing speed of 60
m/min, which corresponds to an exposure dura-
tion of 0.1 s per pass. The maximum light inten-
sity at the sample position was measured by ra-
diometry (IL-390 light bug) to be 600 mW cm22 in
the UV range.
Analysis
The kinetics of the light-induced crosslinking of
the rubber film was studied quantitatively by
FTIR spectroscopy, by following the decrease
upon UV exposure of the absorption band charac-
teristic of the vinyl double bond at 910 cm21. The
degree of conversion was calculated from the ratio
of the corresponding IR absorbance before and
after UV exposure (A0 and At). It should be men-
tioned that even very small variations of the IR
band intensity can be monitored accurately, be-
cause the analysis is performed on the same sam-
ple exposed to UV light for various durations, at
exactly the same spot.
The gel fraction and the degree of swelling (SR)
of the irradiated polymer were determined by
soaking the sample in toluene for 1 day at room
temperature. The insoluble polymer was recov-
ered by filtration and dried at 70°C to a constant
weight. The hardness of the coating was evalu-
ated before and after irradiation by monitoring
the damping time of the oscillations of a pendu-
lum (Persoz hardness). The hardness was shown
to be strongly dependent on the glass transition
temperature,9 with Persoz values ranging typi-
cally from 30 s for soft elastomeric materials to
300 s for hard and glassy polymers.
RESULTS
Photoinitiated thiol-ene polymerization has been
thoroughly investigated over the past 20 years,
the main findings having recently been reviewed
by Jacobine in a comprehensive survey.10 The
polymerization proceeds by a step-growth addi-
tion mechanism which is propagated by a chain-
transfer reaction involving the thiyl radicals
(RSz):
A polymer network will be formed only if a
diene or a polyene reacts with a multifunctional
thiol, such as TRIS:
By contrast to most radical-induced crosslink-
ing processes, the thiol/ene polymerization is not
very sensitive to oxygen inhibition, because the
peroxyl radicals (PO2
z ) formed by O2 scavenging
can also react with the thiol and contribute to the
propagation of the chain reaction:
All the curing experiments described in this
article were carried out on thin films in contact
with air.
1904 DECKER AND VIET
There are only a few reports in the literature
on the photocrosslinking of the thiol–SBS sys-
tem,7,8,11 with essentially no information on the
reaction kinetics. We examined the influence of
the thiol and photoinitiator concentrations on the
UV curing of a low-vinyl SBS sample and on some
commercial SBS rubbers as well.
Influence of the Thiol Concentration
Vinyl Consumption
The SBS samples containing 1 wt % of a photo-
initiator (Lucirin TPO) and various concentra-
tions of thiol (0.2 wt % # [TRIS] # 3 wt %) were
exposed to UV radiation for different times up to
3 s. As the thiol consumption could not be fol-
lowed by IR spectroscopy at the low concentration
used, the polymerization reaction was monitored
through the decrease of the vinyl IR band at 910
cm21. Although this decrease is quite small, it can
still be quantified with great accuracy because
each sample undergoing photocrosslinking was
analyzed by the IR beam at exactly the same spot.
Figure 2 shows some typical kinetic curves of the
vinyl group consumption upon UV exposure and the
effect of the thiol content on the curing reaction which
follows a two-step kinetics. The thiol–vinyl copoly-
merization proceeds rapidly during the first 0.3 s of
irradiation, until the thiol monomer has been con-
sumed. At a 5 wt % concentration of TRIS, the SH
group was found to have essentially disappeared
after a 0.3-s UV exposure. The slow decrease of the
vinyl double bonds observed upon further irradia-
tion parallels that in the thiol-free SBS and was
therefore attributed to the homopolymerization of
these groups.
Although the decrease of the 2-butene double
bond cannot be followed by IR spectroscopy be-
cause of cis–trans isomerization (bands at 3060
and 965 cm21, respectively), it is still possible to
evaluate the contribution of the thiol–butene co-
polymerization to the crosslinking process from
the difference between the loss of the thiol group
D [SH] and that of the vinyl group D [V]:
Loss of vinyl by copolymerization:
D [V]copo 5 D [V]SBS1TRIS 2 D [V]SBS.
Loss of thiol group by copolymerization:
D [SH] 5 D [V]copo 1 D [B]copo.
Loss of butene group [B] by copolymerization:
D [B]copo 5 D [SH] 2 D [V] SB 1TRIS 1 D [V]SBS.
In Figure 3, the amounts of vinyl and butene
groups which have copolymerized with the thiol
after a 0.3-s exposure are plotted as a function of
the SH concentration. The two quantities [V]copo
and [B]copo were found to be quite similar and to
increase linearly with the thiol content, as ex-
pected. The fact that a single curve was obtained
implies that the rate constants kv and kb of the two
copolymerization processes are in the reciprocal ra-
tio of the vinyl and 2-butene concentrations:
Figure 2 Influence of the thiol concentration on the
vinyl polymerization of SBS upon UV exposure. [Luci-
rin TPO] 5 1 wt %.
Figure 3 Thiol dependence of the amount of vinyl (F)
and butene (Å’) double bonds copolymerized after 0.3-s
UV exposure of SBS. [Lucirin TPO] 5 1 wt %.
HIGH-SPEED PHOTOCROSSLINKING OF SBS ELASTOMERS 1905
kv@V# 5 kb@B#
kv
kb
5
@B#
@V# 5 11.7
The higher reactivity of the vinyl double bond
toward thiols is in good agreement with previous
structure–reactivity studies on the addition of
thiols on olefins.12 It was attributed to an increase
in electron density of the olefin which reacts with
a relatively electrophilic thiyl radical.13
These data also show that the copolymeriza-
tion of vinyl groups with a trithiol is favored over
the homopolymerization (D [V]copo/D [V]homo 5 5.3
at [SH]0 5 0.075 mol kg
21), even though the vinyl
concentration is much higher than is the thiol
concentration ([vinyl]0/[SH]0 5 16.8 at 1 wt %
TRIS). This means that the propagating alkyl
radicals are much more reactive toward the thiol
group than toward the vinyl double bond:
Insolubilization
The crosslinks created in the elastomeric phase
by both copolymerization and homopolymeriza-
tion reactions led to insolubilization and solvent
resistance of the UV-irradiated SBS–thiol sys-
tem. Figure 4 shows some typical plots of the
insoluble fraction as a function of the exposure
time for different SBS formulations containing 1
wt % Lucirin TPO and increasing amounts of
TRIS (from 0.2 to 3 wt %). For TRIS concentra-
tions above 1 wt %, essentially all of the polymer
was found to become insoluble after a 0.3-s irra-
diation. At lower TRIS contents, the SBS polymer
remained partly soluble, the gel fraction increas-
ing slowly upon further exposure due to homopo-
lymerization of the vinyl groups, the small
amounts of thiol having already been consumed.
From the quantity of reactive groups consumed
after a 0.3-s exposure (2.2 3 1022 , N , 23
3 1022 mol kg21), one can calculate the concen-
tration of branch points in the elastomeric phase
and the number of crosslinks (X) formed per SBS
chain at the various thiol concentrations used:
Xtotal 5 ~@vinyl#homo 1 @vinyl#copo
1 @butene#copo!
MSBX
1000
the vinyl and butene concentrations being ex-
pressed in mol kg21. This number was found to
increase linearly with the thiol content up to a
value of 34 crosslinks per chain at [TRIS] 5 3 wt
%, as shown in Figure 5. Total insolubilization
requires the formation of about 15 crosslinks per
SBS chain. This is clearly apparent when the gel
fraction, measured after a 0.3-s exposure, is plot-
ted as a function of the number of crosslinks
formed per SBS chain (Fig. 6).
Figure 4 Influence of the thiol concentration on the insolubilization of SBS upon UV
exposure. [Lucirin TPO] 5 1 wt %.
1906 DECKER AND VIET
For the SBS sample that contains no thiol,
crosslinking results only from the homopolymer-
ization of the few pendent vinyl double bonds, as
2-butene double bonds are much less reactive.13
After 0.3 s, the vinyl consumption was measured
to be 7 3 1023 mol kg21, which corresponds to
only one crosslink per SBS molecule. This ex-
plains why the polymer remains soluble at that
stage (Fig. 4), insolubilization requiring the
buildup of a least three bridges per chain.
The amount of solvent retained by the swollen
polymer is directly related to the network
crosslink density or to the number-average mo-
lecular weight of the network chain.14 Because
the interaction parameter of the Flory swelling
equation was unknown for the SBS/toluene cou-
ple studied, it was only possible to make a quali-
tative evaluation of the effect of the UV dose and
the thiol content on the tightness of the polymer
network formed. Figure 7 shows how the swelling
ratio (SR 5 swollen solvent weight/dry polymer
weight) varies with the exposure time for the
various TRIS/SBS formulations. At very low TRIS
concentrations (0.2 wt %), the SR value decreases
from 35 to 15 with increasing exposure time,
which indicates that a very loose polymer net-
work has been formed. As the thiol concentration
was increased to 3 wt %, a tighter network was
formed, the swelling ratio decreasing to a mini-
mum value of 5. Figure 6 shows the swelling ratio
dependence on the number of crosslinks per SBS
chain.
Hardening
The crosslinking caused by the thiol-ene polymer-
ization during the first 0.3 s of UV exposure did
not affect significantly the elastomeric character
Figure 6 Dependence of the gel fraction and swelling
ratio on the number of crosslinks per SBS chain after a
0.3-s exposure. [Lucirin TPO] 5 1 wt %.
Figure 7 Influence of the thiol concentrationon the
swelling of a photocrosslinked SBS. [Lucirin TPO] 5 1
wt %.
Figure 5 Thiol dependence of the amount of vinyl
and butene crosslinks formed per chain after 0.3-s UV
exposure of SBS. [Lucirin TPO] 5 1 wt %.
HIGH-SPEED PHOTOCROSSLINKING OF SBS ELASTOMERS 1907
of the polymer, which remained soft and flexible
as long as the TRIS concentration did not exceed
1 wt %. At higher concentrations, the Persoz
hardness was found to increase from 40 to 75 s, as
shown in Figure 8. As the UV irradiation was
pursued, the SBS hardness increased further, es-
pecially for [TRIS] . 1 wt %. This unwanted
hardening may reduce the adhesive properties
(tackiness) of the UV-cured polymer and should
be strictly controlled.
The influence of the thiol on the pho-
tocrosslinking of the SBS elastomer is illustrated
in Figure 9, which shows the variation with the
TRIS concentration of the gel fraction, the swell-
ing ratio, and the Persoz hardness of a sample
UV-exposed for 0.3 s in the presence of Lucirin
TPO (1 wt %). A TRIS concentration of 1 wt % and
a UV dose of 0.3 s (which generates about 12
crosslinks per chain) appear as the best compro-
mise for achieving a fast curing and still ensuring
the high adhesion requested for pressure-sensi-
tive adhesives.
Influence of the Photoinitiator Concentration
All the UV-curing experiments reported so far
were performed with the same concentration of
the photoinitiator, namely, 1 wt % Lucirin TPO.
To increase the penetration of UV radiation for
curing of thick samples and to reduce the formu-
lation cost as well, the photoinitiator concentra-
tion was decreased to 0.1 wt %. This leads to a
slowing of the polymerization reaction, as ex-
pected from the decreased initiation rate. Figure
10 shows the decay profiles of the vinyl double
bond upon UV exposure of the SBS/TRIS (1 wt %)
system at various concentrations in Lucirin TPO.
Figure 9 Influence of TRIS concentration on the pho-
tocrosslinking of SBS. [Lucirin TPO] 5 1 wt %.
Figure 8 Influence of the thiol concentration on the
hardness of a photocrosslinked SBS. [Lucirin TPO] 5 1
wt %.
Figure 10 Influence of the photoinitiator concentra-
tion on the vinyl polymerization of SBS upon UV expo-
sure. [TRIS] 5 1 wt %.
1908 DECKER AND VIET
As expected, the vinyl conversion was found to be
reduced when the photoinitiator concentration
was decreased, but it was still sufficient to cause
insolubilization of SBS, as long as [Lucirin TPO]
$ 0.2 wt %. Figure 11 shows the corresponding
insolubilization profiles for TPO concentrations
between 0 and 1 wt %.
One may notice that at a TPO content of 0.2 wt
% the thiol-ene polymerization proceeds effi-
ciently enough to yield a nearly completely insol-
uble material within 0.3 s. Very similar polymer-
ization and insolubilization profiles were obtained
by operating under the following experimental
conditions: [TRIS] 5 1 wt % and [TPO] 5 0.2 wt
%, or [TRIS] 5 0.5 wt % and [TPO] 5 1 wt %. The
former formulation has the advantage of provid-
ing a more uniform deep-through cure, especially
for thick samples, while being also more cost ef-
fective.
It can be seen in Figures 10 and 11 that curing
occurs even in the absence of any added photoini-
tiator, although at a much slower pace. This is
probably due to the presence in SBS of some im-
purities which act as initating chromophores, but
also to the high reactivity of the thiyl radicals.
Indeed, no crosslinking could be detected upon
UV-irradiation of neat SBS under the selected
experimental conditions.
The chemical network formed in the elasto-
meric phase of the SBS rubber has a loose struc-
ture when low concentrations of the photoinitia-
tor were used. After a 0.3-s UV exposure, the
value of the SBS swelling ratio was found to in-
crease from 11 to 17 when [TPO] was decreased
from 1 to 0.1 wt %, as shown in Figure 12. The
polymer hardening upon UV exposure is less pro-
nounced at low photoinitiator concentrations, as
expected, but it still occurs, even in the photoini-
tiator-free sample (Fig. 13). At [TPO] 5 0.2 wt %,
the Persoz value increased from an initial value of
35 to 60 s after a 1-s exposure, compared to 100 s
at [TPO] 5 1 wt %. Operating at such low photo-
initiator concentrations, and at irradiation times
below 1 s, is therefore recommended for adhesive
applications, where the polymer should remain
soft and flexible after crosslinking.
Figure 13 Influence of the photoinitiator concentra-
tion on the hardness of a photocrosslinked SBS. [TRIS]
5 1 wt %.
Figure 11 Influence of the photoinitiator concentra-
tion on the insolubilization of SBS upon UV exposure.
[TRIS] 5 1 wt %. Figure 12 Influence of the photoinitiator concentra-
tion on the swelling of a photocrosslinked SBS. [TRIS]
5 1 wt %.
HIGH-SPEED PHOTOCROSSLINKING OF SBS ELASTOMERS 1909
Figure 14 summarizes these results by show-
ing the variation with the photoinitiator concen-
tration of the vinyl double-bond consumption, gel
fraction, swelling ratio, and Persoz hardness, in
an SBS/TRIS (1 wt %) sample UV-irradiated for
0.3 s. A Lucirin TPO concentration of 0.2 wt %
appears to be enough to obtain a crosslinked elas-
tomer showing the required performance with re-
spect to the photoreactivity, the viscoelastic prop-
erties, and the solvent and heat resistance. By
operating at such low photoinitiator concentra-
tions, a few millimeter-thick SBS samples were
readily crosslinked by UV-irradiation. This is due
to both an increased penetration of UV light and
to a fast photobleaching of Lucirin TPO which
promotes a frontal polymerization.15
Photocrosslinking of Kraton Thermoplastic
Elastomers
A similar study was performed on two commercial
thermoplastic styrene–butadiene elastomers,
Kratont D-1102 and Kratont D-1186 from Shell.
The styrene/butadiene ratio (30/70) and the poly-
butadiene vinyl content (;10%) were similar to
that of the SBS sample previously studied. They
differ mainly by the molecular weight of the po-
lybutadiene block and its structure which was
linear or branched, respectively.
