Photodegradation of poly(4-acetyl-α-methylstyrene) in solution

Abstract

Dilute solutions of poly(4-acetyl-α-methylstyrene) (PMVAP) were exposed to long-wave ( λ ⩾ 300 nm) u.v. under high vacuum conditions at 25 ± 1°. The principal gaseous low molecular weight product was ethane, but a smaller amount of methane and a trace of CO were also detected, indicating that PMVAP undergoes simultaneous α-cleavages. Chain scission also occurs and was attributed to β-scission of the radicals formed by H-abstraction at the secondary C-atoms, both methyl radicals and carbonyl triplets (photoreduction) being involved. Chain scission is inhibited by the addition of naphthalene, and the diffusion-controlled quenching of the triplet can be represented by Stern-Volmer kinetics. The rate of chain scission is reduced when good H-donors, such as isopropanol, are present; they compete with the polymer in photoreduction reactions with the carbonyl triplet. The effects of less active H-donors such as methanol are more complex at low concentrations. They appear to act as transfer agents but, as the concentration increases, solvent quality plays a more important role. Tighter coiling of the polymer leads initially to more photoreduction and, with it, more chain scission. However, as the concentration of nonsolvent increases, the diffusive separation and disentanglement of macrofragments becomes more difficult and competition from intramolecular cyclization and perhaps also cross-linking increase, resulting in decreased rate of chain scission.