Studies on propagating species in cationic polymerization of styrene derivatives by acetyl perchlorate or iodine. IV. Common-ion salt effect on the polymerization rate and the steric structure of the polymer obtained by acetyl perchlorate

Effects of a common-ion salt, n-Bu₄NClO₄, on the cationic polymerization of styrene and p-chlorostyrene by acetyl perchlorate were studied in a variety of solvents at 0°C. In polymerization (in CH₂Cl₂) which yielded polymers with a bimodal molecular weight distribution (MWD), addition of the salt suppressed the formation of higher polymers, but affected neither the molecular weight nor the steric structure of the lower polymers. The polymerization rate decreased with increasing salt concentration and became constant at or above a certain concentration. In nitrobenzene, on the other hand, the MWD of the polymers was unimodal and steric structure was unchanged even in the presence of salt at a concentration 50 times that of the catalyst. However, the polymerization rate and the polymer molecular weight decreased monotonically as salt concentration increased. On the basis of these results, it was concluded that the ion pair in methylene chloride differs from that in nitrobenzene, and that the species in the latter solvent is similar in nature to free ions. The fractional contribution of the dissociated and nondissociated propagating species to polymer formation was determined from the rate depression caused by addition of the salt.