Both samples proved to be very reactive when
exposed to UV radiation in the presence of TRIS
(1 wt %) and Lucirin TPO (1 wt %). Insolubiliza-
tion was achieved within less than 1 s, with for-
mation of a soft (Persoz hardness 5 60 s) and
loosely crosslinked (SR 5 10) polymer. In the
absence of thiol, the polymerization of the vinyl
double bond proceeds less efficiently, so that in-
solubilization hardly occurs upon UV exposure, as
shown in Figure 15 for Kratont D-1102.
In the presence of 1 wt % TRIS, the branched
SBS (Kratont D-1186) was found to become more
readily insoluble than was the linear SBS, so that
the photoinitiator concentration could be de-
creased to 0.2 wt % and still achieve insolubiliza-
tion within 0.3 s (Fig. 16). As expected, the
branched structure of the elastomeric phase
causes an increase of the crosslink density of the
UV-cured rubber, as shown by the swelling data:
an SR value of 8 for Kratont D-1186 and of 13 for
Kratont D-1102. This trend was also found when
neat SBS was UV-irradiated in the presence of a
photoinitiator. After a 3-s exposure, the gel frac-
tion was measured to be 70% for Kratont D-1186
(Fig. 17), compared to only 20% for Kratont
D-1102 (Fig. 15).
To assess the efficiency of the thiol/ene system
to crosslink SBS rubbers, in comparison to the
Figure 15 Photocrosslinking of Kraton® D-1102. In-
fluence of the thiol. [Lucirin TPO] 5 1 wt %.
Figure 14 Influence of the photoinitiator concentra-
tion on the photocrosslinking of SBS. [TRIS] 5 1 wt %.
UV exposure: 0.3 s.
1910 DECKER AND VIET
usual acrylate-based system, we UV-irradiated a
sample of Kratont D-1186 containing hexanediol
diacrylate (HDDA) as a crosslinking agent (20 wt
%), under exactly the same conditions. It can be
seen in Figure 17 that, in spite of the much higher
crosslinker content, insolubilization takes place
at a slower pace than for the sample containing 1
wt % TRIS. A gel fraction of 90% was reached
after 3 s with the diacrylate and after only 0.3 s
for the SBS/TRIS system. The crosslink density of
the UV-cured polymer is also much lower in the
SBS/acrylate system, as shown by the swelling
profiles (Fig. 18). After a 1-s exposure, the follow-
ing SR values were measured: 70 for neat SBS, 22
for SBS/HDDA, and 8 for SBS/TRIS. The fact that
the acrylate-based rubber is less crosslinked than
is the thiol-based rubber was attributed to the
competitive homopolymerization of the acrylate
double bonds, which does not generate any addi-
tional crosslinks between the SBS chains.
Another type of thermoplastic rubber (SIS)
consists of a styrenic triblock copolymer where
the elastomeric midphase is made of a polyiso-
prene, such as Kratont D-1107, which is com-
monly used in flexographic printing plates and
adhesives applications. Its styrene/rubber ratio is
14/86, the linear polyisoprene chains containing
no vinyl double bonds. SIS is therefore much less
reactive than is SBS and remains completely sol-
uble after a 5-s UV exposure in the presence of
Lucirin TPO (1 wt %). The addition of a diacrylate
monomer (20 wt % of HDDA) leads to partial
insolubilization (Fig. 19), the crosslinks resulting
from the copolymerization of the amylene and
acrylate double bonds.16,17 Crosslinking was
found to proceed much more efficiently in a SIS/
TRIS (2 wt %) system, UV-irradiated in the pres-
ence of 1 wt % Lucirin TPO. Total insolubilization
of SIS was achieved within 3 s (Fig. 19), by reac-
tion of the thiyl radicals with the amylene double
bond:
Figure 16 Photocrosslinking of Kraton® D-1186 in
the presence of (F) 0.2 wt % or (Å’) or 1 wt % Lucirin
TPO. [TRIS] 5 1 wt %.
Figure 17 Influence of thiol and acrylate monomers
on the photocrosslinking of Kraton® D-1186. [TRIS]
5 1 wt %; [HDDA] 5 20 wt %; [Lucirin TPO] 5 1 wt %.
Figure 18 Influence of thiol and acrylate monomers
on the swelling ratio of photocrosslinked Kraton®
D-1186. [TRIS] 5 1 wt %; [HDDA] 5 20 wt %; [Lucirin
TPO] 5 1 wt %.
HIGH-SPEED PHOTOCROSSLINKING OF SBS ELASTOMERS 1911
The best performance was obtained by combin-
ing the two crosslinking agents: 2 wt % of TRIS
and 20 wt % of HDDA. It should be emphasized
that this SIS-based formulation proved actually
to be more reactive than the SBS-based formula-
tion containing 20 wt % HDDA (Fig. 17), with
formation of a tighter polymer network (SR val-
ues of 12 and 16, respectively).
CONCLUSIONS
Thermoplastic styrene/butadiene elastomers con-
taining a low amount of the vinyl group (8%) can
be readily crosslinked at ambient temperature by
UV irradiation in the presence of very low
amounts (#1 wt %) of a trifunctional thiol
crosslinker. Both the vinyl and butene double
bonds of the polybutadiene chain participate in
the thiol/ene polymerization which proceeds by
step-growth addition, the thiyl radicals acting as
chain-transfer agents. The covalent bonds gener-
ated within the elastomeric phase led to a marked
increase of the solvent and heat resistance of the
UV-cured polymer. Insolubilization takes place
within 0.3 s upon intense illumination, with for-
mation of a soft and flexible material, well suited
for adhesive and flexographic applications.
The photocrosslinking technology offers a num-
ber of advantages, such as on-line processing, high-
speed curing, solvent-free formulations, low-energy
consumption, low cost of chemicals, and selective
cure in the illuminated areas. It is easily applicable
to commercial styrene/butadiene thermoplastic
elastomers, and even to the less reactive styrene/
isoprene rubbers, by adding a diacrylate comono-
mer. The main applications of these photocurable
rubbers are expected to be found in industrial sec-
tors where cure speed and spatial control are a
major concern, such as for the manufacturing of
hot-melt adhesives and sealants, safety glasses,
photoresists, and flexible printing plates.
The authors wish to thank Shell Research (Louvain la
Neuve, Belgium) for a research grant.
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Figure 19 Influence of thiol and acrylate monomers
on the photocrosslinking of Kraton® KX-601. [TRIS]
5 2 wt %; [HDDA] 5 20 wt %; [Lucirin TPO] 5 1 wt %.
1912 DECKER AND VIET
Photocrosslinking of functionalized rubbers IX. Thiol-ene polymerization
of styrene-butadiene-block-copolymers
C. Decker*, T. Nguyen Thi Viet
Laboratoire de Photochimie Générale (UMR N87525), Ecole Nationale Supérieure de Chimie de Mulhouse, Université de Haute Alsace,
3, rue Werner, 68200 Mulhouse, France
Received 16 June 1999; received in revised form 1 September 1999; accepted 10 September 1999
Abstract
A thermoplastic elastomer, polystyrene-block-polybutadiene-block-polystyrene (SBS) with a high vinyl content (59%), was crosslinked
within a fraction of a second by UV-irradiation in the presence of a trifunctional thiol and an acylphosphine oxide photoinitiator. The curing
process was followed by IR spectroscopy, insolubilization and hardness measurements. Both the thiol and the photoinitiator concentrations
were shown to affect the kinetics of the thiol–ene polymerization. Insolubilization was found to occur even at very low concentrations in thiol
(#1 wt%) and in photoinitiator (0.1 wt%). The heat resistance of the photosensitive formulation is high enough at 1508C to allow this system
to be used for hot-melt adhesive applications. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Photochemistry; Crosslinking; UV-curing
1. Introduction
Three-dimensional polymer networks can be readily
generated by photoinitiated polymerization of multi-
functional monomers or of polymers bearing reactive func-
tions [1]. This technology has been mainly used, so far, to
perform a quasi-instantaneous liquid to solid phase change,
a process called UV-curing. But it can also serve to
transform irreversibly a soluble polymer into an insoluble
material showing improved resistance to elevated tempera-
tures and chemicals. Among the various functionalized
polymers studied so far, promising results have been
obtained with rubber-type elastomers bearing epoxy rings
[2–4], acrylate functions [5,6] or vinyl [7,8] double bonds
on the main chain. Crosslinking was achieved within
seconds upon UV exposure in the presence of a cationic
type or radical-type photoinitiator, respectively. The main
advantages of using light to initiate the chain reaction,
besides the rapidity of the curing process, consist in the
solvent-free formulations, the low energy consumption,
ambient temperature operations and control of the location
and time of the process, which occurs specifically in the
illuminated areas.
In a recent paper [8], we have shown that styrene–
butadiene-block-copolymers (SBS) undergo a rapid cross-
linking when they are exposed to UV radiation in the
presence of a radical photoinitiator. Complete insolubiliza-
tion requires the reaction of 17 double bonds per polymer
chain for an SBS containing 8% pendent vinyl double
bonds. These thermoplastic elastomers, known under the
trademark KRATONw, have a two-phase morphology and
are commonly used as pressure sensitive adhesives, in
combination with a tackfying resin [9,10], and as flexo-
graphic printing plates [11]. UV-curing creates covalent
bonds within the elastomeric phase, thus reinforcing irrever-
sibly the already existing physical network. As a result, the
photocured SBS becomes more resistant to high tempera-
tures, the values of the Shear Adhesion Failure Temperature
(SAFT) rising from 808C to over 1608C [10].
The crosslinking process was markedly accelerated by
adding to SBS a diacrylate monomer which copolymerizes
with the polybutadiene vinyl and butene double bonds [8],
but at the expense of the heat resistance, thus making the
photosensitive material ill-suited for hot-melt adhesive
applications. Another approach, which proved to be even
more effective, consisted in incorporating a trifunctional
thiol into SBS to perform crosslinking through a thiol–ene
photopolymerization [12,13], according to the following
reaction scheme:
Photoinitiator 1 RSH!hn RSz
RSz 1 CH2yCH–R
0 ! RS–CH2– C
z
H–R 0
RS 2 CH2–C
z
H–R 0 1 RSH! RS–CH2–CH2–R 0 1 RSz
Polymer 41 (2000) 3905–3912
0032-3861/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0032-3861(99)00649-7
* Corresponding author.
The polymerization proceeds by a step growth addition
mechanism which is propagated by a chain transfer reaction
involving the thiyl radical (RS z) [14,15]. A thiol concentra-
tion of 5 wt% proved to be enough to increase 10-fold the
insolubilization rate [16]. The heat resistance of the uncured
formulation, although superior to the acrylate–SBS system,
was still not sufficient to meet the specifications required for
the manufacturing of hot-melt adhesives or flexographic
printing plates (stability of the unreacted sample during
20 min upon processing at 1508C) [7]. Moreover, one of
the major drawbacks of the polyene–thiol system lies in
the bad smell of mercaptans, which is reduced as polymer-
ization proceeds, the thiol becoming incorporated into the
polymer network.
The main objective of the present study was to determine
how much the thiol concentration can be reduced, without
affecting substantially the reactivity, in order to improve the
heat resistance of the photosensitive formulation, and
reduce its smell as well. For the same reason, the concentra-
tion of the photoinitiator was also decreased, with the
expected additional benefit of allowing thick samples to
be deep-through cured, because of the increased penetration
of UV radiation. The progress of the crosslinking reaction
has been followed quantitatively through the insolubiliza-
tion and hardening of the UV-exposed SBS-thiol sample,
and by monitoring the disappearance of the polybutadiene
double bonds by infrared spectroscopy.
2. Experimental
2.1. Materials
The polystyrene-block-polybutadiene-block-polystyrene
(SBS) from SHELL used in this study contained 59%
pendent vinyl groups, the 2-butenylene units (–CH2–
CHyCH–CH2) located on the polybutadiene backbone
representing 41% of the unsaturation content. Because of
its relatively low styrene content (,15%) the SBS behaves
similarly to a conventionally vulcanized rubber. A tri-
functional thiol was selected as crosslinking agent,
trimethylolpropane mercaptopropionate (TRIS) from
Evans Chemetics. It was added to a toluene solution of
SBS at concentrations between 0.2 and 5% by weight of
SBS.
TRIS CH3 CH2 C
CH2 O C CH2 H2 SH
O
CH2 O C CH2 CH2 SH
O
CH2 CH2 CH2 SHCO
O
C
Among the various radical-type photoinitiators tested,
acylphosphine oxides proved to be the most efficient ones
to photocrosslink SBS samples. All the experiments have
been carried out by using 2, 4, 6-trimethylbenzoyl-diphenyl-
phosphine oxide (Lucirin TPO from BASF) as photo-
initiator, at a typical concentration of 1 wt%.
C P
O
O
Lucirin TPO
2.2. Irradiation
In a typical UV-curing experiment, 20 mm thick films
were cast from a toluene solution containing the SBS
rubber, the thiol monomer and the photoinitiator onto either
a KBr crystal for infrared analysis or a glass plate for
insolubilization and hardness measurements. If a solvent-
free process is required for industrial applications, the
photoinitiator and the thiol can also be incorporated into
the liquid resin heated at 1508C, the usual processing
temperature. Samples were exposed at ambient temperature
in the presence of air to the radiation of a 80 W/cm medium
pressure mercury lamp (IST), at a passing speed of 60 m/
min which corresponds to an exposure duration of 0.1 s per
pass. The maximum light intensity at the sample position
was measured by radiometry (IL-390 light bug) to be
600 mW cm22 in the UV range.
2.3. Analysis
The kinetics of the light-induced crosslinking of the
rubber film was studied quantitatively by FTIR spectro-
scopy, by following the decrease upon UV exposure of
the absorption band characteristic of the vinyl double
bond at 1827 cm21. The degree of conversion (x) was calcu-
lated from the ratio of the corresponding IR absorbance
before and after UV exposure (A0 and At) by using the
following equation:
x…%† ˆ ‰1 2 …At=A0†Š × 100
This value was not corrected for shrinkage which was found
to be insignificant, based on the invariance of the C–H peak
at 2960 cm21. It should be mentioned that even very small
variations of the IR band intensity can be monitored accu-
rately, because the analysis is performed on the same sample
exposed to UV light for various durations, exactly on the
same spot.
The gel fraction and the degree of swelling of the
irradiated polymer were determined by soaking the sample
in toluene for one day at room temperature. The insoluble
polymer was recovered by filtration and dried at 708C to a
constant weight. The hardness of the coating was evaluated
before and after irradiation by monitoring the damping of
the oscillations of a pendulum (Persoz hardness). The
C. Decker, T. Nguyen Thi Viet / Polymer 41 (2000) 3905–39123906
hardness was shown to be strongly dependent on the glass
transition temperature [17], with Persoz values ranging typi-
cally from 30 s for soft elastomeric materials to 300 s for
hard and glassy polymers. The thermal stability of the
various formulations was tested by heating the uncured
sample for up to 1 h at 1508C, the processing temperature
for hot-melt adhesive applications.
3. Results
In a previous study on the photocrosslinking of SBS
rubber [16], we have shown that the thiol-ene polymeriza-
tion proceeds much more efficiently with the pendent vinyl
double bonds arising from 1-2 polymerization of butadiene
than with the backbone 2-butene double bonds of the poly-
butadiene chain (by a factor 16). UV-curing experiments
were therefore conducted on a specially designed SBS
sample where as many as 59% of the polybutadiene
unsaturations were vinyl double bonds.
