Carbonium and carbanionic mechanisms of polymerization induced by gamma‐radiation
Authors: Abkin A. D. , Sheǐnker A. P. , Yakovleva M. K. , Mezhirova L. P. 
Published: November 2014

Data obtained from studies on the radiation polymerisation of isobutylene, styrene, butadiene and other monomers lead to the conclusion that under low temperature conditions a cationic mechanism underlies the polymerization of these monomers. No anionic polymerization mechanism under the action of nuclear radiations has been described in the literature. Recently results indicative of γ‐ray‐induced polymerization were obtained not only by a cationic but also by an anionic mechanism. The capacity for the reaction to proceed according to the one or the other of the mechanisms has been found to be determined both by the chemical structure of the monomer and by the nature of the medium in which the polymerization is taking place. Acrylonitrile polymerizes at −78° in solvents with electron‐donor substituents (triethylamine, dimethylformamide) but not in solvents ordinarily employed in cationic polymerization and containing electrophilic groups, as in ethyl chloride. Styrene polymerizes only in ethyl chloride. The γ‐radiation‐initiated copolyraerization of styrene with isobutylene and methyl methacrylate and acrylonitrile with styrene was studied. It was found that isobutylene‐styrene copolymers (polymerization in ethyl chloride) obtained at −78° are enriched in the isobutylene component for all compositions of the initial mixture: r1 (isobutylene) = 3.5, r2 = 0.33. At −78° isobutylene polymerizes more slowly than styrene. A plot of the relation between the polymerization rate and the initial mixture composition is characterized by a minimum in the region of relatively small isobutylene additions. At 0°, polymerization of the monomers proceeds at considerably lower rates (especially for the case of isobutylene) than at −78°. At 0° styrene polymerizes much faster than isobutylene. The rate of polymerization of isobutylene and styrene has been found to have a first‐order dependence on the dose rate. At low temperatures polymers of relatively high molecular weight are formed: in styrene the molecular weight is equal to 70,000; in isobutylene, 80,000. The molecular weight of the copolymers, depending upon the composition, is from 20,000 to 55,000 (from light scattering data). The polymers obtained at 0° have lower molecular weights. Styrene‐methyl methacrylate copolymers obtained at −78° are greatly enriched in the styrene component for all initial composition mixtures. In this case r1 (styrene) = 20, r2 = 0.005. In the polymerization of monomers initiated by BF3‐type catalysts, the values of r1 and r2 are, respectively, 10.5 and 0.1 (as reported in the literature). Small methyl methacrylate additions to styrene lead to an abrupt fall in the polymerization rate (at −78°), and the rate changes slightly on further addition of methyl methacrylate. The rate of polymerization of acrylonitrile at −78° and dose rates of 150 R./sec. in bulk is 2.1 × 10−6 mole/l.‐sec.; in triethylamine it is 6.7 × 10−6 mole/l.‐sec.; in dimethylformamide, 9.7 × 10−6 mole/l.‐sec. (for a monomer concentration of 3.5 mole/l.). In the copolymerization of acrylonitrile and styrene in dimethylformamide solution at −78° the resultant polymers are richer in acrylonitrile than the initial mixture over the entire range of initial compositions. On the basis of the kinetic relations established, as well as other data, obtained in studies of such systems, a discussion is presented of the cationic and anionic mechanisms of γ‐ray‐induced polymerization.