Irradiated polyethylene II—Free radical formation

Abstract

Allyl free radical formation in Marlex 50 polyethylene at room temperature is correlated with vinyl group disappearance and vinylene group formation. The allyl radicals were generated with 2 MeV electrons from a Van de Graaff accelerator and their concentrations were measured with electron spin resonance techniques. A kinetic analysis of the free radical formation and vinyl disappearance data provides a mechanism for these reactions. The mechanism assumes the formation of a steady state concentration of positive alkyl ions, which are formed initially, followed by charge transfer to vinyl and vinylene groups. A subsequent reaction between the vinyl ion and a neutral vinyl group takes place. The resultant divinyl and vinylene ions form allyl radicals following charge recombination. Kinetic equations derived from this mechanism correlate well with experimental data. The G value for alkyl ion formation calculated from these equations, is the same as that obtained for radical formation in lower molecular weight hydrocarbons and for alkyl radical formation in polyethylene at 77°K and at low doses. This G value also correlates with the G values for the polymerization of olefins. The kinetic data provide evidence that allyl radical formation does not take place through a precursor alkyl radical.