Studies on the Nature of Propagating Species in Cationic Polymerization of Isobutyl Vinyl Ether by Iodine

In order to elucidate the nature of the propagating species in cationic polymerization, isobutyl vinyl ether was polymerized by iodine in solvents of different polarities, both in the presence and absence of a common-ion salt (tetra-n-butylammonium tri-iodide). The molecular weight distribution (MWD) of polymers produced in salt-free methylene chloride at 0°C was broad. With increasing salt concentration, the amount of high-molecular-weight polymer decreased, and the MWD was very similar to that for the polymers produced in carbon tetrachloride. The polymerization rate was also reduced by the salt and approached a constant value. These results indicate the existence of two kinds of propagating species in different dissociation states in methylene chloride: dissociated and non-dissociated species. In spite of the existence of the two propagating species, however, the polymers did not show a bimodal MWD in methylene chloride. This, coupled with the uniformity in tacticity of the polymers, suggests a rapid exchange between the two species. For comparison, polymerizations with acetyl perchlorate were carried out in a variety of solvents.