Mechanistic role of Lewis bases and other additives in quasiliving carbocationic polymerization of isobutylene

lsobutylene was initiated using 5-tert-butyl-1,3-di(2-chloro-2-propyl)benze ne/TiCl4 in 60/40 hexane/methyl chloride: [IB](0) = 1.0 M, [TiCl4] = 0.12 o r 0.24 M, [t-Bu-m-DCC] = 0.0119 M, T = -(40-80) degreesC. Most polymerizati ons contained a Lewis base or other additive, i.e., 2,4-dimethylpyridine, 2 ,6-di-tert-butylpyridine, tetra-n-butylammonium chloride, and/or pyridine h ydrochloride. Polymerizations containing an additive yielded theoretical mo lecular weights, narrow polydispersity index, and apparent absence of irrev ersible chain termination (linear kinetic plots, ATR-FTIR spectroscopic dat a) and chain transfer with two exceptions: coupled product was obtained at -40 degreesC, and protic initiation occurred with n-Bu4NCl alone. Polymeriz ations without an additive produced bimodal molecular weight distributions; however, essentially all chains were initiated from t-Bu-m-DCC. With an ad ditive, E-act for propagation was -(5.3-5.5) kcal/mol. Removal of additives increased polymerization rate moderately at -80 degreesC but dramatically at -60 and -40 degreesC; this yielded higher E-act compared to that of syst ems containing additives. These results indicated that both paired and unpa ired (free) ions are propagating species in absence of additives, with free ions less important at lower temperatures; free ion concentration and life time suggested the presence of adventitious common ions and chain transfer between free ions and tert-chloride-terminated PIB chains. The primary role of additives is suppression of free ions through in situ production, via t he scavenging of protic impurities, of common ions.