Radiation-induced polymerization by free ions. Part 1.—Polymerization of styrene under anhydrous conditions

Abstract

The radiation-induced ionic polymerization of bulk styrene has been studied under extremely dry conditions. The rate R_p of polymerization increases with more rigorous drying of the monomer, and this effect is accompanied by a change in the exponent n for the dose-rate (I) dependence of R_p, from n values near unity to 0.6 (R_p∝Iⁿ). A mechanism involving bimolecular termination of free ions is thereby indicated. The strong retardation of the polymerization by scavengers of high proton affinity such as water, diethyl ether, ammonia and trimethylamine in low concentrations (10^–3–10^–6 M) provides evidence that free cations are mainly responsible for the propagation. The addition of nitrous oxide does not affect R_p, and does not appear to prevent electron capture by styrene. Individual reactions steps for cationic and anionic mechanisms are discussed, and the initiation is considered to occur through the respective monomeric ion radicals M⁺_˙ and M⁺_.. It is concluded that the predominance of cationic over anionic polymerization is due to faster initiation and/or propagation processes in the former case.