Carbocationic Polymerization of Isobutylene Using Methylaluminum Bromide Coinitiators: Synthesis of Bromoallyl Functional Polyisobutylene

The carbocationic polymerization of isobutylene (IB) was studied in conjunction with AlBr3, MeAlBr2, Me1.5AlBr1.5, and Me2AlBr coinitiators in hexanes(Hex)/methyl chloride (MeCl) 60/40 (v/v) solvent mixtures at −80 °C in the presence of a proton trap, 2,6-di-tert-butylpyridine. The observed Mns were directly proportional to monomer-to-initiator ratio with 2-chloro-2,4,4-trimethylpentane (TMPCl) as initiator and MeAlBr2, Me1.5AlBr1.5, and Me2AlBr coinitiators; however, with AlBr3 the Mns are much lower than the theoretical values. Chain extension “incremental monomer addition” (IMA) experiments resulting in bimodal distributions demonstrate that termination is operational with Me2AlBr. With MeAlBr2 the chain-extended PIBs exhibited close to theoretical Mns, but the molecular weigh distribution was broad. Using Me1.5AlBr1.5, Mn of the polymers increased in direct proportion and the molecular weight distributions remained narrow. 1H and 13C NMR spectroscopy of the polyisobutylene (PIB) obtained with Me1.5AlBr1.5 suggested virtually quantitative bromo end functionality. With MeAlBr2 and Me2AlBr the bromo functionality was lower (0.8−0), decreasing with the increase of Lewis acid concentration and polymerization time. The capping reaction of living polyisobutylene cation (PIB+) with 1,3-butadiene (BD) in Hex/MeCl 60/40 (v/v) solvent mixtures at −80 °C was also studied in conjunction with methylaluminum bromide coinitiators. Quantitative crossover reaction from living PIB chain end to BD followed by instantaneous termination and selective formation of 1,4-addition product bromoallyl functional PIB (PIB-AllylBr) was obtained only with Me1.5AlBr1.5 coinitiator.