Nature and ESR anisotropy of trapped radicals in UV-irradiated isotactic polypropylene at 4.2 K and 77 K

A comparative ESR study of low-temperature photolysis at 77 and 4.2 K of isotactic polypropylene, preoxidized under controlled conditions, and of 2.6.8 trimethylnonane-4-one as a model compound of the inchain PP ketones CH2COCH2 is presented. The results show conclusively that the ESR spectra of low-temperature PP photolysis reported in the literature arise from the decomposition of tertiary hydroperoxides, whereas the contribution by the Norrish I photolytic cleavage of ketones is of minor importance; the relative efficiency of radical production at 77 K (λ ≥ 300 nm) by both types of photoactive compound has been measured by ESR and found to be about 15 times greater for the hydroperoxides. The mechanism of solid-state photolysis of PP hydroperoxides is dominated by the nearly complete decay of excited alkoxyls during the photolytic events that yield the chain end radicals CH(CH3)CH2· by β scission (87.8%) and the inchain radicals CH2C(CH3)CH2 by hydrogen abstraction (10.4%). The low yield of methyl radicals detected (1.8%) and their delayed appearance strongly suggest that these species are of secondary origin and that the decomposition of alkoxyls via CCH3 scission is of negligible importance. From the ESR anisotropy of the deliberately prepared peroxyradicals CH(CH3)CH2O2· and CH2C(CH3)(O2)CH2 evidence of the misalignment of terminal chain segments following scission of alkoxyls at 77 K was obtained. An unexpected result of the measurements at 4.2 K is represented by the identification of a clear signal by H atoms: the H atoms apparently are not trapped in perprotiated matrices, even at 4.2 K, because of their unusually high reactivity toward the hydrogen abstraction assisted by tunneling effects.