The rate of polymerization (Rp) of 2-hydroxyethyl methacrylate, at low conversion, using aliphatic azo compounds as photoinitiators is considerably faster in water than in acetonitrile as a solvent. Active radical production in different solvents was analysed in detail. The quantum yield of active radicals showed only a minor dependence on the medium properties. These differences are due to changes in primary radical recombination reactions and they are unrelated to changes of Rp observed in different media. By employing intermittent illumination techniques, it is shown that the increased Rp in aqueous solution is due to a reduced rate of termination. Similar propagation rate constants were obtained in acetonitrile and water, whereas the termination rate constant increases by one order of magnitude when acetonitrile is employed instead of water. These results are discussed in terms of the effect of the cosolvent on the medium viscosity and the macroradical conformation.
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Alvarez‐Idaboy et al. (1999) studied this question.