End-quenching of quasiliving carbocationic isobutylene polymerization with hindered bases:: Quantitative formation of exo-olefin-terminated polyisobutylene

Polyisobutylenes possessing exclusively exo-olefin end groups were created by end-quenching TiCl4-catalyzed quasiliving isobutylene (IB) polymerizations with a hindered base at -60 to -40 degrees C. Polymerizations were initiated from either 2-chloro-2,4,4-trimethylpentane (TMPC1) or 1,3-bis(2-chloro-2-propyl)-5-tert-butylbenzene, in 60/40 hexane/methyl chloride in the presence of 2,6-dimethylpyridine, allowed to reach 98+% isobutylene conversion, and then reacted with either 2,5-dimethylpyrrole, 1,2,2,6,6-pentamethylpiperidine, or 2-tert-butylpyridine for various times ranging from 10 to 170 min. Typical reaction molar concentrations were [IB] = 0.5, [TMPCl] = 0.014, [26Lut] = 0.010, [TiCl4] = 0.083, and [hindered base] = 0.02-0.04 M. In some cases, minor amounts of coupled PIB were produced through reaction of carbenium ions with e.ro-olefin. Coupling was suppressed by higher temperature and lower chain end concentration. 2,5-Dimelhylpyrrole was the most effective quencher under the conditions studied, yielding the most rapid quenching and exhibiting the least tendency toward coupling.