Effect of isotopic composition on free‐radical reactions in polyolefins

Abstract

AbstractOn substitution of hydrogen by deuterium the radiation yield of free radicals in polyolefins decreases ca. 2‐fold. Under ultraviolet illumination (γ < 300 nm) in deuterated polyethylene (DPE), allyl radicals are converted into alkyl radicals (hfs constants a = 3.2 Oe, a = 4.4 Oe); in deuterated polypropylene (DPP), allyl and peroxide radicals are converted into alkyl radicals (a = 3.7 Oe). At 77°K under ultraviolet light in γ‐irradiated polyethylene (PE) and DPE, triene, tetraene, dienyl, trienyl, and tetraenyl radicals are formed. Deuterium substitution has no effect on absorption spectra of polyenyl radicals and polyenes. In polyolefins the quantum yields of photochemical reactions of allyl and peroxide radicals are 0.1–1.0. Photochemical reactions of peroxide radicals result in a radical concentration increase of ca. 3‐fold. In reactions of peroxide radicals in the dark, a kinetic isotope effect has been observed. Activation energies of substitution reactions of peroxide radicals are in PE, 9.3 ± 0.3 kcal/mole; in DPE, 10.2 ± 0.3 kcal/mole; in PP, 12.7 ± 0.4 kcal/mole; in DPP, 14 ± 0.5 kcal/mole. Possible mechanisms of the effect of polyolefin isotope composition on radical formation by high energy irradiation as well as on photochemical and dark reactions of free radicals are discussed. The effect of the energy released from phototransformations and radiationless deactivation of macroradicals on the migration of free valence is also considered.