The formation of the polymer network in the elastomeric
phase upon UV irradiation was followed through the vinyl
group disappearance and the polymer insolubilization.
3.1. Loss of the vinyl group upon photocrosslinking of SBS
The SBS samples containing 1 wt% Lucirin TPO and
various concentrations of TRIS (between 0.2 and 5 wt%)
were exposed to UV radiation for different times, up to
3 s. The crosslinking reaction was monitored through the
decrease of the vinyl IR band at 1826 cm21. Fig. 1 shows
the influence of the thiol concentration on the decay profile
of the vinyl group upon UV irradiation. The initial sharp
drop which occurs within the first 0.3 s was attributed to the
copolymerization with the thiol which is rapidly consumed.
At a TRIS concentration of 5 wt%, as much as 90% of the
thiol was found, by IR spectroscopy (band at 2566 cm21),
to have reacted after this short exposure, as shown in
Fig. 2.
As expected, the amount of vinyl groups polymerized
[V]t after a given exposure (0.3 s) increases with the initial
thiol content [SH]0 (Fig. 3). Vinyl groups are also under-
going a radical induced homopolymerization reaction which
can be quantified by substracting [SH]t from [V]t. Fig. 3
shows how the ratio copolymerization/homopolymeriza-
tion, calculated from the ratio [SH]t/([V]t 2 [SH]t),
increases with the thiol concentration. The fact that the
vinyl-thiol copolymerization is as important as the vinyl
homopolymerization, even though the SH concentration
(0.2 mol kg21) is much lower than that of the vinyl groups
(9.3 mol kg21), clearly shows that the propagating alkyl
radicals (P z) are much more reactive toward the thiol than
toward the vinyl double bond.
C. Decker, T. Nguyen Thi Viet / Polymer 41 (2000) 3905–3912 3907
Fig. 1. Influence of the thiol concentration on the vinyl group consumption
upon UV exposure of the SBS/TRIS system. [LucirinTPO]ˆ 1 wt%.
CH CH2
CH CH2
+ RS
RSH
CH CH2
CH2 CH2 S R
CH CH2
CH
CH CH2
kp
ks
CH CH2
CH CH2 S R
(P )
Copolymerization
Homopolymerization
CH2 CH CH2 S R
The 2-butene double bonds of the polybutadiene chain do
not contribute significantly to the crosslinking process as
they were shown to be 16 times less reactive toward the
thiyl radicals than the vinyl double bonds [16], and as
they are in lower concentration. If allowance is made for
them, the value of the copo/homo ratio in Fig. 3 would be
roughly 10% lower.
3.2. Insolubilization
Both reactions of the vinyl groups, homopolymerization
and copolymerization with the thiol, generate crosslinks
between the polybutadiene chains, with formation of a tri-
dimensional network which is schematically represented in
Fig. 4. As a result, the UV-exposed polymer becomes
insoluble in organic solvents, like chloroform or toluene.
Fig. 5 shows the insolubilization profiles obtained at various
concentrations of the thiol crosslinker, between 0.5 and
4 wt%. At a TRIS concentration of 1 wt%, a single pass
under the lamp at a speed of 60 m/min proved to be suffi-
cient to get a 90% insoluble material. This value did not
change much upon further exposure, even when the TRIS
content was increased to 4 wt%. This limiting value
suggests that the SBS sample contains about 5% non
polymeric materials.
In the absence of TRIS, a 30 times as long UV exposure
(3 s) is needed to reach the same gel fraction, the vinyl
consumption being evaluated to be only 2% of its original
value, i.e. 0.18 mol kg21. This result suggests that the poly-
merization of the pendent vinyl double bonds is not a very
effective crosslinking process in this SBS. The most prob-
able reason is that, because of the high vinyl content (59%
of the polybutadiene units), polymerization proceeds prefer-
entially between neighboring vinyl groups to form six-
membered rings along the polymer chain which remains
soluble. However, because the polybutadiene chain contains
41% butene-2 units, the cyclization process will propagate
along the polymer chain over only a few adjacent vinyl
C. Decker, T. Nguyen Thi Viet / Polymer 41 (2000) 3905–39123908
Fig. 3. Influence of the thiol concentration [SH]0 on the amount of vinyl and
thiol groups polymerized after a 0.3 s UV exposure of the SBS/TRIS
system.
R
S
S S
Fig. 4. Schematic representation of the photocrosslinked SBS/TRIS
polymer network formed in the elastomeric phase.
Fig. 5. Insolubilization profiles of UV-exposed SBS/TRIS films for differ-
ent thiol concentrations. [LucirinTPO]ˆ 1 wt%.
Fig. 2. Thiol conversion and vinyl loss in photocrosslinking of the SBS/
TRIS system. [Thiol]ˆ 5 wt%; [LucirinTOP]ˆ 1 wt%.
units. Larger loops can still be formed by reaction of more
distant vinyl double bonds located on the same polymer
chain. Such an intramolecular process was already found
to occur during the cationic polymerization of epoxidized
polyisoprene at high epoxy contents (.50%) [3,4]. Cross-
links will be formed and the polymer will become insoluble
only if vinyl groups located on two different polybutadiene
chains undergo polymerization. As a result, a very loose
polymer network is generated, as shown by the high value
of its swelling ratio: SRˆ weight of absorbed solvent/
weight of polymer ˆ 50 for UV-cured SBS [8].
R CH2 CH
CH2
CH
R
R
R
INTRA INTER
In the presence of the trifunctional thiol, a branch point
will be created each time a thiyl radical reacts with a vinyl
double bond, which leads to a tightening of the polymer
network. Indeed, the swelling ratio of the UV-exposed
polymer was found to drop down to a value of 8 upon
addition of as little a 1 wt% TRIS to SBS, as shown in
Fig. 6. The SR value was cut by half when the TRIS concen-
tration was further increased to 4 wt%. It should yet be
noticed that, because of the high vinyl content of the SBS
used, two of the three thiol groups of the TRIS molecule
may typically react with two neighboring vinyl double
bonds located on the same polymer chain (Fig. 4). Such
an intramolecular process will have no net effect on the
crosslink density, or on the polymer insolubilization.
When all the thiol crosslinker has reacted, typically after
0.3 s UV exposure, the concentration of branch points will
be equal to the initial concentration of the thiol groups, e.g.,
[SH]0ˆ 0.075 mol kg21 at [TRIS]ˆ 1 wt%. This corre-
sponds to 1 attachment formed for every 124 vinyl double
bonds, or to 10 attachments per SBS chain. Even if this
number is reduced to 7 to take into account the above
mentioned intramolecular process, it is still sufficient to
get total insolubilization of the rubber. After this short UV
exposure, crosslinking is mainly due to the thiol–ene
polymerization, the homopolymerization of vinyl groups
providing only a minor contribution, as shown by the low
value of the gel fraction (8%) found in the neat SBS sample
UV-exposed for 0.3 s.
3.3. Hardening
While crosslinking is proceeding upon UV exposure, the
amount of elastic chains which become bounded increases,
and the hardness of the polymer rises consequently, as
shown in Fig. 7. It is important to control precisely the
crosslink density in order to obtain cohesion, but without
hardening too much the polymer, which would be detri-
mental for its adhesion. At a low thiol concentration
(,1 wt%) and short exposure (#0.5 s), the UV-cured
polymer remains still soft (Persoz value of less than 80 s)
and keeps its elastomeric character. This is important if such
rubbers are to be used as adhesives or as impact resistant
materials to maintain the tack and wetting required to
achieve a good adhesion. Harder but still flexible elastomers
can be produced for flexographic printing plates or coatings
applications, simply by increasing the thiol concentration up
to 5 wt%, and/or the UV-exposure time up to 2 s.
The influence of the thiol concentration on the insolubi-
lization, swelling and hardening processes is illustrated in
Fig. 8 for a SBS sample UV-exposed for 0.3 s in the
presence of 1 wt% Lucirin TPO. It clearly appears that a
TRIS concentration of 0.5 wt% is already enough to obtain a
crosslinked elastomer showing the required characteristics
for adhesive applications.
The outer polystyrene segments of the block copolymer,
which are phase separated, are not expected to participate to
C. Decker, T. Nguyen Thi Viet / Polymer 41 (2000) 3905–3912 3909
Fig. 6. Influence of the thiol concentration and the exposure time on the
swelling of a photocrosslinked SBS/TRIS polymer. [LucirinTPO]ˆ 1
wt%.
Fig. 7. Variation of the polymer hardness upon UV-exposure, for various
thiol contents of the SBS/TRIS system. [LucirinTPO]ˆ 1 wt%.
the crosslinking process, except by a possible chain transfer
reaction if some propagating radicals succeed in abstracting
a labile hydrogen on the PS chain at the interphase.
Increasing the PS chain length should not affect insolubili-
zation, which is ensured through the polybutadiene cross-
links, but it will increase the degree of swelling by lowering
the crosslink density.
3.4. Influence of the photoinitiator concentration
The photoinitiator (PI) plays a keyrole by controlling both
the rate of initiation and the penetration of the incident light.
An increase of the PI concentration will accelerate the cross-
linking reaction, but it will also steepen the cure depth
gradient in the UV exposed sample and lead to insufficient
polymerization at the sample/substrate interface. This radia-
tion inner filter effect is a critical factor, specially for thick
samples, because it is often responsible of the poor adhesion
of UV-cured coatings or adhesives. It is therefore recom-
mended to work at low PI concentrations, but it will be at the
expense of the polymerization speed, and of the cure extent
as well. Only with highly reactive systems will it be possible
to achieve a fast and complete deep-through cure. In this
respect, the Lucirin TPO–TRIS–SBS system shows
outstanding performance because crosslinking appears to
proceed efficiently even at very low concentration in
photoinitiator.
Fig. 9 shows the insolubilization profiles obtained upon
UV exposure of a SBS–TRIS (1 wt%) sample containing
various amounts of Lucirin TPO, between 0.1 and 0.8 wt%.
It is quite remarkable that the crosslinking reaction still
proceeds very rapidly at these very low PI concentrations,
insolubilization being achieved in less than 1 s for a Lucirin
TPO content of 0.1 wt%. The polymer network formed is
almost as tight as that obtained by operating at higher photo-
initiator concentrations, as shown by the swelling curves of
Fig. 10. After 1 s exposure, SR values of 12 and 8 were
measured at PI concentrations of 0.1 and 0.8 wt%,
respectively.
An interesting feature was observed when the SBS-TRIS
system was exposed to UV radiation without any added
photoinitiator. After 1 s, as much as 85% of the polymer
has become insoluble in toluene with formation of a rela-
tively tight network (SRˆ 15), as shown in Figs. 9 and 10.
This result suggests that the SBS sample contains some
C. Decker, T. Nguyen Thi Viet / Polymer 41 (2000) 3905–39123910
Fig. 8. Influence of the thiol concentration on the photocrosslinking of the
SBS/TRIS system. UV exposure: 0.3 s; [LucirinTPO]ˆ 1 wt%.
Fig. 9. Influence of the photoinitiator concentration on the insolubilization
of the SBS/TRIS system upon UV exposure. [TRIS]ˆ 1 wt%.
Fig. 10. Influence of the photoinitiator concentration and the exposure time
on the swelling of a photocrosslinked SBS/TRIS polymer. [TRIS]ˆ 1 wt%.
Fig. 11. Variation of the polymer hardness upon UV exposure, for various
photoinitiator concentrations in the SBS/TRIS system. [TRIS]ˆ 1 wt%.
chromophores which absorb the UV radiation of
l . 250 nm. The free radicals generated upon photolysis
would then produce the propagating thiyl radicals by
abstracting an hydrogen atom from the thiol molecule.
As expected, the hardening of the polymer upon UV
exposure becomes less pronounced as the photoinitiator
concentration is decreased (Fig. 11). After a 1 s irradiation,
the Persoz hardness value was found to increase from
40 to 55 s at [TPO]ˆ 0.1 wt%, and up to 85 s for
[TPO]ˆ 0.8 wt%. The UV-cured elastomer remains soft
and flexible, as required for adhesive applications.
The influence of the photoinitiator concentration on the
properties of the polymer material obtained after a 0.3 s UV
exposure of the SBS–TRIS (1 wt%) system is shown in Fig.
12. It clearly appears that a TPO concentration of 0.1 wt% is
already sufficient to get the expected performance. This
formulation presents a number of advantages:
• high reactivity upon UV-exposure;
• no residual thiol and photoinitiator in the UV-cured
rubber;
• low cost of the tiny amounts of additives used;
• low-modulus polymer well suited for adhesives
applications;
• deep through cure because of the low PI concentration;
• curing of thick samples.
For example, a 2 mm thick sample of SBS was rendered
insoluble within less than 1 s, with formation of a relatively
tight polymer network. Fig. 13 shows the insolubilization
and the swelling profiles obtained with such a sample upon
UV exposure in the presence of air, at ambient temperature.
The % gel increase is slower than in thin films, as might be
expected from the inner filter effect which reduces the
amount of radiation received by the deep-lying layers.
More surprising is the fact that the swelling ratio of the
thick UV-cured sample was found to be lower than that of
the thin film (7 and 11, respectively), thus indicating that a
tighter polymer network has been performed. This could be
due to a less pronounced inhibitory effect of atmospheric
oxygen in the thick sample. An additional reason could be
the larger rise in temperature for the thick sample caused by
the exothermal polymerization, as already shown in UV-
curable resins [17]. The resulting increase in molecular
mobility would favour intermolecular reactions and lead
to a higher crosslink density.
3.5. Thermal stability
For some applications, like hot-melt adhesives and flexo-
graphic printing plates, the highly reactive SBS/thiol system
is to be processed at elevated temperatures (usually 1508C).
It is therefore important to assess the heat resistance of the
formulation before UV-curing. In a previous study on a SBS
formulation containing 5 wt% TRIS and 1 wt% Lucirin
TPO, we found that about one-third of the sample has
become insoluble in toluene after a 10 min heating at
1508C [16]. This poor thermal stability makes this system
unfit for hot-melt adhesive applications.
We have repeated this study with a formulation contain-
ing only 0.5 wt% TRIS and 1 wt% Lucirin TPO. Under
C. Decker, T. Nguyen Thi Viet / Polymer 41 (2000) 3905–3912 3911
Fig. 12. Influence of the photoinitiator concentration on the photocrosslink-
ing of the SBS/TRIS system. UV exposure: 0.3 s; [TRIS]ˆ 1 wt%.
Fig. 13. Photocrosslinking of a 2 mm thick SBS/TRIS sample.
[LucirinTPO]ˆ 0.1 wt%; [TRIS]ˆ 1 wt%.
Fig. 14. Insolubilization of a SBS/TRIS upon heating at 1508C.
[LucirinTPO]ˆ 1 wt%; [TRIS]ˆ 0.5 wt%. –– –: SBS without thiol.
those conditions, the vinyl content remained unchanged and
no gelation was found to occur during the first 20 min of
heating, very much like for neat SBS (Fig. 14). This formu-
lation appears thus to meet the required specifications for
heat resistance. The formation of insoluble polymer, which
was observed upon further heating, could probably be
delayed by the addition of an antioxidant, which should
not interfere with the UV-curing process. Interestingly, the
thermooxidation of SBS, which can be quantitatively
followed by monitoring the formation of carbonyl and
hydroxyl groups by IR spectroscopy, was shown to be
depressed in the presence of the TPO and TRIS additives.
These oxidation products are produced in significant
amounts only after a 30 min heating at 1508C, as shown in
Fig. 15.
4. Conclusion
The photoinduced thiol–ene polymerization has been
successfully applied to crosslink at ambient temperature a
styrene–butadiene rubber with a high vinyl content (59%).
