Photo-induced decomposition of organic peroxides: Ultrafast formation and decarboxylation of carbonyloxy radicals

Recent experimental and theoretical studies shed new light on the ultrafast photochemical decomposition of organic peroxides, R1C(O)O-OR2. This reaction both is of fundamental interest, directed toward a detailed understanding of unimolecular processes, and of practical relevance because of the widespread use of peroxides as initiators in free-radical polymerization. An overview is given on pico- and femtosecond UV pump-VIS/near IR/IR probe experiments on the decomposition of diaroyl peroxides (R-1 = aryl, R-2 = C(O)-R-1), of tert-butyl peroxyesters (R-1 = aryl or benzyl, R-2 = tert-butyl), and of tert-butyl peroxycarbonates (R-1 = aryloxy, R-2 = tert-butyl). Analysis of the time-resolved concentration profiles is carried out in conjunction with quantum-chemical calculations, in particular on the structure and reactivity of the intermediate carbonyloxy radicals, R-1-CO2. A semi-quantitative model is presented which allows for an adequate understanding of the ultrafast processes subsequent to instantaneous peroxide photodissociation.