The initiation and propagation of isobutylene (IB) polymerization initiated by 1,2-cpoxi-2,4,4-trimethylpentane (TMPO-1)/TiCl4 was monitored by a new fiber-optic transmission mid-IR probe. The real-time IR data provided insight into the initiation mechanism. Polyether Formation, isomerization of the TMPO-1 into 2,4,4-trimethylpentanal, and its complexation by TiCl4 were observed, which occurred simultaneously with initiation of IB polymerization. The proposed initiating mechanism involves the formation of tertiary carbocations, which has been claimed to occur in the cationic polymerization of epoxides by the S(N)1 mechanism. On the basis of our results, the competitive occurrence of both SN1 and S(N)2 pathways is proposed. Interestingly, ketone/TiCl4 systems were found to initiate IB polymerization, albeit with very low efficiency. IR monitoring of these systems gave additional insight into the initiating mechanism. IB polymerization was monitored by following the disappearance of the second overtone of the C-H wag in the =CH2 group in IB at 1780 cm(-1) and the C=C stretch at 1655 cm(-1). The linearity of first-order monomer consumption plots and the production of nearly uniform polyisobulylenes (PIBs) (M-w/M-n = 1.06-1.13) indicated living conditions.