Cationic Polymerizations at Elevated Temperatures by Novel Initiating Systems Having Weakly Coordinating Counteranions. 1. High Molecular Weight Polyisobutylenes

Abstract

In-situ prepared \({{(C{{H}_{3}})}_{3}}S{{i}^{ \oplus }}{{[B{{({{C}_{6}}{{F}_{5}})}_{4}}]}^{ \ominus }}\) efficiently produces extremely high molecular weight polyisobutylene (PIB) in close-to-neat systems (15% toluene or 5% methyl chloride to dissolve the Li[B(C6F5)4] coinitiator precursor salt) over the -35 to -8° (reflux) range. In contrast, much lower molecular weight PIBs are obtained when the preprepared initiating system is added to liquid isobutylene (IB). The molecular weights produced by the in-situ initiating system are very much higher (e.g., \({{\overline M }_{n}} \sim 110,000g/mol\) at - 15°C) than those obtained by common systems, e.g., AICl3, and match those obtained by the use of γ-rays. The molecular weight distributions \(({{\overline M }_{w}}/{{\overline M }_{n}} \sim 2)\) and the slopes of log M vs. 1/T plots (ΔHDP ‡ = -5.9 kcal/mol (-24.7 kjoule/mol)) indicate chaintransfer-controlled polymerizations. The de facto initiating entity is probably the arising from adventitious moisture. Due to the extremely nonnucleophilic nature of the weakly coordinating counteranion \({{[B{{({{C}_{6}}{{F}_{5}})}_{4}}]}^{ \ominus }}\), propagation most likely proceeds by unencumbered carbocations and termination is absent. Living polymerization could not be obtained in the presence of \({{[B{{({{C}_{6}}{{F}_{5}})}_{4}}]}^{ \ominus }}\), which suggests that livingness in carbocationic polymerizations requires counteranion assistance.