The photochemistry of methoxy-substituted poly(acrylophenones)

Abstract

Poly(p-methoxyacrylophenone) (PMAP), poly{3,4-dimethoxyacrylophenone) (P34DMAP) and poly(3,5-dimethoxyacrylophenone) (P35DMAP) were exposed in the form of dilute solutions and thin films to long-wave UV radiation (see document) under high vacuum at 25 ± 1°C. Photoreactions of these polymers occurred predominantly from the carbonyl triplets. In addition, the photophysical observations were consistent with the formation of low-lying (see document) triplets which, in turn, was the result of substitution of the aromatic ketones by electron-donating methoxy groups. Quenching by both naphthalene and cis-1,3-pentadiene conformed to Stern-Volmer kinetics. In all cases, methane and ethane were formed, the quantum yields for their formation being lower in solution than those observed for the solid state photolysis. This resulted from methyl radicals formed by O-CH3 fission. The principal photoreaction in solution was a Norrish type II decomposition which resulted in random chain scission. The polymers also underwent colouration (yellow), and this was attributed to the formation of quinonoid entities, their precursors being the phenoxy radicals formed by O-CH3 fission. The effects on chain scission of a number of additives with varying transfer activities were found to be complex as rates of chain scission decreased but not to the extent that would be expected if H-transfer alone was occurring. Solvent quality and polarity were also important. Similarly, the rate of chain scission decreased with increasing polymer concentration, and this has been attributed, to inhibition of separation of macrofragments (from the Norrish II reaction) due to polymer entanglement.