A very low amount of thiol crosslinker (#1 wt%) proved to
be already enough to achieve total insolubilization within
less than 1 s upon intense illumination in the presence of a
acylphosphine oxide photoinitiator. Crosslinking occurs
predominantly by reaction of the thiyl radicals with the
pendent vinyl double bonds and, to some extent, by homo-
polymerization of the vinyl groups.
Hardening is hardly taking place upon UV-curing, so
that the thermoplastic elastomer retains its viscoelastic
properties, while becoming more resistant to heat and
chemicals. The photoinitiator concentration can be lowered
down to 0.1 wt%, thus allowing relatively thick samples to
be rapidly crosslinked because of the increased penetration
of UV radiation. This UV-curable formulation exhibits a
sufficiently high resistance to heating to allow its use in
hot-melt adhesive and flexographic printing plate applica-
tions. Further studies on the photocrosslinking of the thiol/
SBS combination are in progress, aiming at lowering the
SBS vinyl content down to 8%, to be able to cure rapidly
commercial styrene–butadiene rubbers.
Acknowledgements
The authors wish to thank SHELL RESEARCH
(Louvain-la-Neuve-Belgium) for a research grant.
References
[1] Decker C. Progr Polym Sci 1996;21:593.
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[5] Le Xuan H, Decker C. J Polym Sci, Polym Chem Ed 1993;31:769.
[6] Decker C, Nguyen Thi Viet T, Le Xuan H. Europ Polym J
1996;32:549–59.
[7] Huber HF. In: Fouassier JP, Rabek JF, editors. Radiation curing in
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[8] Decker C, Nguyen Thi Viet T. Macromol Chem Phys 1999;200:358.
[9] Dupont M, De Keyser N. WO Patent 9502, 640, 1995.
[10] Dupont M, De Keyser N. Proceedings of RadTech Europe
Conference, Maestricht, 1995;174.
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[14] Morgan CR, Magnolta F, Ketley AD. J Polym Sci, Polym Chem Ed
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[15] Jacobine AF. In: Fouassier JP, Rabek JF, editors. Radiation curing in
polymer science and technology, 3. London: Elsevier Applied
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[16] Decker C, Nguyen Thi Viet T. Macromol Chem Phys 1999;200:1965.
[17] Decker C, Decker D, Morel F. In: Scranton AB, Bowman CN, Peiffer
RW, editors. ACS Symp.Series, 673. Washington, DC: ACS, 1997.
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C. Decker, T. Nguyen Thi Viet / Polymer 41 (2000) 3905–39123912
Fig. 15. Thermal oxidation of the SBS/TRIS system upon heating at 1508C.
[LucirinTPO]ˆ 1 wt%; [TRIS]ˆ 0.5 wt%. – ––: SBS without thiol.
Macromol. Chem. Phys. 200, 1965–1974 (1999) 1965
Photocrosslinking of functionalized rubbers, 8a)
The thiol-polybutadiene system
Christian Decker*, Trieú Nguyen Thi Viet
Département de Photochimie (UMR – CNRS N87525), Ecole Nationale Supérieure de Chimie de Mulhouse
– Université de Haute Alsace 3, rue A. Werner – 68200 Mulhouse, France
(Received: October 6, 1998; revised: January 13, 1999)
SUMMARY: The light-induced crosslinking of polystyrene-block-polybutadiene-block-polystyrene (SBS)
was carried out in the presence of a trifunctional thiol and a phosphine oxide photoinitiator. The curing pro-
cess was followed by infrared spectroscopy, insolubilization and hardness measurements, and was shown to
proceed extensively within less than 1 s under intense illumination in air at ambient temperature. The relative
reactivity of the thiyl radicals toward the polybutadiene vinyl and butene double bonds was determined, as
well as the competition between homo- and copolymerization for the propagating alkyl radical. An increase
of the SBS vinyl content from 8 to 59% has little effect on the crosslinking process, because it enhances
mainly intramolecular reactions. The thiol/ene polymerization is much more effective to crosslink thermopla-
stic SBS elastomers than the copolymerization of the polybutadiene double bonds with a diacrylate mono-
mer.
Introduction
The crosslinking of vinyl-functionalized polymers can be
readily achieved at ambient temperature by UV-irradia-
tion in the presence of a free radical-type photoinitia-
tor1–4). Most of these studies were performed on styrene-
butadiene-styrene (SBS) block copolymers to make such
thermoplastic elastomers more resistant to temperature
and organic solvents by creating covalent bonds between
the polybutadiene chains. In a previous paper of this ser-
ies5), we have shown that a styrene-butadiene rubber hav-
ing only 8% of its unsaturations as pendent vinyl double
bonds can be made totally insoluble within a few seconds
upon UV exposure in the presence of air. The addition of
a diacrylate monomer, which copolymerizes with the
vinyl and butene double bonds, was found to accelerate
markedly the crosslinking process, but at the expense of
the heat resistance of the plasticized rubber. This highly
reactive photosensitive material is therefore ill-suited for
hot-melt adhesive applications or flexographic printing
plates requiring processing at 1508C.
In the present study, another approach has been used to
achieve a rapid photocrosslinking of SBS rubber. It is
based on the photoinduced addition of a thiol (RSH) onto
an olefinic double bond, and it requires incorporation of a
multifunctional thiol into the polyene elastomer. A great
number of kinetic and mechanistic studies have been
devoted to thiol-ene photopolymerization, a topic which
has been recently reviewed by Jacobine in a comprehen-
sive monography6). The chain reaction proceeds by a step
growth addition mechanism which is propagated by a
chain transfer reaction involving thiyl radicals (RS9)7, 8).
The overall process can be formally represented as fol-
lows:
Crosslinking requires the association of a diene or
polyene and of a multifunctional thiol. Because peroxy
radicals formed by the O2 scavenging of alkyl radicals are
also capable of abstracting hydrogen atoms from the
thiol, UV-curable thiol-ene systems are much less sensi-
tive to air inhibition9) than conventional radical-induced
polymerization10):
Macromol. Chem. Phys. 200, No. 8 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 1022-1352/99/0808–1965$17.50+.50/0
a) Part 7: cf. ref.5)
1966 C. Decker, T. Nguyen Thi Viet
There are only a few reports in the literature on the
photocrosslinking of thiol-SBS system11, 12), with little
information on the reaction kinetics. Like in our previous
study5), the main objective of this work was to determine
how fast and how extensively styrene-butadiene rubbers
could be crosslinked by UV-irradiation at ambient tem-
perature in the presence of a thiol monomer. Special
attention has been given to the influence on the cure
kinetics of the three key components: the photoinitiator,
the thiol monomer and the SBS vinyl double bonds. The
progress of the crosslinking reaction has been followed
through the insolubilization and hardening of the UV-irra-
diated sample, and by monitoring the thiol and vinyl
groups disappearance by IR spectroscopy.
Experimental part
Materials
Two types of polystyrene-block-polybutadiene-block-poly-
styrene (SBS), both from SHELL, have been used in this
study. They differ only by their content of pendent vinyl
groups: 8% of the total double bond content for the low-
vinyl polymer (LV-SBS), and 59% for the high-vinyl poly-
mer (HV-SBS). The 2-butenylene units (1CH21CH2
CH1CH21) located on the polybutadiene backbone repre-
sent 92% and 41% of the unsaturation content, respectively.
In some experiments, a paraffinic oil (Nujol from Aldrich)
was added to the formulation to increase the molecular
mobility in the polymer film. A trifunctional thiol was
selected as crosslinking agent, trimethylolpropane tris-
(3-mercaptopropionate) (TRIS) from Evans Chemetics. It
was added to a toluene solution of SBS at a concentration
between 3 and 20 wt.-% of SBS.
Different types of photoinitiators were introduced in the
polymer, at a typical concentration of 2 wt.-% SBS, namely
benzophenone (BZP from Aldrich), isopropylthioxanthone
(Quantacure ITX from BioSynthetics), 4,49-diphenoxybenzo-
phenone (DPB from SNPE) and 2,4,6-trimethylbenzoyl(di-
phenyl)phosphine oxide (Lucirin TPO from BASF).
Scheme 1 shows the chemical formulas of the various com-
pounds used in this study.
In a typical UV-curing experiment, 20 lm thick films were
cast from a toluene solution containing the SBS rubber, the
thiol monomer and the photoinitiator onto either a KBr crys-
tal for infrared analysis or a glass plate for insolubilization
and hardness measurements. Samples were exposed to the
radiation of a 80 W/cm medium pressure mercury lamp, in
the presence of air, at a passing speed of 60 m/min, which
corresponds to an exposure duration of 0.1 s per pass. The
maximum light intensity at the sample position was meas-
ured by radiometry (IL-390 light bug) to be 600 mW N cm–2
in the UV range.
Analysis
The kinetics of the light-induced crosslinking of the rubber
film was studied quantitatively by FTIR spectroscopy, by
following the decrease upon UV exposure of the absorption
bands characteristic of the vinyl double bond at 910.8,
1637.8 and 1827 cm–1, and of the but-2-ene double bond at
964.7 cm–1. The disappearance of the thiol monomer upon
UV-exposure was followed through the IR absorbance of the
SH group at 2569 cm–1. The degree of conversion (x) was
calculated from the ratio of the corresponding IR absorbance
before and after UVexposure (A0 and At) by using the follow-
ing equation:
x (%) = [1 – (At /A0)] N 100
This value was not corrected for shrinkage, as it was found
to account for less than 2%, based on the variation of the
C1H peak at 2960 cm–1. The loss of the photoinitiator in the
UV-irradiated sample was followed quantitatively by UV-
spectroscopy.
The gel fraction and the degree of swelling of the irra-
diated polymer were determined by soaking the sample in
toluene for one day at room temperature. The insoluble poly-
mer was recovered by filtration and dried at 708C to a con-
stant weight. The hardness of the coating was evaluated
before and after irradiation by monitoring the damping of the
oscillations of a pendulum (Persoz hardness). The hardness
was shown to be strongly dependent on the glass transition
temperature13), with Persoz values ranging typically from
30 s for soft elastomeric materials to 300 s for hard and
glassy polymers. The thermal stability of the various formu-
Scheme 1: Chemical formulas of compounds used
Photocrosslinking of functionalized rubbers, 8 1967
lations was tested by heating the uncured sample for up to
1 h at 1508C, the processing temperature for hot-melt adhe-
sive applications.
Results
The thiol-ene polymerization is known to proceed more
efficiently with olefins in which the double bond is
located at the end of the monomer molecule (R1CH
2CH2) rather than in its backbone (R1CH2CH1R)6).
Consequently, a faster crosslinking was expected to occur
with SBS rubbers containing a high content of pendent
vinyl double bonds arising from the 1–2 polymerization
of butadiene. Our first UV-curing experiments were
therefore conducted with a specially designed SBS sam-
ple where as many as 59% of the polybutadiene unsatura-
tions were vinyl double bonds (HV-SBS).
Influence of the photoinitiator on the photocrosslinking
of the thiol-SBS system
The photoinitiator (PI) used in UV-curing applications
can be classified into two major categories, depending on
the way the free radicals are generated:
– by photocleavage of the PI excited states
– through hydrogen abstraction by the PI excited states
As thiols exhibit a strong hydrogen donor character, the
second type of photoinitiator is often used in the thiol-ene
photopolymerization. The benzoyl radical produced by
the photocleavage reaction is also capable to abstract a
hydrogen atom from the thiol to generate the propagating
thiyl radical6):
Among the different types of photoinitiators examined,
the following four were found to give the best perfor-
mance: benzophenone (BZP), isopropylthioxanthone
(ITX), diphenoxybenzophenone (DPB) and 2,4,6-tri-
methylbenzoyl(diphenyl)phosphine oxide (Lucirin, TPO).
Fig. 1 shows how the degree of conversion of the thiol
monomer varies with the exposure time for a HV-SBS
rubber containing 2 wt.-% photoinitiator and 20 wt.-%
TRIS, which corresponds to a double bond/SH molar
ratio of 8.3. The phosphine oxide proved to be the most
efficient initiator and was therefore used in our further
studies. One may still notice that TPO is 10 times more
efficient than benzophenone in the absence of thiol
(Fig. 1 of ref.5)), but only 1.6 times in the presence of
thiol (Tab. 1).
It is quite remarkable that the polymerization of the
thiol proceeds so rapidly in the solid state, reaching 50%
conversion within 0.5 s exposure. The slowing down
observed upon further irradiation was attributed to two
factors: (i) a fast consumption of the photoinitiator and of
the thiol and (ii) molecular mobility restrictions resulting
from the build up of the tridimensional polymer network.
A similar ranking of the photoinitiators was found by
following the disappearance of the vinyl double bond of
SBS (Fig. 1). Here again, the reaction slows down after
0.5 s and the vinyl content is leveling off at around 60%
of its original value after 3 s. At that final stage of the
polymerization, the absolute amount of vinyl groups
which have disappeared is about twice that of the SH
groups consumed. This result suggests that a significant
amount of vinyl double bonds undergoes homopolymeri-
zation, besides their copolymerization with the trifunc-
tional thiol. Both processes will lead to crosslinking and
therefore insolubilization of the polymers, if the vinyl
double bonds are located on different SBS chains. Fig. 2
shows the insolubilization profiles of the HV-SBS/TRIS
system exposed to UV-light in the presence of 2 wt.-%
photoinitiator. With the most efficient initiator (Lucirin
TPO), 80% insolubilization was achieved within 0.1 s. At
that stage of the reaction, 5% of the vinyl double bonds
have polymerized, i. e., 70 groups per SBS chain. As inso-
lubilization requires the build up of only a few crosslinks
(at least 3 per chain), we are left to conclude, like in our
previous study on the UV-curing of the same SBS sam-
Fig. 1. Influence of the photoinitiator (2 wt.-%) on the photo-
crosslinking of the HV-SBS/Tris system. [Tris] = 20 wt.-%.
Light intensity: 600 mW N cm–2
1968 C. Decker, T. Nguyen Thi Viet
ple5), that the polymerization involves mainly neighbor-
ing vinyl groups located on the same polybutadiene
chain. Such an intramolecular process seems feasible in
consideration of the large number of pendent vinyl groups
in HV-SBS (6 for every 10 monomer units). Lucirin TPO
proved to be also the best photoinitiator for reaching
rapidly a high crosslink density, as shown by the swelling
profiles of Fig. 2. A swelling ratio of 5 was measured for
the sample UV-irradiated during 0.1 s in the presence of
this phosphine oxide, compared to values ranging from 7
to 9 with the other photoinitiators. These values are less
than half those measured in UV-cured SBS5), thus show-
ing that a tighter polymer network has been formed upon
UV-irradiation of the thiol-SBS system. A performance
analysis of the photoinitiators studied in UV-curing of the
TRIS/HV-SBS combination (20/80 weight ratio) is
reported in Tab. 1. The two sets of results (rate of poly-
merization and insolubilization data) correlate perfectly
well for the 4 compounds.
Influence of the SBS vinyl content
The photoinduced thiol-ene polymerization of two SBS
samples having very different vinyl group contents (59
and 8%) has been studied in order to evaluate the relative
reactivity of vinyl and butene double bonds, and to quan-
tify the competition between the homopolymerization of
vinyl double bonds and the transfer copolymerization
with the trifunctional thiol. The overall crosslinking pro-
cess can be represented by the following reaction scheme.
