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Recent Evidence for the Association of Initiator with Counter Ion in Cationic Polymerization

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Recent Evidence for the Association of Initiator with Counter Ion in Cationic Polymerization Polymer Journal, Vol. 12, No. 9, pp 65i-659 (1980) Recent Evidence for the Association of Initiator with Counter Ion in Cationic Polymerization Pierre SIGWALT and Georges SAUVET Universite Pierre et Marie Curie, Laboratoire de Chimie Macromoleculaire, 4, Place Jussieu, 75230 Paris, Cedex 05, France. (Received April 15, 1980) ABSTRACT: Spectroscopic studies (by UVand NMR) of the reaction of 1,1-diphenylethylene with trifiuoromethanesulfonic acid have been made in dichloromethane, at temperatures (- 30°C to - 90°C) at which the carbocations are stable. A 33% average yield in carbocations with respect to CF3 S03 H was found in a large concentration range and interpreted as a complexation of the CF3S03 - anion by 2 inactive acid molecules. For 3-phenylindene, the yield was 50%. Further model studies have shown that various trifiic salts (of Ph3 C +, Bu4 N +, Ag +) are strongly solvated by trifiic acid, and the results are in agreement with the formation of complex counter ions of structures A -(HA); A -(HA)2 and A -(HAh. The implications of these results for cationic polymerizations are discussed and particularly the interpretation of various observations made in the literature about the effect of common salt addition and of a variation of the dielectric constant of the medium. KEY WORDS Cationic Initiation I Trifiic Acid I Salt Effects I Anion Solvation 11, 1-Diphenylethylene I Trifiuoromethanesulfonic Acid I Cationic Polymerization I The central problem of cationic polymerization of added water on the polymerization rate. But water ethylenic monomers is still that of the nature and may inhibit the polymerization when it is premixed concentration of the active species, and is linked (in equimolar ratio) to the perchloric acid. Several with the mechanism of initiation. It has not been kinetic data about monomer consumption are in really solved in the case of protonic acid initiation agreement with a solvation of the active perchlorate generally still written as a simple electrophilic ester by 4 styrene molecules, the propagation being proton addition on the double bond as in the written2 : pioneer work of Pepper with the styrene-sulphuric 4M + M -----> acid system. 1 I Ph The system styrene-perchloric acid in various 4M chlorinated solvents has been one of the most I I studied, but the kinetics are quite different at Ph Ph various temperatures. Near room temperature (in Initiation by acetyl perchlorate at ooc also gave a CH2Cl2 or CH2 ClCH2 Cl solution), polymerization reaction without visible species and without kinetic is slow and without kinetic termination, but there is no visible species during the propagation step in the termination, but a bimodal molecular weight (MW) distribution was obtained for the polymer3 and 300--450 nm range. In their pseudocationic theory, attributed to the independent growth of two types of Gandini and Plesch2 proposed that the active active species, a non-dissociated one, giving the low­ invisible species are esters, but this has been molecular-weight peak, and another dissociated challenged by other authors who suggested an one, giving the high molecular weights. The latter equilibrium reaction giving a very low concen­ tration of ionic species, the ester being inactive. could be suppressed by addition of tetraalkylam­ monium perchlorate or by using solvents of lower However, the best argument for the pseudocationic theory remains that there is a very limited effect of dielectric constants. 651 P. SIGWALT and G. SAUVET Bimodal distributions were also observed with of Bu4 N+, S03CF3- to the system. But this effect perchloric acid at ooc, with a reduction of the high­ did also occur in benzene, showing that a simple molecular-weight peak by salt addition.4 "salt effect', cpuld not explain all the results. For At low temperatures (-70°C to - 98°C in dielectric constants of about 6-8, bimodal distri­ CH2Cl2), the kinetics are completely different, butions were obtained for the polymer, and attri­ polymerization being very rapid and completed in a buted to the presence of two types of independent few seconds, with limited yields.4 •5 Stopped-flow active species, dissociated and non-dissociated. methods permitted the detection of ionic species Unimodal distributions were obtained for low absorbing at 340 nm, supposed to be carbocations, (benzene) and high (nitrobenzene) dielectric but with maximum concentrations of about I% of constants. the initiator only. The presence of perchlorate end The use of stopped-flow experiments permitted groups (unreactive with styrene at low temperature) the observation of transient cationic species in the is likely since block copolymers could be prepared system styrene-CF3S03H-CH2 Cl/1 by recording with high yields with N-substituted aziridines ini­ of rapid-scan spectra. A peak at 340 nm was attribut­ tiated at low temperature.6 However, it should be ed to the polystyryl cation, and had very short life­ noted that such type of end groups could not be times of 200-300 ms at 30°C. The maximum con­ detected by the NMR technique for polymerizations centration [P+lmax observed was about 1% of realized in CC14 at room temperature. 