Theoretical study of the oxidation reactions of methylacrylic acid and methyl methacrylate by triplet O2

The oxidations of organic compounds and polymers by triplet O-2 were called "dark oxidation" or "auto-oxidation", in contrast to their "photo-oxidation" by singlet O-2. To study the relevant dark oxidation mechanism we take methylacrylic acid (MAA) and methyl methacrylate (MMA) as prototypes to study their reactions with triplet O-2 by performing density functional theory calculations. Two reaction channels, the C-H bond oxidation and CC bond oxidation, have been characterized in detail. The structures of the initial contact charge-transfer complexes, intermediates, transition states, and final oxides involved in the reactions have been localized at the UB3LYP/6-311+G(d,p) level. It is found that the C-H bond in the methyl group connected to the CC bond presents relatively higher reactivity toward triplet O-2 than the CC bond itself. Thus, the reactions are expected to proceed via the C-H bond oxidation branch at room temperature and also via CC bond oxidation at elevated temperature. In this sense, an effective method for preventing or retarding the dark oxidations of MAA and MMA in a natural environment is to chemically decorate or protect the C-H bond in the methyl connected to the CC bond. The present results are expected to provide a general guide for understanding the dark oxidation mechanism of organic compounds and polymers.