Thiyl radicals are also produced by hydrogen abstrac-
tion on the thiol by any of the secondary free radicals
formed by the addition process, as shown in the introduc-
tion reaction scheme. While vinyl groups undergo both
homopolymerization and copolymerization with the thiol,
the butene double bonds are only reacting with the thiyl
radicals. Indeed, homopolymerization of the butene group
was not observed upon UV exposure of SBS in the
absence of thiol5).
Crosslinking results from both the copolymerization of
the butadiene double bonds with the trifunctional thiol
and the homopolymerization of the pendent vinyl groups.
The structure of the polymer network obtained is repre-
sented schematically in Fig. 3.
Photocrosslinking of HV-SBS rubber
The SBS sample containing 59% pendent vinyl double
bonds has been exposed to UV-radiation in the presence
of the trifunctional thiol ([TRIS] = 20 wt.-%) and Lucirin
TPO (2 wt.-%). Infrared spectra were recorded after var-
Tab. 1. Influence of the photoinitiator on the photocrosslinking kinetics of the thiol/HV-SBS system. [Tris] = 20 wt.-%. Light inten-
sity: 600 mW N cm–2
Photoinitiator (2 wt.-%) Rs
mol Nkgÿ1 Nsÿ1
a† Rv
mol Nkgÿ1 Nsÿ1
b† Gel in %c) Swelling ratioc)
Benzophenone (BZP) 1.5 2.8 64 9
Isopropylthioxanthone (ITX) 1.8 3.0 69 8
Diphenoxybenzophenone (DPB) 2.0 3.3 78 7
Lucirin (TPO) 2.4 3.6 83 5
a) Initial rate of thiol consumtion.
b) Initial rate of vinyl consumption.
c) After 0.1 s UV exposure.
Fig. 2. Influence of the photoinitiator (2 wt.-%) on the photo-
crosslinking of the HV-SBS/Tris system. [Tris] = 20 wt.-%.
Light intensity: 600 mW N cm–2
Photocrosslinking of functionalized rubbers, 8 1969
ious exposure times (up to 2 s) to monitor the disappear-
ance of the three functional groups: thiol, vinyl and
butene. The butene double bond was found to be much
less reactive than the vinyl double bond in the ene-thiol
polymerization. Its initial rate of copolymerization with
the thiol Rb was measured to be 0.1 mol N kg–1 N s–1, com-
pared to 2.3 mol N kg–1 N s–1 for the vinyl copolymerization
rate (Rv)copo (Tab. 2).
The relative reactivity of the two types of double bonds
toward the trifunctional thiol can be determined from the
ratio of the rate constants kv and kb of the addition reac-
tion of the thiyl radical to the vinyl and butene double
bonds, respectively.
(Rv)copo ˆ kv [RS9] [vinyl] (1)
Rb ˆ kb [RS9] [butene] (2)
kv
kb
ˆ
…Rv†copo
Rb
N
‰buteneŠ
‰vinylŠ
ˆ 16 …3†
The higher reactivity of the vinyl double bond toward
thiols is in good agreement with previous structure-reac-
tivity studies of the addition of thiols on olefins14). For
instance, a-methylstyrene was found to be 25 times as
reactive toward thiol glycolate as a,b-dimethylstyrene15).
The enhanced reactivity of the vinyl group can be attribu-
ted to an increase in electron density of the olefin which
is reacting with a relatively electrophilic thiyl radical16).
An additional factor in HV-SBS is the poor accessibility
of the backbone butene double bonds by thiyl radicals.
From the data shown in Fig. 1, one can evaluate the
competition which exists for the vinyl radical between
homopolymerization and copolymerization with the thiol.
The rate equation for the thiol consumption can be
written as:
Rs = ks [P9] [RSH] (4)
where P9 are the alkyl radicals produced by the addition
reaction of the thiyl radical to the double bond (consump-
tion of the thiol by reaction with the photoinitiator was
neglected). Vinyl groups are consumed by reaction with
both P9 radicals and thiyl radicals, which leads to the rate
equation:
Rv = kp [P9] [vinyl] + kv [RS9] [vinyl] (5)
As the thiyl free radical is assumed to react only with
the SBS double bonds to generate a copolymer unit, one
can write:
ks [P9] [RSH] = [RS9] (kv [vinyl] + kb [butene]) (6)
By neglecting kb [butene] which is small in regard of kv
[vinyl] (a 1/23 ratio), the rate equation of the vinyl con-
sumption becomes:
Rv = kp [P9] [vinyl] + ks [P9] [RSH] (7)
The competition between homopolymerization and
copolymerization can then be quantified from the ratio
Rv/Rs:
Rv
Rs
ˆ
kp‰vinylŠ ‡ ks‰RSHŠ
ks‰RSHŠ
…8†
Fig. 3. Chemical structure of the photocured SBS/Tris polymer
network
Tab. 2. Performance analysis of the photocrosslinking of styr-
ene-butadiene rubbers LV-SBS and HV-SBS in the presence of a
trifunctional thiol (20 wt.-%). Photoinitiator: [Lucirin TPO] = 2
wt.-%
LV-SBS HV-SBS
Vinyl content in % 8 59
Ene/thiol molar ratio 8.4 8.4
Initial loss rate in mol N kg–1 N s–1:
Thiol Rs 1.8 2.4
Vinyl Rv 0.83 3.6
(Rv)homo 0.06 1.3
(Rv)copo 0.77 2.3
Butene Rb 0.9 0.1
Relative reactivity:
Vinyl group
copo/homopolymerization: ks/kp – 10
Copolymerization
vinyl/butene: kv/kb 10 16
Insolubilization rate in % N s–1 80 90
Swelling degree (after 0.1 s) 6 5
1970 C. Decker, T. Nguyen Thi Viet
which leads to
kp
ks
ˆ
Rv
Rs
ÿ 1
 
‰RSHŠ
‰vinylŠ
 
…9†
In the photocrosslinking of HV-SBS, the following
values were obtained: Rv = 3.6 mol N kg–1 N s–1, Rs = 2.4
mol N kg–1 N s–1 and [RSH]/[vinyl] = 0.2. The ratio ks/kp
was calculated from Eq. (9) to be as high as 10, which
means that the propagating alkyl radicals P9 are 10 times
as reactive toward the thiol group as they are toward the
vinyl double bond, in agreement with previous observa-
tions6). It should be mentioned that the value of the vinyl
homopolymerization rate in the thiol-SBS system, calcu-
lated from the equation Rhomo = Rv –Rs, is quite similar to
the rate of polymerization of vinyl groups in neat SBS
photocrosslinked under the same conditions5), 1.2 and 1.0
mol N kg–1 N s–1, respectively.
The network formation is not only making the rubber
insoluble in organic solvents, but it also increases the Shear
Adhesion Failure Temperature (SAFT) from 80 to 1608C,
and the hardness of the UV-exposed polymer as well, as
shown in Fig. 4. Because hardening occurs mainly upon
prolonged UV irradiation, for up to 2 s, it is still possible to
obtain a soft and insoluble polymer by shortening the expo-
sure time down to 0.3 s. Another way to get a crosslinked
polymer showing a strong elastomeric character for adhe-
sive or flexographic applications is by introducing a plasti-
cizer in the thiol-SBS formulations, such as Nujol, a paraffi-
nic oil. At a weight concentration of 35%, insolubilization
of SBS was found to proceed as fast as for the unplasticized
sample, but the crosslinked polymer remained soft upon
UVexposure for up to 2 s, as shown in Fig. 4.
Photocrosslinking of LV-SBS rubber
The same study was carried out with a SBS sample con-
taining only 8% vinyl group (LV-SBS), a value which is
more typical of commercial styrene-butadiene thermo-
plastic elastomers. In the presence of 20 wt.-% TRIS, the
insolubilization, swelling and hardening profiles (Fig. 5)
were found to be very similar to those obtained with the
HV-SBS sample. The same behavior was previously
observed in the photocrosslinking of neat SBS copoly-
mers5). This surprising result suggests that, in HV-SBS, a
substantial fraction of the vinyl groups disappear through
intramolecular processes involving neighboring pendent
double bonds, without any net effect on the insolubiliza-
tion and hardening. By contrast, in LV-SBS such intramo-
lecular reactions are less likely to occur, given the aver-
age distance separating the pendent vinyl double bonds
(12 monomer units). Crosslinking will therefore proceed
readily through intermolecular reactions between func-
tional groups located on different polymer chains.
The initial rate of thiol consumption in LV-SBS (Rs =
1.8 mol N kg–1 N s–1) was found to be superior to the initial
rate of vinyl consumption (Rv = 0.83 mol N kg–1 N s–1),
most probably because the thiyl radicals react with the
backbone butene double bonds, which are present in lar-
ger amounts and are more accessible than in HV-SBS.
From these data and by using the ks /kp ratio determined in
the previous section, one can evaluate the relative reactiv-
ity of the thiyl radicals toward the vinyl and butene dou-
ble bonds of polybutadiene in LV-SBS.
Vinyl groups are disappearing by both homopolymeri-
zation and copolymerization with the thiol, so that one
can write:
Rv = (Rv)homo + (Rv)copo (10)
The contribution of each process can be evaluated by
the equation
…Rv†copo
…Rv†homo
ˆ
ks‰RSHŠ
kp‰vinylŠ
…11†
Fig. 4. Hardening of the HV-SBS/Tris system upon UV expo-
sure. [Tris] = 20 wt.-%; [Lucirin TPO] = 2 wt.-%
Fig. 5. Photocrosslinking of the LV-SBS/Tris system. [Tris] =
20 wt.-%; [Lucirin TPO] = 2 wt.-%
Photocrosslinking of functionalized rubbers, 8 1971
From the value of the ratio ks /kp = 10, evaluated pre-
viously, and the thiol/vinyl molar ratio (1.5), one can
determine the rate of copolymerization of the vinyl
group:
(Rv)copo = 0.933 Rv
= 0.77 mol N kg–1 N s–1
As the thiol is consumed by reaction with both the
vinyl and butene double bonds, one can write:
Rs = (Rv)copo + (Rb)copo (12)
Butene double bonds do not undergo significant homo-
polymerization upon UV exposure of LV-SBS in the pre-
sence of Lucirin TPO, so that (Rb)copo X Rb. In the pre-
sence of 20 wt.-% TRIS, they disappear at an initial rate
of 0.9 mol N kg–1 N s–1, a value in good agreement with that
calculated from Eq. (12):
Rb = 1.8–0.77 = 1.03 mol N kg–1 N s–1
The relative reactivity of thiyl radicals toward vinyl
and butene double bonds in LV-SBS was calculated from
the ratio of the two addition rate constants:
kv
kb
ˆ
…Rv†copo
Rb
6
‰buteneŠ
‰vinylŠ
…13†
ˆ
0:77
0:9
611:7 ˆ 10
This value confirms that thiyl radicals are much more
reactive toward the pendent vinyl double bonds than
toward the backbone butene double bonds. The larger
value found in HV-SBS (kv/kb = 17) may result from a
decrease of kb caused by the more difficult access by thiyl
radicals of the butene double bonds squeezed between the
many pendent vinyl groups.
In Tab. 2, we have summarized our kinetic data on the
photocrosslinking of the two types of SBS rubber studied,
in the presence of 20 wt.-% TRIS and 2 wt.-% Lucirin
TPO.
Influence of the thiol concentration
All the photocrosslinking experiments reported so far
have been carried out with SBS samples containing the
trifunctional thiol at a weight concentration of 20%,
which corresponds to a double bond/SH molar ratio of
8.4. As insolubilization of polymers requires the creation
of only a few bridges between the polymer chains, we
have reduced the content of TRIS down to 10 wt.-% and
5 wt.-%, i.e., ene/thiol ratios of 19 and 40, respectively.
Fig. 6 shows the vinyl loss and thiol conversion profiles
for the two HV-SBS samples exposed to UV radiation for
up to 2 s in the presence of 1 wt.-% Lucirin TPO. The
crosslinking reaction appears to proceed as efficiently as
at the higher thiol concentration, thus leading to a rapid
insolubilization of the SBS rubber upon UV irradiation
(Fig. 7). A 0.1 s exposure proved to be sufficient to get a
90% insoluble polymer showing nearly the same value of
the swelling ratio (6) as the polymer photocured with 20
wt.-% TRIS (swelling ratio of 5).
Lowering the thiol concentration down to 5 wt.-% has
therefore no detrimental effect on the cure kinetics and on
the crosslinking extent of the SBS rubber. Besides reduc-
ing the cost of the formulation, it will also lessen some of
the disadvantages associated with the use of sulfur com-
pounds, in particular the bad smell. In this respect, it
should be emphasized that the photocured polymer shows
Fig. 6. Influence of the thiol content on the photocrosslinking
of the HV-SBS/Tris system. [Lucirin TPO] = 1 wt.-%. Thiol con-
version (0), vinyl loss (h)
Fig. 7. Influence of the thiol content on the photocrosslinking
of the HV-SBS/Tris system. [Lucirin TPO] = 1 wt.-%. Gel frac-
tion (0, h); swelling ratio (9, H)
1972 C. Decker, T. Nguyen Thi Viet
no unpleasant odor anymore, because essentially all the
thiol groups have reacted and are chemically bonded to
the polymer network. The few unreacted SH groups are
still linked to the polymer because of the trifunctional
character of the mercaptan used.
Very similar results have been obtained with the LV-
SBS sample UV-irradiated in the presence of 3 wt.-%
TRIS and 1 wt.-% Lucirin TPO. Here again crosslinking
of this thermoplastic elastomer was found to proceed
readily, to yield a tight tridimensional polymer network.
Fig. 8 shows the polymerization, insolubilization, harden-
ing and swelling profiles of the UV-exposed sample.
Insolubilization was achieved within 0.1 s at a vinyl con-
sumption of 5%, i.e., 0.05 mol/kg SBS, which corre-
sponds to 8 bridges per polybutadiene chain. Nearly the
same amount of crosslinks was produced by reaction of
the thiol with the butene double bonds, which leads to an
average molecular weight between crosslinks of 7000.
The drastic effect of the trifunctional thiol on the
photocrosslinking of SBS is illustrated in Fig. 9, which
shows the variation of the sol fraction and the hardness
with exposure time, for both systems. Insolubilization
required 20 times less energy than in the thiol-free SBS,
with formation of a much harder crosslinked polymer.
Moreover, the sharp drop of the swelling ratio of UV-
cured SBS, from 25 to 4 in the presence of TRIS, con-
firms that a much more efficient crosslinking process is
taking place in the thiol/polybutadiene system.
In the presence of 5 wt.-% TRIS, insolubilization was
found to occur not only much faster than in the neat rub-
ber, but even faster than in an SBS sample containing 20
wt.-% of a very reactive diacrylate monomer5), as shown
in Fig. 10. This result was unexpected, based on the
monomer functionality: f = 3 for the thiol and f = 4 for
the diacrylate. Indeed, a linear polymer will be formed by
using either a monoacrylate (f = 2) or a difunctional thiol
(f = 2) associated to a diene, because the latter chain reac-
tion proceeds by a step growth addition. If the thiol com-
pound contains only 1 SH group, there will be no poly-
merization and no crosslinking, but simply an addition of
the thiol to the SBS double bond, as shown by the reac-
tion scheme given in the introduction section.
