The photochemistry of poly(4 ′-ethoxyacrylophenone) in the solid state

Abstract

The photolysis of poly(4 ′-ethoxyacrylophenone) was studied in the solid phase, thin films being exposed to long-wave (i.e. λ⩾300 nm) UV radiation under high vacuum at 298 K. The principal photo-induced reaction was random chain scission, which in turn, was the result of a Norrish type II decomposition involving the carbonyl triplet. However, the quantum yield for scission was more than an order of magnitude lower, than that for the corresponding reaction in dilute solution, and this was associated with conformational restrictions imposed upon the collapse of the biradical. Low molecular weight products were also formed, but in low quantum yields ( Φ≈10 −4 mol (Einstein) −1), and their presence indicates the formation of ethyl radical intermediates. This observation is consistent with O–Et bond fission, and it also confirms that long-wave UV irradiation of ring substituted poly(acrylophenone)s leads to fission of the O-alkyl bonds. The polymer also underwent colouration (yellow), and this was attributed to the formation of quinonoid entities, the precursors being the phenoxy radicals formed simultaneously with the ethyl radicals. Because of their stability and their long lifetimes in the solid polymer at sub- T G temperatures, phenoxy radicals may also participate in other reactions, such as cross-linking, which can be brought about by reaction of these with each other, and by their interaction with radicals formed on the polymer backbone by ethyl radical abstraction of H-atoms. Cross-linking, which manifests itself by insolubilization and gel formation, is more apparent after longer exposures. It does, however, reduce the effective rate of chain scission, as does self-quenching, the new terminally unsaturated carbonyl species acting as effective triplet quenchers. Quenching of the triplet, which also occurs when naphthalene and 1,3- cis-pentadiene are added, conforms to the Perrin relationship. However, the high volume of the quenching sphere for naphthalene indicates that the process is subject to steric inhibition, the adjacent polymer chain restricting access of the quenchers to the carbonyl triplet.