| Abstract: | 1-Phenyl-2-acetoxy-2-methyl-propanone-1 (III) and 1-phenyl-2-methoxy-2-methyl-propanone-1 (IV) were irradiated at room temperature with 15 or 25 ns flashes of 265 nm or 347 nm light in Ar-saturated cyclohexane or benzene solution. Transient absorptions and emissions were recorded as a function of time between 300 and 550 nm. Triplet lifetimes of III in cyclohexane and benzene and of IV in p-dioxane were determined as ca. 2 μs. In contrast to the photolysis of 1-phenyl-2-hydroxy-2-methyl-propanone-1 (compound I) and of 1-[4-(2-propyl)-phenyl]-2-hydroxy-2-methyl-propanone-1 (compound II), α-cleavage occurs upon UV irradiation of III and IV to a minor or negligible extent. Triplets of III were found to react effectively with N-methyldiethanol amine, but a reaction with cyclohexane did not become noticeable. They react with O2 with k(T + O2) = 1.6 · 109 l/mol s. Triplets of IV react with cyclohexane with k(T + CH) = 8.6 · 105 l/mol s and with O2 with k(T + O2) = 1.1 · 109 l/mol s. They are inert towards p-dioxane. Photocuring of a transparent acrylate coating system yielded the following results on the basis of pendulum hardness measurements: I and II exhibit excellent initiator effectivity, III and IV are not capable of initiating curing if applied without coinitiator, e.g. N-methyldiethanol amine. In the presence of the latter III and IV exhibit satisfactory curing capability. In conclusion: the high curing effectivity of I and II is due to the high yield and the high rate of α-cleavage. III and IV do not undergo α-cleavage and therefore do not initiate curing. The curing capability of III and IV in the presence of a coinitiator is due to the formation of ketyl radicals by a hydrogen transfer mechanism. |
