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Abstract

Laser flash photolysis studies and stationary polymerizations were carried out with the aim to elucidate kinetics and mechanisms of photoinitiation processes based on thioxanthone (TX)/amine interactions. During these studies prominence was given to reactions which significantly influence the initiator efficiencies. Thus, rate constants of the reactions 3TX* + monomer (kq), 3TX* + amine (kXH), and ketyl radical + monomer (kmath image) were determined; kq decreases according to the series: styrene (St) > N-vinyl-2-pyrrolidone (VP) > methyl methacrylate (MMA) > acrylonitrile (AN) > butyl vinyl ether (BVE) > vinyl acetate (VA) from 6·109 l·mol−1·s−;1 to 2·104 l·mol−1·s−1. Substitution in the 2-position of TX by methyl (MTX), isopropyl (ITX) and Cl (CTX) generally causes a decrease of kq, except for the systems CTX/VP and CTX/BVE. The kXH values range from 6 to 8·109 l·mol−1·s−1 for typical amines, e.g. ethyl 4-((dimethylamino)benzoate). Ketyl radicals react rather slowly with monomers; for CTX kmath image in l·mol−1·s−1: 7·103 (St), 4·102 (VP), 4·101 (MMA), ⩽ 10 (BVE). O2 reacts rapidly with ketyl radicals: kmath image = 2·109 l·mol−1·s−1 (TX). Rates of polymerization ν (monomer consumption) determined at [M] = 5 mol/l and [amine] = 10−1 mol/l are equal for TX, ITX, MTX, and CTX, but increase according to the series AN < MMA < VP; ν depends on the thioxanthone and the monomer concentration according to ν ∝ [TX]1/2[M].