Photochemistry of ketone polymers. 16—Studies on model α-substituted 2-alkanones

Abstract

Studies have been made of the photochemistry of a series of α-substituted 2-alkanones of the general structure CH 3C(O)CHR(n-C 18H 37) where R = H, CH 3, C 2H 5, n-C 3H 7 and n-C 18H 37. It is demonstrated that all of these compounds have total quantum yields of approximately 0·15, both in hydrocarbon solution and in amorphous hydrocarbon polymer matrices. When the keto group is attached to a tertiary carbon the total quantum yield is made up of about equal proportions of type I, type II and cyclobutanol reactions, and the quantum yields of these are relatively independent of temperature (from 7 to 82°C), the molecular weight of the hydrocarbon chain, or the location of the keto group on it. There is no evidence for enhanced reactivity for chromophores at chain ends, nor for enhanced cage effects in the polymer matrices above their glass transitions. When R = H, the keto group is attached to a secondary carbon atom and the quantum yield of the free-radical forming type I reaction is reduced to only 1–2% of the total quantum yield, compared to 20–40% for the tertiary ketones. From quenching studies with cis,cis-1,3-cyclooctadiene it is inferred that 80–90% of the type I reaction arises from the excited triplet rather than the singlet state. The significance of these observations to the mechanism of photooxidation of hydrocarbon polymers is discussed.