Reactions of free radicals with polymers—II: Abstraction reactions of methyl and acetyl radicals with poly(vinyl acetophenone)

Abstract

The kinetics of H abstraction by methyl and acetyl radicals from poly(vinyl acetophenone) (PVAP) films (4 × 10 3 mm thick) have been investigated, both radicals being derived from the polymer by photolysis ( λ ≥ 300 nm) under high vacuum conditions (pressure < 10 −4 Pa). Differential equations have been obtained to describe the simultaneous diffusion and reaction of each of the radicals, and the solutions (both steady and non-steady state conditions) have been used in conjunction with experimental data (including yields of methane and acetaldehyde) to obtain values of rate constants for abstraction. which it is argued is likely to occur predominantly at the α-carbon atoms in the polymer. Both steady and non-steady state calculations yield the same values of rate constants. Values of these constants have been compared with each other and that for methyl radical abstraction is compared with data obtained for abstraction from other styrene polymers. PVAP is less reactive than polystyrene towards methyl radicals. Factors accounting for these differences, including diffusant volume, polymer free volume and the energetics of formation of the transition state for abstraction in the various polymers, are considered. Theoretical rates of product formation, based on the solutions of the equations, are compared with the experimental yields of methane and acetaldehyde; a good correspondence is observed for approx. 3 hr reaction time. Subsequent discrepancies between the two sets of data are attributed to the radiation modified diffusion and optical characteristics of the polymer.