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 y radiolysis of water, which allows virtually instantaneous cessation of the production of primary radicals after removal from the y 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.

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The effects of 9‐vinyl anthracene (VAn) on solution and emulsion polymerization of styrene are examined, to gain information about the mechanism of particle formation in emulsion polymerizations. Styrene solution polymerization in ethyl benzene is found to be inhibited by small amounts of VAn. In an emulsion system, the effects of VAn are found to depend on surfactant concentration [S]. With [S] = 0, addition of VAn engenders some inhibition and very little change in the steady‐state polymerization rate. As [S] is increased from zero to above the critical micelle concentration (cmc), a slight increases in the induction period and an increasing retardation (reduction in the steady‐state rate) are observed. Below the cmc, VAn has no significant effect on the particle number, but above the cmc it considerably increases the latter with concomitant appearance of very small (5–15 nm radius) particles. The data strongly support the supposition that the coagulative/homogeneous nucleation mechanism dominates particle formation for [S] below the cmc. Above the cmc, the data are compatible with both the micellar entry and coagulative/homogeneous mechanisms.