Carbocation Rearrangement in Controlled/Living Isobutylene Polymerization

Abstract

Isobutylene polymerization was initiated using the cumyl chloride/TiCl4/2,4-dimethylpyridine system ([IB] ) 1.0 M, [CumCl] ) 2.3 × 10 -2 M, [TiCl4] ) 0.24 M, [DMP] ) 2.4 × 10 -3 M). Polymerization kinetics in 60/40 hexane/MeCl at -80 °C showed that an IB conversion of ∼98% was reached in 70 s. The active centers were monitored over time at high conversion, by 1 H 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 1 H NMR of an active polymerization mixture also indicated depletion of tert-chloride end groups. GPC results showed that coupling of PIB 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 TiCl 4-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; 1 H 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 × 10 -5 s -1 . The ratio of rate constants for propagation and rearrangement, kp/kr, was calculated to be 3 × 10 4 M -1 .

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“…Deprotonation of the individual carbocations via chain transfer to monomer gives rise to unsaturated structures of the A, A 1 , A 2 , B, B 1 , C 1 types (Scheme ). Storey et al45 recently also reported double‐bond migration in living IB polymerization. In addition, also unsaturated structures C and D can exist.…”

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“…[17] In situ attenuated total refection Fourier transform infrared (ATR-FTIR) spectroscopy was also employed to study the kinetics of carbocation-and ring-opening-polymerization processes as well as metallocene-catalyzed copolymerizations, polycondensations, and post-polymerization modifications ( Figure 1). [18][19][20] Figure 1. Decrease in absorbance at 1635 cm -1 and FTIR waterfall profiles showing the disappearance of the 1640-1620 cm -1 vibrations upon monomer conversion, corresponding to the vinyl bond of the acrylate monomer, upon the photoinduced SUMI reaction.…”

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“…3(a)]. So, the observed broadening of MWD s in conjunction with some deviation of experimental M n s values from theoretical ones at the later stage of polymerization indicates that some side reactions (chain transfer or the formation of dead chains due to the irreversible termination or the rearrangement of growing species26, 39) can operate in the investigated system.…”