Halogen-free polyisobutylene (PIB) was synthesized by in situ methylation of living PIE using dimethylzinc. Quantitative methylation of living PIE was achieved within 60 min using a ratio of [(CH3)(2)Zn]/[TiCl4](0) = 1 without any side reactions. Under similar conditions, living PIE capped with 1,1-diphenylethylene (PIB-DPE+) yielded close to 1:1 mixture of methoxy- and methyl-functionality. By using the ratio of [(CH3)(2)Zn]/[TiCl4](0) greater than or equal to 3, however, quantitative methylation of PIB-DPE+ could be achieved in 120 min without any side reactions as confirmed by spectroscopic and chromatographic analyses.