Examination of the preparation of a block copolymer from macroradicals produced in viscous media

Abstract

Extensive experiments on copolymer syntheses as a type of dispersion polymerization in viscous poor solvents (e.g., silicone oil) have been reported (Seymour, R. B.; Kincaid, P. D.; Owen, D. R. Adv. Chem. Ser. 1973, 129, 230) which, based on a trapped-macroradical model, were taken to suggest that such conditions promoted the formation of block copolymers. Here we report the results of a series of tests devised to elucidate the mechanisms in these systems. One such test used initiation by γ radiolysis of water, which allows virtually instantaneous cessation of the production of primary radicals after removal from the γ source. This resulted in a very rapid drop in the polymerization rate. This observation constitutes what seems to be irrefutable evidence against the trapped macroradical model, which would predict that the polymerization should continue at a moderate rate after removal from the source. Furthermore, it is shown that in these viscous-solvent dispersion polymerizations, there is a significant rate of production of free radicals under conditions where the original interpretation implied that this would not be the case. Moreover, it is shown that the formation of graft copolymers can result from the transfer reaction of primary free radical with the preformed homopolystyrene backbone. The original erroneous interpretation (in terms of formation of block copolymers from trapped macroradicals) can thus have arisen because the distinction between block and graft copolymer was beyond the resolving power of the analytical technique used to characterize the products. The results of the present experiments and those of Seymour can be readily explained in terms of rapid termination between two long macroradicals encountering each other through reaction/diffusion and/or between an entangled macroradical and a mobile oligomeric free radical formed by transfer.