Polymer International 47 (1998) 345È350

Addition–Fragmentation Type Initiation ofCationic Polymerization using

Allyloxy-pyridinium Salts

Vildan Bacak,1 Ivo Reetz,1 Yusuf Yagci1,* & Wolfram Schnabel2

1 Istanbul Technical University, Organic Chemistry Department, Maslak, Istanbul 80626, Turkey2 Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, D-14109 Berlin, Germany

(Received 28 April 1997 ; revised version received 15 August 1997 ; accepted 18 February 1998)

Abstract : Allyloxy-pyridinium salts with various substituents on the allylicmoiety are shown to be very efficient coinitiators in radical promoted cationicpolymerization of cyclohexene oxide. Depending upon the radical initiatorchosen, cationic polymerizations may be initiated by either heat or light. For themost part, the mechanism of initiation involves the addition of free radicalsformed by the radical initiator, and a subsequent fragmentation of the energy-richintermediate yielding initiating pyridinium type radical cations. In additionÈfragmentation polymerization, the substituent at the allylic moiety does not sig-niÐcantly inÑuence the polymerization rate, thus implying that fragmentation isthe rate determining step. In some cases, oxidation of the primarily formed freeradicals contributes to the formation of initiating species. The salts under investi-gation are also able, to various extents, to initiate cationic polymerization uponexternal stimulation (heating or UV irradiation) without added radical initiators.

Society of Chemical Industry( 1998

Polym. Int. 47, 345È350 (1998)

Key words : pyridinium salts ; radical promoted cationic polymerization ;addition-fragmentation reactions ; photopolymerization

INTRODUCTION

Cationic polymerizations induced by external stimu-lation,1 such as heating or photoirradiation are of greatpractical interest due to their applicability for the curingof coatings and printing inks, and for resist technology.Recently, we introduced systems consisting of speciallydesigned allyloxy-onium salts (addition-fragmentationagent) and a radical initiator for radical promotedcationic polymerization. In most cases under investi-gation, initiation follows the radical additionÈfragmentationÈpolymerization scheme. Upon choosingappropriate radical initiators, one can easily tune poly-merization conditions to a desired wavelength or tem-perature range for cationic photo- and thermal

* To whom all correspondence should be addressed.Contract/grant sponsor : Istanbul Technical UniversityResearch Fund.

polymerization, respectively. The allyloxy-onium saltsfound to be useful so far include an allylesterÈtetrahydrothiophenium compound,2,3 a N-allyl pyri-dine salt4 and various N-allyloxy pyridinium deriv-atives.5h7 The latter are particularly suitable foradditionÈfragmentation reactions, because the NwObond is very weak and may easily be ruptured in thecourse of fragmentation, giving rise to initiating pyridin-ium radical cations. This paper describes the inÑuenceof substitution on catalytic activity for the followingallyloxy-pyridinium salts :

R\ H (AP) this paperCH3 (MAP) see also ref 5C(O)OCH2CH3 (EAP) see also ref 6Ph (PAP) see also ref. 7

3451998 Society of Chemical Industry. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

346 V . Bacak et al.

In contrast to previous studies,5h7 this work deals withexperiments that allow conclusions to be drawn regard-ing the inÑuence of substituents, thus enabling a deeperinsight into the reaction mechanism to be gained.

In order to demonstrate the role of additionÈfragmentation type initiation, polymerization capabilityis viewed in comparison with N-ethoxy-a-picoliniumhexaÑuorophosphate (EMP).8h12

Regarding the initiating species formed (pyridiniumradical cations), EMP resembles allyloxy-picoliniumsalts. However, it is not able to undergo additionÈfragmentation reactions.

EXPERIMENTAL

Materials

Synthesis of AP. Allyl bromide (0É5 g, [98% Fluka) and2-picoline N-oxide (0É45 g, 96%, Aldrich) were stirredfor 20 h at room temperature. The whitish precipitateformed was Ðltered o†, washed with diethyl ether anddried. Then it was dissolved in 2 ml distilled water towhich (0É6 g ; Merck) was added. After 2 h stir-AgNO3ring in the dark, AgBr was removed by centrifugationand to the aqueous solution was added 0É6 g of NaSbF6in one portion. After short stirring, AP precipitated aswhitish, bulky crystals. These were washed carefullywith water and diethyl ether before drying (m.p. 45¡C,yield 9É4%). 1H NMR (d in ppm) : 8É7È7É8, 6É0,Harom ;5É4 and 4É9, 2É7, 1É7,Holef ; CH2wOwN` ;

IR (main bands, l in cm~1) : 3133, 3110,CH3wpyridine.3081, 3072, 3035, 2987, 1734, 1684, 1652, 1616, 1558,1506, 1500, 1466, 1429, 1368, 1299, 1267, 1170, 115, 998,962, 939, 920, 838, 783, 693, 654. UV: jmax\ 265 nm,

Elemental analysis :e265 nm \ 6.2 ] 103 l mol~1 cm~1.(385.7 g mol~1). Calculated : C, 28É0%;C9H12NOSbF6

H, 3É11%; N, 3É63%. Found: C, 29É4%; H, 3É35%; N,3É49%.

