The counterion effect on the steric course of the cationic polymerization of isobutyl vinyl ether

The cationic polymerization of isobutyl vinyl ether was conducted with a variety of triphenylmethyl salts as initiators, and the steric structure of the polymer was determined from the methylene dyad peaks in the¹³C-NMR spectra. The fraction of the isotactic dyad increased with the increase in the counteranion size from BF4-to CIO₄⁻ to BC1₄⁻, and then decreased: A trend similar to that observed for ɑ-methylstyrene. The correlation seems to be more complex for larger counteranions, and a periodical influence was clearly noticed. These results were explained by considering the tightness of the propagating ion-pair. Pentacoordinate counteranions showed peculiar influences in that the polymer structure was influenced by the counteranion concentration. The molecular weight distribution was unimodal, suggesting that a single propagating species is present.