A novel approach to the understanding of the solvent effects in radical polymerization propagation kinetics

Propagation kinetics of isobornyl methacrylate (iBoMA) and methyl methacrylate (MMA) free-radical homopolymerization were studied by the PLP-SEC method, which consists of pulsed laser initiated polymerizations and subsequent polymer analysis by size-exclusion chromatography. The reactions were carried out in bulk and in solution of several organic solvents: toluene, tetralin, tetrahydrofuran (THF), methyl isobutyrate, and phenethyl isobutyrate. In all polymerization systems, the activation energy of the observed propagation rate coefficient, k(p),(app), is not affected by either solvent type or solvent concentration. However, kp,app may be significantly different from the corresponding bulk values. In MMA polymerizations, kp,app values in solution were higher than in bulk, with the strongest enhancement of 50% observed for tetralin as solvent. In contrast, a solvent-induced lowering in propagation rate of up to 52% for THF as solvent was seen for iBoMA. The observed changes in propagation rate were assigned to the occurrence of a local monomer concentration in the vicinity of the free-radical chain end rather than to an intrinsic kinetic effect. The variation in local monomer concentrations is suggested to originate from differences in molar volume of the monomer and the solvent. Critical analysis of the results led to a linear correlation between solvent-induced changes in propagation rate and the difference in molar volumes of monomer and solvent. The correlation holds for both monomers.