Other materials. The synthesis of EMP,9 MAP,5 EAP6and PAP7 has been described elsewhere.

Cyclohexeneoxide (CHO) was dried over andCaH2distilled. Benzoin (Fluka) was recrystallized fromethanol ; benzoyl peroxide (BPO; Fluka) was rec-rystallized from diethyl ether. Phenylazotriphenylme-thane (PAT) was prepared as described previously.13

Polymerization

For thermal polymerizations, the monomer, theallyloxy-picolinium salt and, in some experiments, the

respective free radical initiator, were mixed in Pyrextubes that were closed with a TeÑon stopcock after bub-bling through with nitrogen. These tubes wereimmersed for a given time in an oil bath kept at con-stant temperature. Quartz tubes were used for photopolymerizations. Photo polymerizations at 280 nm wereperformed with a monochromatic light source equippedwith a xenon lamp (XBO75 W/2) and a mercury lamp(HBO100 W/2). For the experiments with benzoin, amerry-go-round type photo reactor with 16 Philips8W/06 lamps was used, emitting overwhelmingly lightat about 367 nm.

Characterization

Polymers were obtained from the reaction mixture byprecipitation with methanol. In some cases, the meth-anol soluble fractions were analysed with a gas chro-matograph operated in conjuction with a massspectrometer (GCÈMS). UV-vis spectra were recordedon a Perkin Elmer Lambda 2 spectrophotometer.

RESULTS

For the investigations described here, cyclohexene oxide(CHO) was deliberately chosen as a model monomer.CHO is not polymerizable by a radical mechanism anddoes not form oxidizable radicals in the course of poly-merization. Also, it is not polymerizable by pyridiniumsalts in the dark at room temperature, like vinyl estersor N-vinyl carbazol.5,8

Thermal polymerization of CHO

Polymerization in the absence of additional radical initi-ators. As seen in Fig. 1, in the absence of additionalradical initiators, the polymerization of CHO takesplace with satisfactory yield only at relatively hightemperatures. The efficiency in this mode of cationic

Fig. 1. Thermal polymerization of CHO: [pyridinium salt]\5]10~1moll~1, reaction time 1h ; conversion\(M0[M)/M0 .

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Initiation using allyloxy-pyridinium salts 347

Fig. 2. Thermal polymerization of CHO with pyridinium saltsin conjunction with BPO as radical source : [pyridiniumsalt] \ [BPO]\ 5 ] 10~3mol l~1, at 70¡C; conversion \

(M0 [ M)/M0 .

polymerization rises in the orderEMP\ EAP\ AP\ MAP\ PAP. Notably, withEMP there is only little polymer formed after 1 h at125¡C.

Fig. 3. Thermal polymerization of CHO with pyridinium saltsin conjunction with PAT as radical source : [pyridiniumsalt] \ [PAT]\ 5 ] 10~3mol l~1, at 70¡C; conversion \

(M0 [ M)M0 .

Fig. 4. Photo-polymerization of CHO: 20¡C;jinc\ 280 nm,[AP]\ 5É6 ] 10~4mol l~1 ; [MAP]1]OD280 nm \ 0É95.

10~3mol l~1 ; [PAP]\ 3É4 ] 10~4mol l~1 ; [EAP]\ 1 ]10~3mol l~1 ; [EMP]\ 1É3 ] 10~3mol l~1 ; conversion \

(M0 [ M)M0 .

E†ect of added radical initiator. The time conversioncurves for CHO polymerization at 70¡C presented inFigs 2 and 3 clearly suggest that for all allyloxy pyridin-ium salts, the addition of a free radical initiator pro-motes cationic polymerization. On the contrary, in thecase of EMP, no e†ect of the added radical initiators onthe initiation efficiency was found.

The two radical initiators chosen, BPO and PAT,show slightly di†erent behaviours. Firstly, poly-merizations in the presence of BPO are slower than inthe presence of PAT owing surely to the di†erentradical forming capability of the two initiators (thedecomposition rate constants are 1É1 ] 10~5 s~1 (70¡C)and 3É5 ] 10~4 s~1 (55¡C) for BPO14 and PAT,15respectively). In the case of BPO as radical initiator,similar polymerization rates were obtained with all ally-loxy salts ; for AP only a slightly longer inductionperiod was observed. With PAT, however, very fastpolymerization was found for AP and EAP, whereaslower rates were detected with PAP and MAP.