Consequently, the copolymerization of a diacrylate (or
a telechelic acrylate) with the SBS double bonds will gen-
erate 4 branch points, instead of only 3 in the case of the
trifunctional thiol. The lower crosslink density actually
found in the SBS/acrylate system (swelling ratio of 12)
was attributed to the competitive homopolymerization of
Fig. 8. Photocrosslinking of LV-SBS in the presence of 3
wt.-% of Tris. [Lucirin TPO] = 1 wt.-%. Vinyl loss (0); gel frac-
tion (f); swelling ratio (f) and hardness (h) Fig. 9. Influence of a trifunctional thiol (3 wt.-%) on the
photocrosslinking of LV-SBS. [Lucirin TPO] = 1 wt.-%. Soluble
fraction (0, 9) ; hardness (h, H)
Fig. 10. Photocrosslinking of HV-SBS, neat (f), in the pre-
sence of a diacrylate (20 wt.-%) (h), or of a trifunctional thiol
(5 wt.-%) (0). [Lucirin TPO] = 1 wt.-%
Photocrosslinking of functionalized rubbers, 8 1973
the acrylate double bonds, which does not generate any
additional crosslinks between SBS chains.
Heat resistance of UV-curable SBS
If these photocurable thermoplastic elastomers are to be
used as hot-melt adhesives, they must exhibit a fair heat
resistance at the processing temperature, usually 1508C.
A 20 lm thick film of HV-SBS containing 5 wt.-%, TRIS
and 1 wt.-% Lucirin TPO was heated at 1508C for up to
30 min. The thiol content was found to drop rapidly dur-
ing this thermal treatment, 50% loss after 5 min, as shown
in Fig. 11. This loss is mainly due to volatilization of the
liquid TRIS out of the thin film, rather than to a chemical
reaction, as indicated by the following three observations:
– the thiol disappears as rapidly in an SBS film contain-
ing no photoinitiator,
– the thiol loss is much reduced in thick SBS samples,
– gelation of SBS does not occur during the first 5 min
of heating when half of the TRIS is lost.
After 10 min heating in the dark, one third of the sam-
ple has become insoluble in toluene (Fig. 12), which
makes this very reactive system unfit for hot-melt adhe-
sive applications. In the absence of photoinitiator, gela-
tion started to proceed 5 min later, but insolubilization
did follow a similar profile as in the presence of Lucirin
TPO, as shown in Fig. 12. In a previous study, we found
neat SBS to be stable for over 20 min at 1508C5). Reduc-
ing the thiol content and adding an antioxidant might
therefore help in improving the heat resistance of the
photocurable thiol/SBS formulation.
Conclusion
The light-induced thiol/ene polymerization is a very
effective method to crosslink rapidly styrene-butadiene
block copolymers at ambient temperature. Under intense
illumination, the curing reaction proceeds within a frac-
tion of a second by a step growth addition mechanism to
yield an insoluble polymer network. Both the butene and
the vinyl double bonds of the polybutadiene chain are
attacked by the thiyl radicals, which are at least 10 times
more reactive toward the pendent vinyl groups. It is how-
ever not necessary to increase the vinyl content of SBS
above 8% because it will mainly favor intramolecular
homo- and copolymerization reactions between neighbor-
ing vinyl double bonds and will not accelerate the net-
work formation. This result is important from an econom-
ical point of view because it makes this UV-technology
directly applicable to commercial SBS rubbers which
have generally a low vinyl content. Another interesting
feature is that the thiol content can be reduced down to 5
wt.-%, without detrimental effect on the cure speed.
The main applications of this photocurable rubber are
expected to be found in industrial sectors where spatial
control and cure speed are a major concern, such as for
the manufacturing of protective coatings, adhesives,
photoresists and printing plates. Further studies on the
photocrosslinking of the thiol/SBS combination are in
progress, aiming at lowering both the thiol and the photo-
initiator content, to make this system even more econom-
ically attractive, more resistant to heat, and to extend its
field of applications to thick samples and composite
materials.
Acknowledgement: The authors wish to thank SHELL-
Research (Belgium) for a research grant.
Fig. 11. Loss of thiol group upon heating at 150 8C of the HV-
SBS/Tris system. [Tris] = 5 wt.-%; [Lucirin TPO] = 1 wt.-% (0)
or 0% (h)
Fig. 12. Insolubilization of HV-SBS upon heating at 150 8C.
[Tris] = 5 wt.-%; [Lucirin TPO] = 1 wt.-% (0) or 0% (h); neat
SBS (f)
1974 C. Decker, T. Nguyen Thi Viet
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Org. Chem. 52, 1720 (1987)
Photocrosslinking of functionalized rubbers, 7a
Styrene-butadiene block copolymers
Christian Decker*, Trieu Nguyen Thi Viet
Laboratoire de Photochimie Générale (URA N8431-CNRS),
Ecole Nationale Supérieure de Chimie de Mulhouse – Université de Haute Alsace,
3, rue Werner – 68200 Mulhouse; France
(Received: May 23, 1998; revised: July 13, 1998)
SUMMARY: The photocrosslinking of polystyrene-block-polybutadiene-block-polystyrene (SBS) was stu-
died by means of infrared spectroscopy, by monitoring the disappearance of the pendent vinyl double bonds,
which was shown to proceed within a few seconds upon UV exposure in the presence of an acylphosphine
oxide photoinitiator. Complete insolubilization requires the reaction of 17 double bonds per polymer chain
for an SBS sample containing 8% vinyl groups. An increase of the vinyl content has little effect on the cross-
linking process, because it enhances mainly intramolecular reactions. A paraffinic oil proved to be an effec-
tive plasticizer for the photocrosslinked elastomer. The addition of a telechelic acrylate oligomer causes a
substantial increase of both the reaction rate and the final degree of conversion of the SBS double bonds. The
light-induced copolymerization of the vinyl and butene double bonds with the acrylate double bonds leads to
the formation of a hard and flexible polymer material within a fraction of a second.
Introduction
Light-induced polymerization is one of the most effective
methods to generate tridimensional polymer networks,
because of the high initiation rates reached under intense
illumination1–4). In most UV-curing applications, a sol-
vent-free liquid resin is converted quasi-instantly into a
highly crosslinked polymer, selectively in the exposed
areas, to produce protective coatings, quick-setting adhe-
sives or high-resolution relief images. The same photo-
chemical process has been successfully used to crosslink
solid polymers bearing polymerizable functional groups
on their backbone chain5), e.g. cinnamates6), chalcones7),
epoxides8, 9) or acrylates10). A distinct advantage of photo-
initiation is to afford a precise temporal control of the
chemical process. The crosslinking reaction will start
immediately, as soon as light is shed on the sample, and it
can be stopped at any time by switching off the UV lamp.
Moreover, the reaction rate can be varied in a large range,
simply by changing the intensity of the UV-beam.
In the previous articles of this series, we have shown
that epoxy11, 12) or acrylate13) functionalized polyisoprene
can be “photovulcanized” within seconds at ambient tem-
perature by UV light in the presence of a cationic or radi-
cal-type photoinitiator, respectively. Under the same irra-
diation conditions, polyisoprene (natural rubber) did not
undergo any crosslinking, because of the low reactivity of
the amylene double bond. This is not the case for polybu-
tadiene which contains the more reactive butene-2 and
vinyl double bonds formed by 1–4 and 1–2 polymeriza-
tion of butadiene, respectively. There are, however, only
a few reports in the literature on the photocrosslinking of
polybutadiene-based rubbers14–18). Most of these studies
have been performed on styrene-butadiene-styrene (SBS)
tri-block copolymers19), known under the tradename
KRATON, which are commonly used in pressure-sensi-
tive and hot-melt adhesive applications20). These copoly-
mers, which have a two-phase morphology, combine the
properties of elastomers and thermoplastic materials, the
styrenic domains acting as physical crosslinks below the
Tg . Photocuring proved to be an effective method to
make these thermoplastic elastomers more resistant to
solvent and temperature, while improving greatly the
adhesive strength14). By creating covalent bonds within
the elastomeric phase, UV-curing is reinforcing irreversi-
bly the already existing physical network.
The main objective of the present study was to deter-
mine how fast and how extensively styrene-butadiene
rubbers can be crosslinked by UV irradiation at ambient
temperature. We have examined in particular the influ-
ence of the photoinitiator and of the polybutadiene vinyl
content on the kinetics of the crosslinking process and on
some of the properties of the UV-cured polymer network.
An attempt has been made to overcome the rate limita-
tions inherent to solid state reactions by incorporating in
the starting material a plasticizer or an acrylate monomer.
The photooxidation process which is known to occur
Macromol. Chem. Phys. 200, No. 2 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 1022-1352/99/0202–0358$17.50+.50/0
a Part 6: cf. ref.12)
358 Macromol. Chem. Phys. 200, 358–367 (1999)
Photocrosslinking of functionalized rubbers, 7 359
upon UV irradiation of this type of rubber in the presence
of air21) was minimized by the rapid consumption of oxy-
gen during the short exposure.
Experimental part
Materials
Two types of polystyrene-block-polybutadiene-block-poly-
styrene (SBS), both from SHELL, have been used in this
study. They differ only by their content of pendent vinyl
groups: 8% of the total double bond content for the low-
vinyl polymer (LV-SBS), and 59% for the high-vinyl poly-
mer (HV-SBS). The butene-2 double bonds located on the
polybutadiene backbone represent 92% and 41% of the unsa-
turation content, respectively. In some experiments, a paraf-
finic oil (Nujol from Aldrich) was added to the formulation
to increase the molecular mobility in the polymer film. Such
a plasticizing effect was also achieved by using a diacrylate
monomer (Ebecryl 150 from UCB) which, in addition, was
expected to act as a crosslinking agent by copolymerizing
with the polybutadiene double bonds.
Different types of photoinitiators were introduced in the
polymer, at a typical concentration of 3 wt.-% SBS, namely
benzophenone (BZP from Aldrich), 2,6-bis(azidobenzyli-
dene)-4-methylcyclohexanone (bisazide from Aldrich), 2,2-
dimethoxy-2-phenylacetophenone (Irgacure 651 from Ciba),
1-benzoyl-1-hydroxycyclohexane (Irgacure 184 from Ciba)
and 2,4,6-trimethylbenzoyldiphenylphosphine oxide (Lucirin
TPO from BASF). 20 lm thick films were cast from toluene
solution of SBS and the photoinitiator onto either a KBr
crystal for infrared analysis or a glass plate for insolubiliza-
tion and hardness measurements. Samples were exposed to
the radiation of a 80 W/cm medium pressure mercury lamp,
in the presence of air, at a passing speed of 50 m/min. The
maximum light intensity at the sample position was mea-
sured by radiometry (IL-390 light bug) to be 600 mW cm–2.
Analysis
The kinetics of the light-induced crosslinking of the rubber
film was studied quantitatively by FTIR spectroscopy, by
following the decrease upon UV exposure of the absorption
bands characteristic of the vinyl double bond at 910.8,
1637.8 and 1827 cm–1, and of the butene-2 double bond at
964.7 cm–1. In formulations containing the acrylate mono-
mer, the disappearance of this double bond upon UV-expo-
sure was followed at 812 cm–1 (CH22CH twisting). The
degree of conversion (x) was calculated from the ratio of the
corresponding IR absorbance before and after UV exposure
(A0 and At) by using the following equation:
x(%) = [1 – (At /A0)] N 100
This value was not corrected for shrinkage, as it was found
to account for less than 5%, based on the variation of the
C1H peak at 2960 cm–1. The loss of the photoinitiator in the
UV-irradiated sample was followed quantitatively by UV
spectroscopy. The gel fraction and the degree of swelling of
the irradiated polymer were determined by soaking the sam-
ple in toluene for one day at room temperature. The insoluble
polymer was recovered by filtration and dried at 708C to a
constant weight. The hardness of the coating was evaluated
before and after irradiation by monitoring the damping of the
oscillations of a pendulum (Persoz hardness). The hardness
was shown to be strongly dependent on the glass transition
temperature22), with Persoz values ranging typically from
30 s for soft elastomeric materials to 300 s for hard and
glassy polymers. The physical significance of pendulum
hardness has been reviewed by Sato23) who considered it as a
reliable measure of the viscoelastic properties of a polymer
coating. The adhesion of the UV-cured coating on glass or
other substrates was evaluated by using the standard cross-
hatch adhesive tape test. The thermal stability of the various
formulations was tested by heating the uncured sample for
up to 1 h at 1508C, the processing temperature for hot-melt
adhesive applications.
Results
The photoinitiator (PI) is a key factor for achieving a fast
and extensive crosslinking by means of UV light. Indeed,
both SBS samples were found to remain soluble when
they were exposed to UV radiation in the absence of a
photoinitiator, for doses up to 5 J N cm–2. It was therefore
important to select the most efficient photoinitiator for
the given SBS rubber, and to determine its optimum con-
centration. The role of the pendent vinyl double bond was
also investigated to know whether it is necessary to
increase its content to form readily a 3D polymer net-
work. Finally, the effect of an acrylate crosslinker on the
reaction kinetics was examined, in an attempt to enhance
the reactivity of both the vinyl and the butene double
bonds through copolymerization.
Influence of the radical photoinitiator
Most of the radical-type photoinitiators used in UV-cur-
able systems consist of aromatic ketones which undergo a
carbon-carbon bond cleavage when exposed to UV radia-
tion:
Various factors govern the initiation efficiency of these
compounds: their intrinsic absorbance of the incident
light, the lifetime of the excited states, the rate of photo-
lysis, the efficiency of the cleavage reaction and the reac-
tivity of the radical fragments.
Insolubilization profiles were determined for the two
SBS samples UV-irradiated in the presence of different
types of radical photoinitiators. Fig. 1 shows some char-
acteristic curves obtained by plotting the gel fraction ver-
360 C. Decker, T. Nguyen Thi Viet
sus the exposure time for five photoinitiators in HV-SBS,
at a weight concentration of 3%. They can be classed in
the following order of increasing efficiency:
Bisazide a benzophenone a Irgacure 651
a Irgacure 184 a Lucirin TPO
The bisazide, commonly used as photoinitiator to
crosslink cyclized polyisoprene, appears to be the least
efficient of the five initiators tested. The best perfor-
mance was reached by using the phosphine oxide initia-
tor, nearly all the polymer becoming insoluble after a 1 s
exposure. The same classification was obtained when the
crosslinking process was followed through the loss of the
vinyl double bond or the increase of the rubber hardness.
Consequently, this photoinitiator, Lucirin TPO, was
selected for all of our further studies.
Kinetics of the crosslinking reaction
IR analysis shows that only minor chemical modifications
are taking place in SBS upon UV exposure, mainly a
small decrease of the three peaks of the vinyl double
bond at 910.8, 1637.8 and 1827 cm–1. After 1 s, only 4%
of the pendent vinyl groups of HV-SBS were found to
have reacted, whereas the concentration of the backbone
double bonds absorbing at 964.7 cm–1 remained essen-
tially unchanged. It can be seen in Fig. 2 that the SBS
vinyl content drops rapidly at the very beginning of the
UV irradiation and levels off after 3 s to a constant value
(95% of the original content). This apparent loss of reac-
tivity of the remaining vinyl double bonds can be attribu-
ted to the rapid photolysis of the photoinitiator which has
essentially disappeared after a 2-second exposure (see
below). Mobility restrictions of the reactive sites in the
network under formation are also likely to contribute to
the premature stop of the polymerization.
