Cationic polymerizations by aromatic initiating systems. II. Correlation between cationic model and polymerization reactions with the p-CH₃C₆H₄CH₂Cl/Et₃Al system

Abstract

Model experiments and subsequent polymerization runs have been carried out to elucidate the mechanism of cationic olefin polymerizations initiated by aromatic carbenium ions. Thus, the p-methylbenzylation of 2,4,4-trimethyl-1-pentene, a nonpolymerizable model olefin for isobutylene, was investigated by using the p-CH₃C₆H₄CH₂Cl/Et₃Al initiating system and CH₂Cl₂ solvent under various conditions. All the important organic reaction products were identified and most of them, quantitatively determined. Analysis of the nature of the products and their distributions gave important mechanistic information about the chemistry of elementary events and their relative rates; for example, hydridation and ethylation by the Et₃AlCl^⊖ counteranion of various cations occurs and these processes mimic termination in carbocationic polymerizations. Hydridation and ethylation are much faster than proton elimination (chain transfer in polymerization) and experimental conditions (e.g., Al/Cl ratio and temperature) influence the relative rates of these processes. According to model experiments, the p-CH₃C₆H₄CH₂Cl/Et₃Al system may initiate olefin (isobutylene) polymerization; chain transfer to monomer should be relatively unimportant, but rapid hydridation or ethylation should reduce the kinetic chain and give low conversions. Predictions derived from model experiments were substantiated by polymerizations with isobutylene and the aromatic initiating system.