In all time-conversion curves, short induction periods(usually of several minutes) were observed. These havepreviously been seen for a variety of pyridinium typecationic initiators.4,9 They are attributable to traces ofimpurities remaining in the system even after extensivecleaning of the initial compounds and purging the solu-tions with nitrogen. At the beginning of polymerization,these impurities react with initiating species which areobviously consumed by this process.

Photo-polymerization of CHO

Excitation of the pyridine moiety. The absorptionspectra of allyloxy pyridinium salts show a nÈn*absorption band with a maximum at about 265 nm,characteristic of pyridine derivatives. The wavelength ofthe incident light chosen in this study for directly excit-ing the pyridine moiety (280 nm) lies within the absorp-tion tail which reaches up to approximately 290 nm.The salt concentrations were adjusted in order to obtainidentical optical density values in all experiments

As Fig. 4 shows, fast poly-(OD280 nm\ 0É95).merizations were found for all allyloxy salts. EMP, incontrast, initiates relatively slowly.

Use of additional radical type photo initiators. In photo-polymerization with allyloxy salts, additional radicalinitiators may not only serve to improve the time con-version behaviour, they also enable light of higherwavelengths to be used for initiating polymerizations.As stated in the previous paragraph, if allyloxy pyridin-ium salts are to be excited directly, light of wavelengthsbelow 290 nm has to be employed. With, for example,benzoin as a radical initiator, light of up to 400 nm canbe used. Figure 5 shows that with benzoin in conjunc-tion with all allyloxy pyridinium salts, CHO poly-

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348 V . Bacak et al.

Fig. 5. Photo-polymerization of CHO with benzoin : jinc B367 nm, 20¡C; [Pyridinium salt]\ 5 ]OD367 nm\ 0É10.10~3mol l~1 ; [benzoin]\ 2É4 ] 10 ~2mol l~1 ; conversion\

(M0 [ M)/M0 .

merizes very quickly. The polymerization rate isexceptionally high for EAP.

DISCUSSION

Thermal initiation

It is believed that for most allyloxy-onium salt/radicalinitiator combinations, the polymerization is initiated byan additionÈfragmentationÈpolymerization scheme, asdepicted in eqns (1È4).

Support for a polymerization according to eqns (1È4)may be derived from the fact that high polymerizationrates are obtained only in the presence of free radicals,i.e. by thermally stimulating free radical initiators. ForEMP, where additionÈfragmentation reactions can beexcluded but where pyridinium radical cations are alsoformed as initiating species, a much lower initiation effi-ciency was observed.

Low molecular weight products, predominantly ofthe epoxy type, formed in reaction (3) were detected bygas chromatography in conjunction with mass spectros-

copy (GCÈMS) in the case of EAP. With PAP forexample, GCÈMS investigations under similar condi-tions gave no evidence of the formation of epoxide pro-ducts, giving rise to the assumption that in some casesthe epoxide groups may react with the growing polymerchains, or directly with pyridinium radical cations, andthus be incorporated into polymer chains. Evidence ofpyridinium terminated poly(CHO) was found on thebasis of UV measurements. The UV spectrum of thepolymer obtained shows an absorption maximum atabout 270 nm which may be ascribed to pyridiniumgroups. Bulk poly(CHO) has no intrinsic absorption inthis region.

An interesting insight into the reaction mechanisms isgained from the fact that for BPO there is virtually noinÑuence of the substituent at the allylic double bond.From radical chemistry, it is known that the rate con-stant for the addition of radicals to oleÐnic bondsdepends strongly on the substituents at the doublebond. As an example, for the addition of cyclohexyl rad-icals to substituted ethenes the relative(CH2xCHwR),rate constants are 1, 84 and 3000 for n-butane, phenyland carboxymethyl, respectively.16 Because no signiÐ-cant inÑuence of the substituents is found in the addi-tion fragmentation initiation with allyloxy pyridiniumsalts, addition cannot be the rate determining step. Itseems likely that fragmentation is rate determining. Theinitiation itself, i.e. the reaction of pyridinium radicalcations with monomer, is a relatively fast process withbimolecular rate constants of k \ 106È107 l mol~1 s~1for CHO.12

In contrast to BPO, PAT forms radicals(triphenylmethyl type radicals), which are relativelyeasily oxidized. The oxidation of triphenylmethyl rad-icals by various onium salts has previously been usedfor initiating cationic polymerizations.17,18 With EAPand AP, characteristic UV absorption bands of tri-phenylmethyl carbocations were found between 410 and440 nm upon heating solutions consisting of PAT andthe respective allyloxy salt (see Fig. 6). For all otherallyloxy initiators and for EMP, this absorption did notoccur under similar reaction conditions. Most inter-estingly, it is EAP and AP which show an extremelyrapid polymerization. Therefore, it is expected that amechanism di†erent from additionÈfragmentation,namely oxidation of triphenylmethyl radicals asdepicted in eqns (5È7), accounts to a large extent for theinitiation.

