Non-classical free-radical polymerization: Degradative addition to monomer in the polymerization of 1-vinylimidazole

A study of the free-radical polymerization of 1-vinylimidazole at 70°C is described; most attention has been devoted to the (homogeneous) reaction in ethanol, but polymerizations in N,N-dimethylformamide (DMF), water and bulk monomer are also considered. The polymerization in ethanol is unusual in that the rate becomes effectively zero-order in monomer M at moderately high [M]. Kinetic results indicate the occurrence of a degradative reaction between propagating radicals and monomer (see below); the available molecular weight data suggest that bimolecular termination takes place by radical combination. Polymerization in DMF is generally similar although the kinetic treatment is rather less satisfactory. At high [M] there are indications of occlusion phenomena in DMF and these latter are marked in the (heterogeneous) bulk polymerization. In water, significant interactions between solvent and monomer, evidenced by viscosity-composition behaviour, affect the kinetics of polymerization, which, however, resemble those with the other solvents. Of considerable interest is the influence of pH on rates and degrees of polymerization, which change in the same sense and to nearly the same extent. We believe these findings imply suppression of the degradative reaction by protonation of the monomer. The degradative reaction between propagating chains and monomer is thought to be formation of a relatively unreactive radical by addition to position 2 of the monomer, rather than by chain transfer. Evidence for this is adduced from molecular weight data, which are not consistent with transfer, the existence of occlusion phenomena and the observations on the pH dependence in aqueous solution. Molecular weight distributions expected in polymerizations of this type are calculated in the Appendix. Reinitiation by the adduct radical, which probably occurs only to a limited extent under non-occlusion conditions, becomes important when occlusion is significant. A kinetic treatment of these phenomena is presented.