Multiplicity of active centres in the cationic polymerization of p‐methoxystyrene

Abstract

AbstractA common ion salt effect on the propagation rate constant was studied in the cationic polymerization of p‐methoxystyrene initiated by Ph3C+SbCl−6 in CH2Cl2 at + 10°C. The polymodal molecular weight distribution of the polymers was resolved into four distinct populations, of which three are formed simultaneously at the beginning of the reaction. Influences of conversion, temperature, initial monomer concentration, degree of dissociation of the active centres and composition of solvent mixtures on the proportions of the three populations led to their attribution to three types of slowly exchanging propagating species. From most reactive to least reactive, the species are: 1) monomer‐solvated cations (free ions and ion pairs); 2) dichloromethane‐solvated cations (free ions and ion pairs); 3) polymer‐solvated free cations (intramolecular interaction between the delocalized carbocation and the aromatic ring of the penultimate unit) in equilibrium with contact ion pairs. Absolute propagation rate constants were estimated for each species. Equilibria shifted towards the most reactive species at low temperature, which explains the apparent negative activation energy previously observed.