The photoinitiated polymerization of vinyl groups
located on different polybutadiene chains leads to the for-
mation of a tridimensional polymer network and accounts
for the observed insolubilization of the irradiated rubber
(Fig. 3). The polymer network presents, however, a fairly
loose structure, as indicated by its high degree of swelling
(Psolvent /Ppolymer = 20). A plot of the gel fraction versus the
degree of conversion (Fig. 4) shows that insolubilization
is achieved once 5% of the vinyl double bonds have
reacted, which corresponds to 62 pendent groups per
polybutadiene chain. Theoretically, only three bridges
need to be formed per polymer chain to obtain a cross-
linked material of infinite molecular weight. The fact that
insolubilization of SBS requires the reaction of a much
larger number of vinyl double bonds strongly suggests
that, in addition to the intermolecular reaction, an intra-
molecular polymerization process is taking place between
vinyl double bonds located on the same polybutadiene
chain. It should lead to the formation of cyclic struc-
tures24) along the polybutadiene chain, which would
Fig. 1. Influence of the photoinitiator (3 wt.-%) on the insolu-
bilization profile of a 20 lm thick HV-SBS film exposed to UV
radiation (I = 600 mW N cm–2)
Fig. 2. Photocrosslinking of a SBS rubber with a high vinyl
content (59%) in the presence of Lucirin TPO (3 wt.-%)
Fig. 3. Insolubilization profile of HV-SBS exposed to UV
radiation in the presence of Lucirin TPO (3 wt.-%)
Photocrosslinking of functionalized rubbers, 7 361
therefore remain soluble. This is feasible because of the
high vinyl content of the HV-SBS sample (59% of the
total number of double bonds). However, because the
polybutadiene chain contains also cis and trans 2-butene
units, the cyclisation process will propagate along the
polymer chain over only a few adjacent vinyl units. The
two inter- and intramolecular polymerization processes
are represented formally in Scheme 1.
As crosslinking proceeds upon UV exposure, the
amount of bounded elastic chains becomes higher and the
hardness increases, as shown in Fig. 2. The UV-cured
polymer remains still soft (Persoz value of 80 s), which is
important for adhesive applications to maintain the tack
and wetting required to achieve a good adhesion.
The outer polystyrene segments of the block copoly-
mer, which are phase separated, are not expected to parti-
cipate in the crosslinking process, except by a possible
chain transfer reaction if some propagating radicals suc-
ceed in abstracting a labile hydrogen on the PS chain at
the interphase. Increasing the PS chain length should not
affect insolubilization, which is ensured through the poly-
butadiene crosslinks, but it will increase the degree of
swelling by lowering the crosslink density.
It should be noted that a fraction of the vinyl groups is
likely to disappear by a photooxidation process21, 25, 26).
While no significant changes in the hydroxyl and ether
region of the IR spectrum could be detected after the
short UV exposure, there was indeed a small but distinct
increase of a carbonyl band at 1694 cm–1 during the first
0.5 s of irradiation. Its absorbance was found to level off
at a constant value of 0.015 upon further exposure, prob-
ably because most of the oxygen dissolved in the film has
then been consumed by reaction with the initiator radi-
cals. The concentration of the carbonyl group was calcu-
lated to be on the order of 2610–2 mol N kg–1, which cor-
responds to less than 10% of the total amount of the vinyl
groups lost. The photoinitiated polymerization appears
thus as the main process responsible for the vinyl double
bond consumption during the UV irradiation in the pre-
sence of air. Under O2-free conditions, the vinyl groups
polymerize only slightly faster than in air (Fig. 2), which
shows that O2 inhibition is not very important in these
solid films exposed to intense radiation.
Influence of the photoinitiator concentration
In light-induced reactions, the photoinitiator concentra-
tion [PI] controls the rate of initiation (ri) which obeys
the following equation:
ri ˆ Ui I0…1ÿ eÿel‰PIŠ†
where Ui is the initiation quantum yield, I0 the incident
light intensity, e the molar extinction coefficient of PI,
and l the thickness of the sample. Because of the limited
penetration of the incident radiation in the UV-absorbing
medium, crosslinking within the SBS sample will follow
a surface to depth gradient which is directly dependent on
[PI]. Consequently, an increase in the PI concentration
will both accelerate the crosslinking reaction and steepen
the cure depth gradient in the irradiated sample. Depend-
ing on the considered application, the best compromise
must be found between cure speed and cure depth, the
two extremes being either a uniform but slow deep-
through cure for low absorbing samples (low [PI]), or a
rapid but differential through-cure for highly absorbing
sample (high [PI]).
Fig. 5 shows the insolubilization profiles of a 20 lm
thick HV-SBS film containing 1 to 5 wt.-% Lucirin TPO
exposed to UV radiation for up to 3 s. The rate increase
with the PI concentration is particularly important for
values up to 2 wt.-%. At higher [PI], the accelerating
effect becomes less pronounced, because of the reduced
penetration of the incident light in the deep-lying layers
(inner filter effect).
An increase of the PI concentration leads also to the
formation of a tighter polymer network, because of the
increased number of initiating radicals. The swelling ratio
Fig. 4. Dependence of the gel fraction and hardness of a photo-
crosslinked HV-SBS on the degree of vinyl conversion
Scheme 1:
362 C. Decker, T. Nguyen Thi Viet
of UV-cured HV-SBS was found to drop from 55 to 25
when [TPO] was increased from 1 to 5 wt.-%. Fig. 6
shows the variation of the gel fraction and of the swelling
ratio with the photoinitiator concentration, for a 20 lm
thick sample exposed for 1 s to UV radiation. It should be
mentioned that, when the TPO concentration was
increased above 6 wt.-%, the overall cure extent started
to decrease because of incomplete crosslinking of the bot-
tom layer. The [PI] value where maximum efficiency is
reached depends directly on the sample thickness, e.g. at
a 10 times as low PI concentration for a 10 times as thick
film.
A similar behavior was observed by following the
hardness increase upon UV exposure (Fig. 7), and the
vinyl loss as well. The variation of these two parameters
with the TPO concentration is shown in Fig. 8. Here
again, maximum efficiency was reached for a photoinitia-
tor concentration of about 5 wt.-%. For a 20 lm thick
film, a TPO concentration of 2 wt.-% appears to give the
best overall performance, with respect to deep-through
cure, insolubilization, network density and moderate
hardness increase.
It should be emphasized that the depth of cure gradient
due to the PI light absorbance is progressively smoothing
off upon UV irradiation, because the photoinitiator is con-
verted in non-absorbing photoproducts. Fig. 9 shows the
exponential decay of Lucirin TPO, monitored by UV
spectroscopy at 380 nm, upon UV exposure in a SBS
matrix at different concentrations. Such a first-order
kinetic law was expected for a direct photolysis process,
where the reaction rate depends primarily on the prob-
ability of a chromophore molecule to be hit by a photon
and is therefore proportional to the chromophore concen-
tration, throughout the reaction. From the slope of the
straight line obtained in the semi-log plot, ln [PI]t /[PI]0 =
–k N t, the decay rate constant k was calculated. Its value
Fig. 5. Influence of the photoinitiator concentration (Lucirin
TPO) on the insolubilization profiles of HV-SBS upon UV expo-
sure
Fig. 6. Dependence of the gel fraction and swelling ratio of the
photocrosslinked HV-SBS on the photoinitiator concentration.
UV exposure: 1 s
Fig. 7. Influence of the photoinitiator concentration (Lucirin
TPO) on the hardening of HV-SBS upon UV exposure
Fig. 8. Dependence of the vinyl loss and the hardness of photo-
crosslinked HV-SBS on the photoinitiator concentration. UV
exposure: 1 s
Photocrosslinking of functionalized rubbers, 7 363
was found to drop from 4.3 s–1 to 2.1 s–1 when [PI]0 was
increased from 1 to 5%, thus showing the importance of
the radiation inner filter effect27).
One of the main consequences of this photobleaching
process is to make the top layer of the irradiated sample
more transparent to UV light, which can therefore pene-
trate deeper into the sample, thus inducing a frontal poly-
merization28). Up to a few millimeter thick specimens have
been crosslinked within less than 1 min by this UV tech-
nology.
Influence of the vinyl content of SBS
The photocrosslinking experiments reported so far have
all been carried out on a SBS sample with a high content
(59%) of pendent vinyl double bonds, which are known
to be more reactive than the in-chain butene double
bonds. As SBS rubbers usually contain a lower amount of
pendent vinyl groups, we have repeated some of these
experiments with a SBS sample where only 8% of the
unsaturations are vinyl double bonds (LV-SBS). Surpris-
ingly, the crosslinking of this rubber was found to pro-
ceed nearly as fast upon UV exposure as that containing
59% vinyl group.
Fig. 10 shows the decrease of the vinyl double bond in a
20 lm thick LV-SBS film containing 3 wt.-% Lucirin TPO
exposed for up to 3 s to UV-radiation. Although the vinyl
double bonds seem to disappear faster in LV-SBS than in
HV-SBS (Fig. 10 and 2), this is not the case. This mislead-
ing result is due to the chosen representation (relative loss
of vinyl group versus exposure time). The initial rate of
polymerization of the vinyl group, expressed in mol N kg–
1 N s–1, is still 3 times greater in the sample containing a
large amount of vinyl double bonds (1 mol N kg–1 N s–1 ver-
sus 0.3 mol N kg–1 N s–1). After 0.5 s and a 10% vinyl loss,
the reaction is slowing down, most probably because of
the fast consumption of the photoinitiator. At the same
time, only a slight decrease (2%) of the butene double
bond at 967 cm–1 was found to occur upon UV irradiation,
very much like in HL-SBS. Crosslinking was character-
ized by both an increase of the polymer hardness (Fig. 10)
and a nearly complete insolubilization of the irradiated
sample (Fig. 11). Although the initial rate of gelation was
lower in LV-SBS than in HV-SBS, the crosslink density of
the polymer network was similar for both samples, as
shown by the values of the swelling ratio: 27 and 25 after a
2 s exposure for LV-SBS and HV-SBS, respectively.
All these results, which are summarized in Tab. 1, indi-
cate that it is not necessary to increase the vinyl content
to achieve a fast photocrosslinking of SBS rubber. In LV-
SBS, gelation occurs when about 10% of the vinyl groups
have reacted, which corresponds to 17 double bonds per
polybutadiene chain. An increase of the vinyl content of
SBS will mainly favor the intramolecular polymerization
between neighboring pendent double bonds, without
much effect on the crosslinking process.
Fig. 9. Disappearance of the photoinitiator (Lucirin TPO)
upon UV irradiation of HV-SBS
Fig. 10. Loss of the polybutadiene double bonds and hardness
increase in a UV-irradiated SBS rubber with a low vinyl content
(8%). Photoinitiator: [Lucirin TPO] = 3 wt.-%
Fig. 11. Insolubilization profile of a LV-SBS rubber exposed to
UV-radiation in the presence of Lucirin TPO (3 wt.-%)
364 C. Decker, T. Nguyen Thi Viet
Influence of plasticizers
Like any reaction carried out in the solid state, the photo-
crosslinking of SBS is hampered by the restricted mobi-
lity of the reactive sites (initiator and polymer radicals, as
well as vinyl double bonds). The addition of a plasticizer
like Nujol, a parrafinic oil, was thus expected to acceler-
ate the polymerization process. Such an effect was actu-
ally observed only for Nujol concentrations above 20%,
probably because of the elastomeric character of the SBS
matrix which provides already enough molecular mobi-
lity in the neat polymer. For a 50/50 mixture by weight of
LV-SBS and Nujol, the vinyl double bonds were found to
disappear almost twice faster than in the neat SBS sample
(Fig. 12). Insolubilization was accelerated in the same
manner (Fig. 13), but the photocrosslinked polymer
remained very soft because of the presence of the plastici-
zer occluded in the polymer network (Fig. 12).
It should be mentioned that the gel fraction of the UV-
cured polymer, which reaches close to 100% in 2 s, was
calculated on the basis of the SBS content, i. e. 50% of
the irradiated sample had become insoluble in toluene.
Interestingly, the degree of swelling of the polymer net-
work was cut by half when LV-SBS was UV-cured in the
presence of Nujol (Fig. 13), thus showing that the cross-
link density has increased. This result is in good agree-
ment with the observed doubling of the vinyl polymeriza-
tion rate and suggests that the plasticizer is favoring inter-
molecular polymerization by increasing the molecular
mobility of the SBS chains.
Similar results were obtained when Nujol was added to
the SBS sample containing 59% pendent vinyl groups.
The effect of the plasticizer concentration on the vinyl
conversion, insolubilization and hardness is illustrated in
Fig. 14 for a HV-SBS sample exposed to UV radiation
during 1 s. For both SBS rubbers, a Nujol concentration
of 30 wt.-% appears to be an optimum value to achieve
an efficient crosslinking and obtain a polymer showing a
strong elastomeric character and excellent adhesion on
various substrates (glass, metals, plastics). It should be
noticed that the plasticized SBS films remain perfectly
clear, up to a Nujol concentration of 70%, thus showing
the good compatibility of these two compounds.
Tab. 1. Performance analysis of SBS photocrosslinking
Thermoplastic elastomer LV-SBS HV-SBS
Initial vinyl content in % 8 59
Rate of vinyl loss
% sÿ1
mol N kgÿ1 N sÿ1

25 10
0.3 1.0
Rate of gelation (% s–1) 150 240
Rate of hardness increase 60 80
Swelling ratio after 2 s 27 25
Fig. 12. Photocrosslinking of a blend of LV-SBS rubber and a
paraffinic oil ([Nujol] = 50 wt.-%)
Fig. 13. Insolubilization profile of a 50/50 LV-SBS/Nujol
blend exposed to UV irradiation in the presence of Lucirin TPO
(3 wt.-%). - - - -: neat LV-SBS
Fig. 14. Influence of the plasticizer content (Nujol) on the
characteristics of a 1 s UV-irradiated HV-SBS rubber
Photocrosslinking of functionalized rubbers, 7 365
Influence of an acrylate crosslinker
In an attempt to further increase the crosslinking effi-
ciency, an acrylate monomer, was introduced in the SBS
rubber, together with the photoinitiator (Lucirin TPO). At
20% by weight of a diacrylate bisphenol A telechelic oli-
gomer (Ebecryl 150), both the pendent vinyl and the
backbone butene double bond were found to react rapidly
upon UV exposure, as shown by Fig. 15. A 0.1 s exposure
proved to be sufficient to induce the polymerization of
14% of the vinyl groups and 6% of the butene-2 double
bonds, compared to 1% and 0% in neat HV-SBS, respec-
tively. At the same time, 70% of the acrylate groups had
polymerized, as shown by the sharp drop of the character-
istic absorption peak at 812 cm–1. This result strongly
suggests that an effective copolymerization is taking
place between the acrylate double bonds and the vinyl
double bonds. A strong correlation was found to exist
between the rate of polymerization of the polybutadiene
unsaturation and that of the acrylate double bonds, during
the whole reaction time. The same behavior was observed
with the LV-SBS sample. Fig. 16 shows schematically the
structure of the net-polybutadiene-i-diacrylate formed
upon UV irradiation of the SBS-diacrylate mixture. As
expected from the kinetic profiles of Fig. 15, insolubiliza-
tion of the SBS rubber is rapidly taking place with forma-
tion of a relatively tight polymer network (Fig. 17). After
a 1 s UV exposure, the swelling ratio of HV-SBS was
measured to be 12, compared to 32 for the neat rubber.
The additional crosslinks generated by the diacrylate
monomer are causing a substantial increase of the poly-
mer hardness (Persoz values of 200 s). Because of the
elastomeric character of the polybutadiene chains, the
UV-cured film still maintains a high flexibility and passes
successfully the severe zero-T-bend test. These highly
reactive UV-curable systems are therefore more suited for
protective coating than for adhesive applications, spe-
cially on flexible substrates.
Thermal stability of the UV-curable formulation
When these UV-curable rubbers are used as hot-melt
adhesives or as flexographic printing plates, the formula-
tion is processed at temperatures up to 1508C19, 20). It was
therefore important to evaluate the thermal stability of
the SBS rubber containing the photoinitiator and the addi-
tive (Nujol or diacrylate). The photoinitiator selected
(Lucirin TPO) was found to be relatively stable at that
temperature, its concentration in the sample dropping by
only 20% after a 30 min heating at 1508C in the presence
of air (Fig. 18). The UV-curing performances were there-
fore not very much affected by this thermal treatment.
