The effect of the counterion composition on the chain restriction in the cationic polymerization of isobutylene

The effects of the nature of halogens in the initiating tert-butyl halide-aluminum-containing Lewis acid system on the number average molecular weight (M) over bar(n), and the structure of end groups of polyisobutylene macromolecules obtained in the cationic polymerization of isobutylene in hexane at -78 degrees C were studied. An increase in (M) over bar(n) is observed in the transition from chlorine to bromine and iodine, accompanied by a decrease in the fraction of end C=C groups and an increase in the relative content of C-Hal groups (Hal = Cl, Br, and I). When atoms of different halogens are present in the counterion, more bulky atoms preferentially participate in the formation of the end groups. The results are interpreted within the framework of the principle of hard and soft acids and bases.