ESR spectra of sterically hindered piperidine derivatives embedded in polypropylene are superpositions of signals arising from radicals of different mobility. Analysis of the spectra at different temperatures gives information on the mobility and distribution of radicals in solution and in polymer media. At the temperature of irradiation, radicals are homogeneously distributed also in a polymer matrix. During irradiation the concentration of radicals first increases then, after reaching a maximum, decreases. This intermediate character is the result of simultaneous formation and reaction of nitroxyl radicals with radicals arising from polypropylene photo-oxidation. Maximum radical concentration is reached at higher dosage, and subsequent decrease in concentration is slower when hydroxy-benzophenone is applied together with the piperidine derivative. In case of joint application of the latter with a nickel complex, on the other hand, no radical formation can be detected in the polymer before, or in the course of irradiation.
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