Fig. 15. Polymerization of the acrylate and polybutadiene dou-
ble bonds, upon UV exposure of a 20/80 blend of diacrylate
monomer (Ebecryl 150) and HV-SBS rubber. [Lucirin TPO] = 3
wt.-%
Fig. 16. Structure of polymer network formed by photopoly-
merization of acrylate and polybutadiene double bonds
Fig. 17. Insolubilization profile of HV-SBS/Ebecryl 150 blend
(80/20 by weight) exposed to UV radiation in the presence of
Lucirin TPO (3 wt.-%). - - - -: neat HV-SBS
366 C. Decker, T. Nguyen Thi Viet
The concentration of vinyl double bonds remained essen-
tially unchanged during the first 20 min of heating, and
started to decrease upon further treatment, as shown in
Fig. 18 for the LV-SBS sample. The butene double bonds
proved to be somewhat more resistant to this thermal
treatment, as expected from their lower reactivity. The
loss of double bonds is accompanied by polymerization
and oxidation reactions which both occur after a certain
induction period. The SBS rubber started to become
partly insoluble after a 20 min heating (Fig. 19), the gel
fraction reaching 50% after 40 min, with formation of a
relatively tight polymer network (swelling ratio of 10).
Different types of functional groups (alcohol, ketone,
ether), resulting from thermooxidation were detected by
infrared spectroscopy for heating periods exceeding
20 min, as shown in Fig. 20. Similar results have been
obtained with the SBS sample containing 59% vinyl dou-
ble bonds (HV-SBS) which proved to be nearly as resis-
tant to heat as LV-SBS. If necessary, the thermal stability
of the UV-curable rubber can be increased by the addition
of a phenolic antioxidant, like Irganox 1010 from Ciba15).
Interestingly, the addition of paraffinic oil (Nujol) was
found to improve the heat resistance of the SBS rubber
(Fig. 19). This is not the case of the very reactive diacryl-
ate monomer (Ebecryl 150) which promotes the thermal
curing, as shown in Fig. 21. After a 20 min heating at
1508C in air, 30% of both the butene and vinyl double
bonds had polymerized, and as many as 70% of the acryl-
ate double bonds, with formation of a totally insoluble
polymer. The decreased stability of the butene double
bond in the presence of an acrylate monomer is an argu-
ment in favor of its copolymerization.
The heat resistance of SBS rubber containing a phos-
phine oxide initiator is strong enough to allow this UV-
curable system to be used in hot-melt adhesive applica-
tions. This is not true for the much more reactive acryl-
Fig. 18. Heat resistance at 150 8C of a LV-SBS rubber contain-
ing 3 wt.-% Lucirin TPO
Fig. 19. Insolubilization of LV-SBS upon heating at 150 8C in
the presence of Lucirin TPO (3 wt.-%)
Fig. 20. Oxidation products formed upon heating at 150 8C of
a LV-SBS rubber containing 3 wt.-% Lucirin TPO
Fig. 21. Loss of acrylate and polybutadiene double bonds upon
heating at 150 8C of a LV-SBS rubber containing 3 wt.-%
Lucirin TPO
Photocrosslinking of functionalized rubbers, 7 367
ate-loaded SBS formulation which is best suited for the
production of fast-drying protective coatings showing
excellent adhesion and impact resistance.
Conclusion
Polystyrene-block-polybutadiene-block-polystyrene can
be readily crosslinked at ambient temperature by UV irra-
diation in the presence of a phosphine oxide photoinitia-
tor. The radical-induced polymerization of the pendent
vinyl groups occurs within seconds to yield an insoluble
elastomeric material. The reaction kinetics can be finely
controlled through the photoinitiator concentration which
governs also the cure depth profile. Increasing the vinyl
double bond content of the polybutadiene chain from 8 to
59% has little effect on both the cure speed and the cross-
link density of the polymer network, as it mainly favors
intramolecular reactions between neighboring pendent
double bonds. The crosslinking reaction can be markedly
accelerated by the addition of a telechelic acrylate oligo-
mer which promotes an effective copolymerization with
the polybutadiene double bonds, but at the expense of the
formulation heat resistance.
The photocrosslinking technology offers a number of
advantages29), such as cure speed, low energy consump-
tion, ambient temperature operations, no emission of sol-
vent and spatio-temporal control of the initiation step.
Because of its process facility and efficiency, it has been
successfully applied to crosslink a commercial-type ther-
moplastic elastomer and modify, selectively in the illumi-
nated areas, its physico-chemical characteristics. Such
UV-curable rubbers are particularly well suited to produ-
cing fast-drying coatings, hot-melt adhesives and flexible
printing plates.
Acknowledgement: The authors wish to thank SHELL
RESEARCH (Louvain la Neuve – Belgium) for a research grant.
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2452 S. 0. JONES AND
moved by the reaction of the organic halide with
the adjacent atoms of magnesium. If the film is
magnesium hydroxide its disappearance on stand-
ing might be explained by the reaction with the
magnesium bromide from the Grignard equilib-
rium to produce basic magnesium bromide which
is soluble in ether.
Summary
(1) Magnesium amalgam electrodes gave zero
dark voltage and no light sensitivity with ether
solutions of either phenylmagnesium bromide or
E. EMMET REID Vol. 60
ethylmagnesium bromide. (2) Magnesium elec-
trodes which had been cleaned by reacting with
the organic halide gave the same results. (3)
Magnesium electrodes which had not been cleaned
in this manner showed variable dark voltages and
variable photo-voltaic responses. Both of these
could be destroyed by adding the organic halide
or by allowing the cell to stand for several clays.
(4) "Cleaned" magnesium electrodes were made
sensitive to light by exposure to oxygen.
C O I . U N B I A , MU "?CEIVt:D JUNE 23, 19.78
._ . . . .- . . .
[CONTRIBUTION FROM THE CHEMICAL LABORATORY OF TAB JOHNS HOPKINS UNIVERSITY]
The Addition of Sulfur, Hydrogen Sulfide and Mercaptans to Unsaturated
Hydrocarbons
BY S. 0. JONES' AND E. EMMET REID
'There have been many investigations of the
action of sulfur and hydrogen sulfide on unsatu-
rated hydrocarbons* and a few on the addition of
mercaptans3 but so many points have not been
cleared up that further study seemed desirable.
Curiously enough the end-products are much
the same whether an unsaturated hydrocarbon
is treated with sulfur, hydrogen sulfide or a mer-
captan. Thus with ethylene, sulfur gives hy-
drogen sulfide which reacts with more ethylene
to form ethyl mercaptan which then adds to more
ethylene to form ethyl sulfide.
Our results help to explain the presence of
ethyl, i-propyl and other mercaptans and the
corresponding sulfides in petroleum distillates but
do not account for the presence of methvl mer-
captan and methyl sulfide.
The Action of Sulfur.-Quite different results
are obtained from the reaction with sulfur ac-
cording to the conditions and whether free sulfur
or a compound which readily liberates sulfur is
used. When ethylene was passed over pyrites at
350' about 1% of thiophene was isolated along
with hydrogen s a d e and ethyl mercaptan, while
when it was bubbled through sulfur at 325' much
hydrogen sulfide was formed dong with 3% of
ethyl mercaptan and small amounts of carbon di-
sulfide and ethyl suEde. We found it advan-
(1) Taken from the Ph. D. dissertation of S. 0. Jones, R. J . Rey-
nolds Tobacco Co. Fellow, The Johns Hoplrins University, June, 1936.
(2) Mdlhe and Renaudie, Compt. rend., 196, 891 (1932); Duffey,
Snow and Keyes, Ind. Eng. Chem., 16,91 (1934).
(3) Posner, Bcr , 86, 646 (1905)' Nicolet. THIS J O I T R N A L , 67, 1008
(1935)
tageous to use ethyl tetrasulfide as a sulfur donor.
This is a liquid which mixes well with organic com-
pounds and decomposes on heating, giving off
what may be assumed to be atomic sulfur. The
proportions were calculated on the basis of the
tetrasulfide going down to the disulfide. A note-
worthy difference is that with it, appreciable
yields of the cyclic sulfides were obtained.
When ethylene was bubbled slowly through
ethyl tetrasulfide kept a t about 150°, the main
product isolated was ethyl mercaptan with some
ethyl sulfide and in addition some ethylene sulfide.
The results of heating several hydrocarbons in a
bomb with ethyl tetrasulfide are given in Table I.
TABLE J
Hydrocarbons with Et&, 10 hrs. at 180'
-- Yields of rducts ,
IIydrocarboit Mercaptan Surfide 3-
Ethylene 5 1s 1
Propylene ti 20 15
Heptene- 1 80
Octene- I 19
Cyclohexeue b 8
The mercaptans, except from ethylene, and sul-
fides were all secondary.
The Addition of Hydrogen Sulfide.-The
addition of hydrogen sulfide was effected by heat-
ing in the bomb for ten hours at 180'. Sulfur
was added as a catalyst for without it there was
little if any addition. The results are given in
Table 11. The sulfur is found on the secondary
or tertiary carbon atom in accordance with Mar-
kownikow's rule.
Oct., 1938 ADDITION OF SULFUR AND SULFIDES TO UNSATURATED HYDROCARBONS 2453
TABLE I1
Addition of hydrogen sulfide, 10 hrs. at 180'
--Yields of products, %-
Hydrocarbon Mercaptan Sulfide
Ethylene 11 80
Propylene 7 90
&Butylene 23 6
Octene-1 9 35
Cyclohexene 7 5
The addition takes place according to Mar-
kownikow's rule. In all cases the mercaptan that
is first formed adds to a second molecule of the
hydrocarbon to form the sulfide. In the case of
the simple unsaturates this second reaction must
be rapid as compared to the first addition as little
of the mercaptans is left over. &Butyl and cyclo-
hexyl mercaptans evidently do not add so readily
to the hydrocarbons.
The Addition of Mercaptans.-The unex-
pected observation was made in the course of our
experiments that peroxides influence the mode of
addition of mercaptans to unsaturates just as
Kharasch4 found that they influence the addition
of hydrogen bromide. When we heated ethyl
mercaptan with propylene we obtained ethyl i-
propyl sulfide, but with octylene the product was
ethyl n-octyl sulfide. The propylene had never
been exposed to the air while the octylene had been
stored for some time in a partly filled bottle and
gave a strong test for peroxides. By adding
ethyl mercaptan to propylene in the presence of
added peroxides, we obtained ethyl n-propyl sul-
fide.
It has been observed by Posner and Nicolet3
that thiophenol and p-thiocresol add to unsatu-
rates contrary to Markownikow's rule. We have
verified this in a large number of cases, but by
using freshly distilled thiophenol and p-thiocre-
sol we were able to reverse the mode of addition.
Kharasch4 observed that p-thiocresol was not
effective as an anti-oxidant unless it was freshly
distilled. It appears that only very small amounts
of peroxides such as are usually present in the
hydrocarbon or in the p-thiocresol itself are suf-
ficient to influence the addition of mercaptans
while for hydrogen bromide much larger amounts
are required. In the absence of catalysts scarcely
any addition takes place even on heating for
twenty-four hours at 180'. Sulfur catalyzes the
normal addition and peroxides the abnormal.
It is difficult to suppress the abnormal addition
entirely. Into each of 3 tubes of 2-cc. capacity
(4) Kharasch and Hannum. THIS JOURNAL, 66,712 (1934).
were placed 0.25 g. of p-thiocresol and 0.4 cc. of
tridecene. The p-thiocresol had been distilled
recently in vacuo and the tridecene had just
been distilled over sodium. To the first tube
was added a trace of ascaridole, to the second a
trace of sulfur and nothing to the third. After
the usual heating the product in the first tube was
solid and on recrystallization gave 0.3 g. which
melted at 39.9', the melting point of p-thiocresyl
n-tridecyl sulfide made from n-tridecyl bromide
in the usual way. From the liquid products
in the other two tubes it was possible to isolate
0.01 and 0.08 g., respectively, of the sulfide melt-
ing at the same point.
In a number of experiments the products were
identified by their boiling points and by oxidation
to known solid sulfones but for positive proof of
structure unsaturated hydrocarbons6 and mer-
captans were chosen that would give known crys-
talline sulfides which previously had been pre-
pared from the potassium salt of the mercaptans
and the normal alkyl bromides.6 The tubes were
of 2-cc. capacity and the hydrocarbon was in
slight excess. The heating was for ten hours and
180'. The results given in Table I11 show con-
clusively that the abnormal addition took place
with these hydrocarbons which had been exposed
to air for some weeks.
In another series tridecylene was added to
dimercaptan~.~ The results are given .in Table
IV.
In one experiment lauryl mercaptan was added
to allyl lauryl sulfide CI2Hz6SCH2CH=CH2 and
the product was C B H ~ ~ ( C H ~ ) ~ S C ~ Z H ~ ~ , m. p.
47O, proved by melting point and mixed melting
point to be identical with that from lauryl bro-
mide and trimethylene mercaptan.
Experimental
Ten moles of ethylene bubbled at the rate of 50 cc. per
minute through a 25-cm. layer of sulfur in a vertical tube
kept at 325' gave much hydrogen sulfide. The 25 cc. of
condensate gave 15 g. of ethyl mercaptan, b. p. 34-37',
Hg(SEt)s, m. p. 76', 2 g. of carbon disulfide and 3 g. of
ethyl sulfide, b. p. 91-92', Et&HgCle, m. p. 76.5". Five
moles passed at the rate of 20 cc. per minute through a 20-
cm. column of ethyl tetrasulfide at 140-150" gave 30 g. of
condensate, largely ethyl sulfide but containing 10 g. of
ethyl mercaptan Hg(SEt)*, m. p. 76', and 1.5 g. of ethylene
b. p. 54-57', S calcd. 53.36, found 53.09 and 53.12.
(5) Prepared by Kozacik and Reid, ibid. , 60, 2436 (1938).
(6) The alkyl bromides were those described by Meyer and Reid,
(7) Hall and Reid, unpublished results.
( 8 ) DelCpine. Bull. SOC. chim., 788, 03 (1923).
ibid. , 66, 1574 (1933).
Vol. 60 2454 s. 0. JONES AND E. EMMET REID
TABLE 111
MELTING POINTS AND ANALYSES OF SULFIDES OBTAINED BY ADDITION COMPARED WTE THOSE OF SYNTHETIC SULFIDES
Un-aturata
CHa( CHz)aCH=CHz
CH3( CHi)sCH=CHz
CHs( CHZ)ECH=CHZ
CH3( CHz)aCH=CH?
CHa( CHz)ioCH=CHn
CHa( CHz)ioCH=CHz
CHs(CHz)ioCH=CHz
CH3( CHz)ioCH=CHs
CH3( CHz)izCH=CHz
CH~(CH~)I~CH=CHI
CHa(CH1)13CH=CHz
CHB( CHi)izCH=C€Iz
CH,(CHz)i4CH=CHi
CHa( CHn)irCH=CHs
CHa(CHz)i4CH=CH?
CH3(CHz)irCH=CHz
C€Is( CHi)isCH=CHs
CH3(CHz)isCH=CHs
CH3( CHz)lsCH=CHx
CH3(CHz)isCH=CHz
BY synthetic addition
Sulfide M . p , ' C . hl. p., OC
33.8
20.8
46.8
37.2
43 .8
40.2
54.6
39.2