Photolysis of poly(o‐acetylstyrene) in solution

Abstract

AbstractDilute solutions of poly(o‐acetylstyrene) (POAS) were exposed to long‐wave (λ ≥ 300 nm) UV radiation under high vacuum at 25 ± 1°C. Methane and much smaller amounts of ethane were formed, indicating α‐cleavage (Norrish Type I). The quantum yield for CH4 formation (5 × 10−5 mol einstein−1) was an order of magnitude lower than that observed for similar studies of POAS films. Molecular weight (Mn) measurements indicate that chain scission occurs, and this is attributed to β‐scission of the macroradicals formed by H‐abstraction at the α‐C atoms by the carbonyl triplet, and to a lesser extent, by the CH3 radicals. Quenching by both naphthalene and cyclooctadiene conformed to Stern–Volmer kinetics. The effects on chain scission of a number of additives with varying transfer activities were found to be complex and unexpected. Rates of scission not only increased, even when substances with high transfer activity (e.g., cumene) were present, but also varied with the concentration of additive, being higher at lower additive concentrations. It would appear that solvent quality has a dominant influence (these additives are poor solvents). Tighter coiling of the polymer in the solutions containing poorer solvents results in more segment‐segment contacts and with them more photoreduction and chain scission. However, at higher nonsolvent concentrations, diffusive separation of the fragments (and chain scission) becomes more difficult, and the balance is shifted in favor of cyclization (and perhaps also intermolecular crosslinking). The relative high photostability of POAS in solution (cf. thin film) has been interpreted in terms of increased competition from photoisomerization. © 1992 John Wiley & Sons, Inc.