Abstract: A modifier composition useful for the preparation of a modified polymer which includes:(1) a compound represented by the general formula (1): ##STR1## wherein R is a hydrogen atom or a methyl group, A is an alkylene group having 2-4 carbon atoms, (OA).sub.n indicates a polyoxyalkylene group, M is a monovalent or divalent cation, n is 2-30, and m is 1 or 2;(2) 50-3,000 ppm, based on the weight of the modifier composition, of an organic polymerization inhibitor; and(3) 0.01-20 ppm of copper ion, based on the weight of the composition;a modified polymer containing the modifier composition and method for producing the modified polymer are provided, wherein the modifier composition has excellent thermal shelf stability at ordinary temperatures.

Abstract: A process for producing a colorless unsymmetrical-diester of maleic acid, useful as a raw material of an emulsifier for emulsion polymerization and as a modifier for a polymer, which comprises:(1) reacting maleic anhydride with a monohydric alcohol in the presence of an inert organic solvent, in a molar ratio of 1:(1.0-1.2) to obtain maleic acid mono ester containing substantially no unreacted maleic anhydride, and(2) reacting said monoester with an alkenyl halide in the presence of a tertiary amine and water, in a molar ratio of said monoester:water of 1:(0.01-0.20).