Some photo-reactions of substituted poly(acrylophenones)

Abstract

Poly( p-methoxyacrylophenone) (PPMA) and poly(3′,4′-dimethoxyacrylenephenone) (PDMA) were exposed in the form of thin films to long-wave u.v. radiation ( λ ⩾ 300 nm) under high vacuum. The low molecular mass products from both were ethane and methane, the quantum yields ( Φ i) for their formation being in the range 1 × 10 −4 < φ 4 × 10 −4 mol(E) −4. The polymers also underwent coloration (yellow), and this was attributed to the formation of quinonoid entities, their precursors being the phenoxy radicals formed by the photolysis of the OCH 3 bonds. Experiments with filtered u.v. radiation, indicate that the CO group is not the sole long-wave chromophore. The first π → π ∗(S 0 → S 1) transition of the phenyl group is sufficiently red-shifted, by the presence of the methoxy substituents, to cause absorption in the long-wave region ( λ > 300 nm). In addition, there is a considerable interaction between n → π ∗ and π → π ∗ orbitals, and this produces a composite chromophore. For the purposes of comparison, the plausible model compound 3, 4 dimethoxybenzylalcohol (DMBA) was irradiated under identical conditions. Not only were the gaseous products identical and the quantum yields similar, but DMBA underwent similar spectral changes, including coloration. In this case the S 0 → S 1 absorption is the chromophore. The results have some bearing on the coloration of lignin, which contains similar structural elements.