Photooxidative free-radical pathway

There is much interest at present in the reactivity of excited states of dioxygen and various oxygen species such as superoxide ion and the perhydroxyl radical, and hydroxyl radicals. The photooxidation of acetone azine proceeds via a free radical pathway initiated by electron transfer taking place from the azine to singlet oxygen ( 02). This generates superoxide and azine cation radical,... 

Fig. 6. The initial degradation pathway for thermooxidation and photooxidation. The free radical X is generated by the effect of heat or light on impurities,...

We have recently found that this free radical oxidation of the methyl groups is in fact not a major pathway in the photooxidation of poly(phenylene oxide). Instead, the oxidation apparently occurs through an electron-transfer mechanism on the backbone of the polymer not chemically involving the methyl groups at all. In this paper, we present evidence inconsistent with the free radical mechanism and supporting this novel pathway for polymer photooxidation. 

The mechanism of photooxidation of petroleum probably includes significant contributions from both radical and singlet oxygen pathways. Petroleum contains polycyclic aromatic hydrocarbons and other compounds with known photosensitizing activity Lichtenthaler et al. (1989) demonstrated quantum yields for O2 formation ranging from 0.5 to 0.8 for a number of crude oils. The photooxidation of a 2 fuel oil was inhibited by /3-carotene, an excellent quencher of O2 and free radicals, at concentrations > lO" M (Larson and Hunt, 1978). A mechanism incorporating... 

Lipid oxidation in foods is a complex chain of reactions that first consist of the introduction of a functional group containing two concatenated oxygen atoms (peroxides) into unsaturated fatty acids, in a free-radical chain reaction, that afterward gives rise to secondary oxidation products. Different pathways for lipid oxidation have been described radical mechanism or autoxidation, singlet oxygen-mediated mechanism or photooxidation, and enzymatic oxidation. 

To conclude, the transition metal residues of the pcljmerization catalysts act as sensitizers in the photooxidation of polyolefins and of polymers in general according to a mechanism that should involve light absorption and the production of free radicals throu a photoexcited d tion transfer from the anion to the cation. After rapidly reaching thermal equilibrium, tte free radicals should start the oxidation of the sutetrate following the pathways discusred in Section I. 

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