N-Methylpyrrole-Terminated Polyisobutylene through End-Quenching of Quasiliving Carbocationic Polymerization

Abstract

Quasiliving isobutylene polymerization initiated by 2-chloro-2,4,4-trimethylpentane/TiCl4/2,6-dimethylpyridine in 60/40 n-hexane/methyl chloride at −70 °C was allowed to reach 98+% monomer conversion and then reacted with N-methylpyrrole. All polyisobutylene (PIB) chains alkylated the N-methylpyrrole ring to form a mixture of 46% 2-PIB−N-methylpyrrole and 54% 3-PIB−N-methylpyrrole. GPC indicated the absence of coupled PIB, confirming that exclusively monosubstitution had occurred. Complete 1H and 13C NMR chemical shift assignments were made for both isomers. The product was converted exclusively to mixed 2- and 3-PIB−N-methylpyrrolidine by catalytic hydrogenation using PtO2 in glacial acetic acid. Quantitative 1H NMR integration of PIB initiated from the difunctional aromatic initiator, 5-tert-butyl-1,3-di(2-chloro-2-propyl)benzene, showed exactly two N-methylpyrrole end groups per aromatic initiator residue. Quantitative reaction of PIB chains with N-methylpyrrole could not be obtained with BCl3 systems. In methyl chloride diluent at −45 °C, <10% N-methylpyrrole capping was obtained after 70 min; in 1,2-dichloroethane at −10 °C, 77% of the PIB chains reacted with N-methylpyrrole after 15 min, and no further reaction was observed up to 18 h. In both BCl3 systems, GPC analysis showed the product to be unimodal, indicating the absence of coupling through dialkylation of N-methylpyrrole.