Carbocation rearrangement in controlled/living isobutylene polymerization

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Polymers and High Performance Materials


Isobutylene polymerization was initiated using the cumyl chloride/TiCl4/2,4-dimethylpyridine system ([IB] = 1.0 M, [CumCl] = 2.3 x 10(-2) M, [TiCl4] = 0.24 M, [DMP] = 2.4 x 10(-3) M). Polymerization kinetics in 60/40 hexane/MeCl at -80 degrees C showed that an IB conversion of similar to 98% was reached in 70 s. The active centers were monitored over time at high conversion, by H-1 NMR after quenching with MeOH or allyltrimethylsilane (ATMS), and by GPC after a second addition of IB. Quenching with MeOH or ATMS at times up to 70 s yielded quantitative tert-chloride or allyl chain ends, respectively. For longer periods up to 12 h, MeOH-quenched PIB showed increasing depletion of tert-chloride chain ends of the normal structure (but no appearance of olefin) and ATMS quenching yielded a gradual reduction in allylation to 78% at 12 h. Real-time H-1 NMR of an active polymerization mixture also indicated depletion of tert-chloride end groups. GPC results showed that coupling of PIE chains was not occurring and that a second charge of IB was slowly initiated. The latter process caused a fraction of the chain ends to be restored with normal tert-chloride groups. It was hypothesized that normal tert-chloride chain ends slowly undergo carbenium ion rearrangement to form a mixture of isomerized chain end structures. This process represents the dominant chain interrupting event in TiCl4-co-initiated IB polymerization under these conditions. The rearranged structures were hypothesized to result from combinations of 1,2-hydride and 1,2-methide shifts; H-1 NMR analysis of a dehydrochlorinated sample was consistent with this interpretation. The rate of depletion of tert-chloride end groups was shown to follow first-order kinetics with an apparent rate constant of 8 x 10(-5) s(-1). The ratio of rate constants for propagation and rearrangement, k(p)/k(r), was calculated to be 3 x 10(4) M-1.

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