Mechanistic Role of Lewis Bases and Other Additives in Quasiliving Carbocationic Polymerization of Isobutylene

Isobutylene was initiated using 5-tert-butyl-1,3-di(2-chloro-2-propyl)benzene/TiCl4 in 60/40 hexane/methyl chloride: [IB]0 = 1.0 M, [TiCl4] = 0.12 or 0.24 M, [t-Bu-m-DCC] = 0.0119 M, T = −(40−80) °C. Most polymerizations contained a Lewis base or other additive, i.e., 2,4-dimethylpyridine, 2,6-di-tert-butylpyridine, tetra-n-butylammonium chloride, and/or pyridine hydrochloride. Polymerizations containing an additive yielded theoretical molecular weights, narrow polydispersity index, and apparent absence of irreversible chain termination (linear kinetic plots, ATR−FTIR spectroscopic data) and chain transfer, with two exceptions: coupled product was obtained at −40 °C, and protic initiation occurred with n-Bu4NCl alone. Polymerizations without an additive produced bimodal molecular weight distributions; however, essentially all chains were initiated from t-Bu-m-DCC. With an additive, E act for propagation was −(5.3−5.5) kcal/mol. Removal of additives increased polymerization rate moderately at −80 °C but dramatically at −60 and −40 °C; this yielded higher E act compared to that of systems containing additives. These results indicated that both paired and unpaired (free) ions are propagating species in absence of additives, with free ions less important at lower temperatures; free ion concentration and lifetime 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 the scavenging of protic impurities, of common ions.