On the photoinitiation of free radical polymerization‐laser flash photolysis investigations on thioxanthone derivatives

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 3 TX * + monomer ( k q ), 3 TX * + amine ( k XH ), and ketyl radical + monomer ( k ) were determined; k q 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 ·10 9 l·mol −1 ·s −;1 to 2·10 4 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 k q , except for the systems CTX/VP and CTX/BVE. The k XH values range from 6 to 8·10 9 l·mol −1 ·s −1 for typical amines, e.g. ethyl 4‐((dimethylamino)benzoate). Ketyl radicals react rather slowly with monomers; for CTX k in l·mol −1 ·s −1 : 7·10 3 (St), 4·10 2 (VP), 4·10 1 (MMA), ⩽ 10 (BVE). O 2 reacts rapidly with ketyl radicals: k = 2·10 9 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].