Capping of polyisobutylene with 1,1-diphenylethylene and its derivatives - Kinetic and mechanistic studies

Fundamental investigations have been carried out on the capping reaction of living polyisobutylene with 1,1-diphenylethylene (DPE) and its derivatives. Capping is a reversible reaction and the corresponding equilibrium constants have been determined. In accord with theoretical considerations, higher K-e was obtained at lower temperature, at higher solvent polarity, and for better stabilized diphenyl carbenium ions. With DPE Delta H was found to be -85.2 kJ/mole in hexane:methyl chloride 40:60 (v:v) solvent mixture and -83.1 kJ/mole in hexane:methyl chloride 60:40 (v:v) solvent mixture. Capping involves about -300 J/mole/K-0 of entropy change-due to the decrease in the number of molecules involved. A simplified kinetic equation was developed to describe the time dependence of uncapped living chain end concentration. By fitting the experimental data the corresponding kinetic parameters have been calculated, and found to be in satisfactory agreement with that obtained from capped/uncapped living chain end concentration at equilibrium.