free radicals capable of initiating polymerization reactions of u03b1 u03b2 ethylenically unsaturated monomers activation of polymerization reactions initiated by peroxide initiators with tertiary amines which induce the decomposition of peroxides into free radicals are also well known to those skilled in the art use of mercaptans to increase the rate of polymerization of hemi perester initiated polymerization and curing of methyl methacrylate containing syrups has been described in u s pat no 3 362 942 hereby incorporated by reference the activator of this invention is a salt of an oxo acid of sulfur or the free acid while the exact mechanism of activation is not known it is thought that those members of this group whose anions are capable of reacting with the double bond of the maleic hemi perester are effective in increasing the rate of polymerization and cure among useful salts are included bisulfites thiosulfates and metabisulfites the latter when dissolved in water are known to generate bisulfites thought to be the active species which co react with the hemi peresters of maleic acid of these preferred salts are those having cations selected from the group consisting of group ia metals group iia metals and ammonium the activator is present in a weight ratio to the hemi perester of maleic acid of from approximately 1 7 to 4 1 preferably from 1 7 to 1 1 and even more preferably for optimum cure rates in a ratio of approximately 1 3 to 1 1 most preferred are the weight ratios of from 1 1 2 to 1 1 the activator can be added to the syrup in the form of an aqueous solution increased levels of water can further increase the rate of.