Photochemical transformation of benzil pendant groups of polystyrene copolymers into benzoyl peroxide moieties and their subsequent thermal decomposition.: Cross-linking or chain scission?

Films of the copolymer, 11 wt% 1-{4-(2-methacroyloxyethoxyphenyl}-2-phenyl-1,2-ethanedione (BZMA) and styrene (S) (BZMA/S), as well as polystyrene (PS) doped with either the BZMA monomer (BZMA-PS) or 1-{4-(2-acetyloxyethoxy-phenyl}-2-phenyl-1,2-ethanedione (BZAc-PS) in concentrations that match the composition of the copolymer, have been irradiated (lambda > 400 nm) in the presence of molecular oxygen at ambient temperatures. The rates of consumption of BZMA and BZAc and the concurrent formation of the corresponding benzoyl peroxide-containing units (BPMA and BPAc) were followed by infrared spectroscopy. The rates of benzil-group consumption and peroxide formation matched each other and were virtually the same in the three film types. Larger concentrations of oxygen increased the rate of consumption of BZMA. From a kinetic treatment of data at two concentrations of oxygen in PS, it is concluded that BZMA photooxidation is more than 10 times faster than that of benzil. At 91 degrees C, the first-order rate constants for thermal decomposition of BPMA and BPAc in PS are equal to and are larger than in the BPMA/S copolymer. The lower rate constant of BPMA/S, as well as the worse fit of the rate data from BPMA and BPAc in PS to a unimolecular decomposition model, is ascribed to some bimolecular decomposition, probably from aggregated peroxides. There is no indication of a bimolecular decomposition component in the copolymer. A very large portion (91 wt %) of BPMA/S, from irradiation of BZMA/S, is THF insoluble (i.e., cross-linked). The insoluble part increases to about 99 wt % after the BPMA/S film is treated at 91 degrees C for 6 h. Both of the corresponding doped polymers remain completely soluble in THF after irradiation and thermolysis. Cross-linking during the irradiation and heating is ascribed to formation of cater linkages (through abstraction of H atoms from -O-CH2-CH2-O- groups by acyloxy radicals) and combination of pendant acyloxy radicals with radical sites on neighboring chains; abstraction from benzylic carbon atoms along PS chains leads to scission. By contrast, irradiation and subsequent heating of BZMA-PS or BZAc-PS films results in more chain scissions than cross-linking since the average molecular weights are decreased.