Abstract
The light-induced oxygenation of lignin was studied with the aid of lignin model compounds in dimethoxyethane solution saturated with air or oxygen. The results are obtained from experiments with tert-butylguaiacyl-carbinol and aromatic α carbonyl compounds (acetoguaiacone, polymer-bound acetoguaiacone, p-methoxyacetophenone, p-methoxypropiophenone and benzophenone), and aromatic compounds containing ring-conjugated double bonds (anethole and isoeugenol) and the biphenyl compound bicreosol were added as sensitizers. The rates of disappearance of the reactants were determined by gas chromatography and the formation of peroxides was measured by iodometric titration. The kinetics were also studied in the presence of β-carotene and diazabicyclo[2.2.2]octane. The reactivity of the non-phenolic lignin model compound tert-butylveratrylcarbinol was also examined. The importance of the different chromophoric structures in lignin is discussed. The results indicate that the mechanism for photo-oxygenation is very complicated, with many pathways operating simultaneously, one of which is attributed to a reaction involving singlet oxygen. The relative significance of the different pathways is discussed.
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