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

Abstract

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 (R₁ = aryl, R₂ = C(O)–R₁), of tert-butyl peroxyesters (R₁ = aryl or benzyl, R₂ = tert-butyl), and of tert-butyl peroxycarbonates (R₁ = aryloxy, R₂ = 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, R₁–CO₂. A semi-quantitative model is presented which allows for an adequate understanding of the ultrafast processes subsequent to instantaneous peroxide photodissociation.