7 CF3S03H. From the visible active species con­ Several other features of these polymerizations by centrations kP values could be obtained (2 x 105- perchloric acid are not completely explained, such 4 x 105 M -l s -l at 30°C). But contrarily to what as the increase of the apparent kP with an was found by Higashimura et a!. for the same increase of [M]0 and with an in salt system, bimodal distribution was not observed in concentration for the low-temperature reaction.4 this case. More recent studies have used the strongest Very recently, Kunitake et a!Y published the protonic acid available, trifluoromethanesulfonic results of their investigations between - I oc and (triflic) acid, that gives high polymerization rates 30°C, giving a positive activation energy for the even at high temperatures. Three main series of propagation of 7 kcal mol-1. The effect of [P+lmax studies have been made, by Chmelir et a!., concentration on kP was studied, and assuming an Higashimura et at., and Kunitake et al. ion-pair-free ions equilibrium, kP values of0.5 x 105 Styrene polymerization by triflic acid ([C] 10-4 to 1 x 105 were obtained at ooc. The effect on kP of moll-1) in dichloromethane solution at -l5°C8 •9 an addition of a common ion salt (Bu4 N+, was rapid and completed in a few minutes when the CF3S03 -) was also studied, and assuming a initiator concentration was high enough. S-shaped straightforward concentration effect of Bu4 N+, curves were observed for low [C]. There was also a CF3S03- kp ( +) (about 3 X 105 at ooq and ko of strong negative effect of monomer concentration on the ionic species ( X 10-6 at 0°C) COUld be the rate8 that was attributed to an inactivation of the calculated (with an estimated dissociation constant initiator through complexation with the monomer K,=5 x 10-5 for the ammonium salt). However, a ([M] varying between 0.05 and 0.5 mol1-1). very strange result was obtained for the activation In order to explain the kinetic results and energy on free ions: EP(±)= 14 kcal mol-1 (whereas conductimetry experiments, initiation was supposed the activation energy on ion-pairs was to occur through ionization of acid aggregates, such EP( ±) = 5 kcal mol-1). The first value looks quite as unlikely if one compares those obtained for cationic polymerizations under radiation. It should be observed that in most studies10 - 13 Similar results were observed by Higashimura et the initiator was used without further purification, a!., 10 who, however, found a strong positive effect of and that in one case10 the water concentration was an increase of the dielectric constant of the medium estimated to be ca. 3 x I 0-4 moll-1 which was of on the rate constant kP and attributed the monomer the same order as that of the initiator. concentration effect to this phenomenon. These Another recent study is that of p-methoxystyrene authors also found a decrease of kP on the addition polymerization by stopped-flow methods.13 The 652 Polymer J., Vol. 12, No. 9, 1980 Association of Initiator with Counter Ion active centers for this monomer were known to be received in graduated tubes equipped with break much more stable than for styrene14 near room seals, that were sealed. It remained prefectly temperature and the maximum [P+lmax was observ­ colourless even after several months. ed at 30°C for longer times and with much higher Tetrabutylammonium triflate was purified ac­ concentrations. This permitted the measurement of cording to Higashimura et a/. 10 kP on these visible species (about 105 M -l s -l ). The Triphenylmethyl triflate samples could not be catalyst efficiency ([P+lmax/[CF3 S03 H]) varied from isolated in the pure state and were studied in situ by 36 to 42% in CH2 Cl2 . Two observations were, UV and NMR. Ph3C+, CF3 S03 - was prepared by however, made: the maximum concentration of p+ reacting Ph3 CC1 with AgCF3 S03 in CH2Cl2 , and decreased considerably with the dielectric constant Ph3C+, (CF3S03 -, CF3 S03H) was obtained by D of the solvent, while the corresponding kP reacting Ph3C-OH with triflic anhydride. increased when D became lower. This was explained by assuming the formation, in solvents of low Spectrophotometry polarity, of active "invisible species." A similar Measurements were performed under vacuum in effect was observed on the addition of a common sealed vessels, using Cary 15 and Cary 118C ion electrolyte, which led to a decrease of [P+lmax spectrophotometers. intensity and to an increase of kv. The initiation reaction was followed using an From this discussion, it may be seen that the apparatus for relatively fast spectrophotometric nature, the relative proportions and the reactivities measurements, 19 the acid phial being broken inside of ionic and non-ionic species involved in protonic the monomer solution.
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