As Fig. 1 suggests, cationic polymerization may alsobe initiated by allyloxy pyridinium salts without addi-tional radical sources. Furthermore, it is assumed thatin this case, radicals are formed by thermal decomposi-tion of the salt. They subsequently add to double bondsof intact initiator units. This assumption is strongly sup-ported by the Ðnding that EMP is signiÐcantly lessprone to cationic initiation than allyloxy pyridiniuminitiator salts. If direct initiation via thermally induced

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Initiation using allyloxy-pyridinium salts 349

Fig. 6. UV changes in the system at 80¡C: [AP]\ [PAT]\ 5 ] 10~3mol l~1. Absorption occurring at aboutAP/PAT/CH2CI2440 nm is ascribed to conversion(Ph)3C` ; \ (M0[ M)/M0 .

rupture of the NwO bond were to account for the poly-merization, initiation should also occur to a similarextent with EMP. However, it is difficult to decidewhich radicals are formed and how di†erences in initi-ation between the various allyloxy salts can beexplained. Respective GCÈMS investigations did notyield conclusive results. An alternative explanationwould be that upon heating, the double bonds of theallyloxy initiators open, as is known for a variety ofmonomers, which would give rise to the formation of

biradicals. These species might also be active in addingto allylic double bonds.

Photo initiation

If the polymerization is initiated by UV light, poly-merization rates are also high in the absence of addi-tional radical sources. From EMP and other alkoxypyridinium type initiators, it is known that the NwObond is photosensitive and undergoes easily bond disso-ciation when the salt absorbs UV light.10 By this reac-tion, pyridinium radical cations are produced which can

initiate cationic polymerizations. NwO bond rupture isalso feasible with the allyloxy pyridinium salts present-ed here (as shown in eqn 8).

However, because the polymerization rates are drasti-cally higher as compared with EMP, there must beother processes accounting for the formation of initi-ating species. It is highly likely that the oxygen centredradicals formed upon NwO bond dissociation add toallylic double bonds of intact salt units, thus triggeringfragmentation and production of another pyridin-ium radical cation (according to eqns 2È4). Thus, uponabsorption of one photon, up to two initiating speciescould be produced, provided the quantum yield of theNwO bond rupture is unity, and the yields in addi-tion and fragmentation 100%. Again, there is no signiÐ-cant inÑuence of the substituent on the initiation effi-ciency, supporting the assumption stated earlier thatfragmentation rather than addition is rate determining.The photolytical NwO bond rupture is probably nota†ected much by the substituent at the allylic bond.

Benzoin has been chosen as an additional radical ini-tiator because it absorbs light in the near UV region,thus tuning the initiating system to wavelengths applic-able for practical purposes. The high efficiency in thiscase is again understood to be proof of the efficiency ofadditionÈfragmentation. Interestingly, the poly-merization rate for the initiation with EAP is higherthan for all other allyloxy pyridinium salts. As men-tioned above, EAP is most suitable for oxidizing freeradicals. In other words, the mechanism shown in eqns(9) and (10) involving the oxidation of hydroxyphenylradicals stemming from benzoin by EAP probablyaccounts for the faster initiation in the case of EAP.

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350 V . Bacak et al.

CONCLUSIONS

Allyloxy pyridinium salts are efficient coinitiators forcationic polymerization when used in conjunction withfree radical initiators. The mechanism follows eitherthe additionÈfragmentationÈpolymerization route, orinvolves the oxidation of free radicals, depending on thefree radical source used and the pyridinium salt. EAPand AP are the two representatives with the highest sus-ceptibility to oxidation reactions. Where addition frag-mentation is concerned, there is no inÑuence of thesubstituent, giving rise to the conclusion that fragmen-tation rather than addition is the rate determining step.Therefore, if an increase in initiation efficiency is sought,it is the ease of fragmentation that has to be improvedby choosing appropriate leaving groups and weakbonds in proximity to the allylic double bond. Suchinvestigations are now in progress.

ACKNOWLEDGEMENTS

One of the authors (I.R.) is grateful to the Alexandervon Humboldt-Stiftung of Germany for generous Ðnan-

cial support through the Feodor Lynen program. V.B.was kindly granted a stipend by the Turkish ScientiÐcTechnical Research Council (Tu� bitak). Financialsupport from the Istanbul Technical UniversityResearch Fund is also gratefully acknowledged.

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