SOME PHOTO-REACTIONS OF SUBSTITUTED POLY(ACRYLOPHENONES)

Poly(p-methoxyacrylophenone) (PPMA) and poly(3',4'-dimethoxyacryleneph enone) (PDMA) were exposed in the form of thin films to long-wave u.v. radiation (lambda greater-than-or-equal-to 300 nm) under high vacuum. The low molecular mass products from both were ethane and methane, th e quantum yields (phi(i)) for their formation being in the range 1 x 1 0(-4) < phi < 4 x 10(-4) mol(E)-1. The polymers also underwent colorat ion (yellow), and this was attributed to the formation of quinonoid en tities, their precursors being the phenoxy radicals formed by the phot olysis of the O-CH3 bonds. Experiments with filtered u.v. radiation, i ndicate that the CO group is not the sole long-wave chromophore. The f irst pi --> pi(S0 --> S1) transition of the phenyl group is sufficien tly red-shifted, by the presence of the methoxy substituents, to cause absorption in the long-wave region (lambda > 300 nm). In addition, th ere is a considerable interaction between n --> pi and pi --> pi* orb itals, 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 gas eous products identical and the quantum yields similar, but DMBA under went similar spectral changes, including coloration. In this case the S0 --> S1 absorption is the chromophore. The results have some bearing on the coloration of lignin, which contains similar structural elemen ts.