Short‐chain branching in γ‐radiation‐induced polymerization of ethylene

Abstract

Abstract In an attempt to elucidate the mechanism of chain‐branch formation in the polymerization of ethylene, the effect of reaction conditions on short‐chain branching in γ‐radiationinduced polymerization of ethylene was investigated by using infrared spectroscopy. The concentration of methyl groups, i.e., the frequency of short‐chain branching, increases with temperature and pressure and is independent of ethylene conversion to polymer and radiation intensity. The number of methyl groups per polymer molecule increases almost proportionally with the degree of polymerization. These facts indicate that short‐chain branching occurs mainly by the mechanism of intramolecular hydrogen transfer. The effect of pressure on the rate of chain branching can be postulated by considering the transition state to be six‐membered rings in hydrogen transfer reactions. The activation energy of chain branching is found to exceed that of propagation by 6 kcal./mole.