Mechanistic study of drying of alkyd resins using (Z,Z)- and (E,E)-3,6-nonadiene as model substances

The cobalt-catalysed autoxidative drying of alkyd resins was studied using (Z,Z)-3,6-nonadiene and (E,E)-3,6-nonadiene as model compounds. A large number of reaction products were isolated from the autoxidation mixture using HPLC and preparative size exclusion chromatography and identified with ¹H- and ¹³C-NMR. The identified compounds comprised C₉ hydroperoxides, endoperoxides, epoxides, aldehydes and ketones and some other oxidation products. Their chemical structures pointed to three different types of oxidation processes taking place. Besides the main radical autoxidation reaction, evidence was found for photo-sensitized oxidation involving singlet oxygen. Thirdly, epoxidation occurs via peracids or hydroperoxides formed as intermediates. Because of the large number of possible isomers having very similar physical properties, isolation of dimers required considerable effort. Nevertheless, two dimers were isolated and characterized. Their structures indicate crosslinking to occur by recombination of radicals as termination reaction.