The cationic polymerization of a-methylstyrene was carried out at-78°C by using Lewis acids and triphenylmethyl salts as initiator. The polymerization with triphenylmethyl salts was much slower than that with Lewis acids. The triad tacticity data of the polymer determined by NMR spectroscopy fitted the theoretical curve of one parameter (Bovey's O' value). The O' value, though almost independent of the initiator used in CH2Clz-CH"CN (0'~0.04), increased with decreasing polarity of polymerization solvents and varied moderately (0.07-0.34) with the initiator in nonpolar solvents. In the case of triphenylmethyl salt initiators, the order of counteranions with increasing O' value (SnCI5-:2;SbCl5->AlBr4-> AICI.->BF4-) was the same as that observed for the isotactic propagation of isobutyl vinyl ether. These results were interpreted within the framework of the cationic propagation scheme proposed by us earlier. The steric influence of counteranions was discussed, and the unusual effect of SnCh-was considered in connection with its coordination state. The variation of the (}' value with Lewis acids (SnCI. excluded) was much smaller than that with triphenylmethyl salts.

ABSTRACT:The cationic polymerization of isobutyl vinyl ether was carried out at -75°C by using triphenylmethyl salts (Ph3C+X-, X-=BF4-, AlCl4-, AlBr4-, SbCk and SnCh-J as initiator. The polymer was shown to contain the triphenylmethyl group. The extent of chain transfer, determined from the content of the triphenylmethyl group, was 0-4.4 in all the systems studied. These values are much smaller than those reported with BF3OEt2. The steric structure of the polymer was inferred from the precipitation temperature of the polymer in methyl ethyl ketone, Tp, as suggested by Okamura, et al. Tp decreased with increasing polarity of polymerization solvents. In less polar media, Tp decreased in the following orderThe observed order of Tp agreed with that of anion sizes, except for SnCh-. Thus it is suggested that the steric effect of the counteranion is an important factor in deciding the steric course of propagation for tight ion-pairs. These results were explained within the framework of the steric course of the cationic propagation which we had proposed previously.

Cyclopolymerization of o‐divinylbenzene was studied by using triphenylmethyl fluoroborate as initiator. The cyclization constant rc (ratio of the rate constant for intramolecular cyclization to that for intermolecular propagation) decreased with increase in polarity of the solvent. This solvent effect as well as the effect of catalysts reported previously could be accounted for by a revised model of the propagating species, where both the counter anion and the o‐vinyl group were coordinated to the carbonium ion. The difference in activation energy between intramolecular cyclization and intermolecular propagation was 6.2 kcal/mole. This value was close to that obtained previously with BF3·OEt2 catalyst but greater than those for anionic and radical polymerizations. When tetraalkylammonium flouroborates were added to the polymerization system in methylene chloride, rc and the molecular weight of the polymer decreased. These results were ascribed to the decrease in the amount of the free propagating cation by the common ion effect.