Photoinduced Change of Organic Free Radicals as Studied by ESR. I. Conversion of Allyl-Type to Alkyl-Type Radicals in Irradiated Polyethylene

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Abstract
The allyl-type radical –CH2–ĊH–CH=CH–CH2– in irradiated polyethylene was converted into the alkyl-type radical –CH2–ĊH–CH2– upon uv illumination (λ 2537 Å). During the conversion total concentration of the radicals remained constant. The production curve of the alkyl-type radical reached a saturation at 43% conversion. Initial rate of production of the alkyl-type radical was found proportional to the light intensity, while the saturation concentration was the same for illuminations of different intensities. The initial rate was independent of the illumination temperature and of presence of added hydrogen gas. A reaction mechanism which involves an intramolecular hydrogen atom shift was put forward: –CH2–CH2–ĊH–CH=CH–⇌ lim k2k1–CH2–ĊH–CH2–CH=CH–.The corresponding kinetic equation was solved and the rate constants k1 and k2 as well as the quantum yield were calculated from experimental results. It was found that this mechanism would explain all the features of the reaction satisfactorily. In addition to conversion to the alkyl-type radical, minor reaction of the allyl-type radical also took place simultaneously to produce polyenyl radicals of shorter length, evolving hydrogen gas. These ESR results were consistent with the evidences obtained from uv—visible spectra.