The Stability of Free Radicals in Densely Crosslinked Acrylate Polymer Networks

Abstract

The four types of crosslinked polymer samples based on pentaerythritol tetraacrylate (PETA) and propylene glycol-425-diacetoacetate (PGDAA) were prepared in two steps. At first the Michael dark addition of vinyl groups of PETA to PGDAA results in polymer network (cf. Scheme 1). The higher conversion of residual vinyl monomer and pendant double bonds built up in the Michael network was achieved by a photo-initiated free radical polymerization in the second step of polymer network synthesis. The lifetime of trapped radicals was estimated by following the decay of radicals in the dark period of intermittent illumination directly in the resonator cavity of ESR spectrometer. The determined values of bimolecular termination rate constants kt are of order 10–1 to 102 kg/mol·s in dependence on composition, structure of network, and environment. It was observed that oxygen facilitates the decay of free radicals. The kt values are of about 1 to 2 decimal orders higher than that determined in nitrogen. Moreover, as in each subsequent light–dark cycle the kt was increased, we believe that it is a consequence of enhancement the diffusion of free radicals by assisted oxygen chain reaction with formation of hydroperoxides. The exception is the sample B with the highest final crosslinks density and conversion of monomers to network. More tightly trapped free radicals were present in polymer networks photo-cured in nitrogen. The kt values in all four types of networks decreased in each consecutive light–dark cycle.