Irradiated polyethylene III—Evidence for an ionic mechanism in allyl-free radical formation

Abstract

Radiation interacting with polyethylene forms alkyl ions which in turn provide allyl-free radicals. Charge and radical migration mechanisms are considered for the processes whereby reactivity is transferred from the initial alkyl ion to the allyl radical site. The following observations provide evidence for a charge migration mechanism. (1) An ionic mechanism which would involve a charge migration process would be expected to be intramolecular in character whereas a radical mechanism would not be restricted to intramolecular migration. Statistical models are considered for charge and radical migration and compared with experimental results. The comparative results favour a charge migration process. (2) The G values for vinyl decay in polyethylene and for the polymerization of lower molecular weight 1-olefins, which are known to undergo ionic rather than radical reactions, are sufficiently close to suggest that the same mechanisms are operative in both materials. (3) Radiation-induced ionic polymerizations show no dose-rate effect whereas radiation-induced free radical polymerizations are proportional to the square root of the radiation intensity. As allyl radical formation shows no dose-rate effect in this study, it provides additional evidence for an ionic process.