Electron-pair-donor reaction order in the cationic polymerization of isobutylene coinitiated by AlCl3

The cationic polymerizations of isobutylene (IB) initiated by an H2O/AlCl3 system were carried out in a mixture of n-hexane/methylene dichloride of 60/40 v/v at -50 degrees C in the presence of various external electron pair donors (EDs), including methyl benzoate (MB), ethyl benzoate (EB), and methyl acrylate (MA). The effects of the concentrations of EDs ([ED]) and polymerization time on monomer conversion and kinetics of IB polymerization were also investigated. The complexes between AlCl3 and the esters were soluble in the polymerization system at -50 degrees C. The polymers with high molecular weights and relatively narrow molecular weight distributions were obtained in the presence of the EDs. The rate of polymerization decreased with increasing [ED]. The kinetic orders of the EDs were remarkably dependent on their chemical structure, steric hindrance from moieties, and concentration in the polymerization system. The reaction order of MB was determined to be around -1.3 when [MB] <= 0.40 mmol/L, whereas it was -12.9 when [MB] >= 0.40 mmol/L. The reaction order of EB was determined to be -1.36 when [EB] <= 0.41 mmol/L, whereas it was -3.36 when [EB] >= 0.41 mmol/L. The reaction order of MA was determined to be -1.85 when [MA] <= 0.48 mmol/L, whereas it was -16.7 when [MA] >= 0.48 mmol/L. (c) 2007 Wiley Periodicals, Inc.