The phase-transfer catalyzed graft copolymerization of methyl methacrylate onto polychloroprene was carried out using tricaprylylmethylammonium chloride as a phase-transfer catalyst in a two-phase system of an aqueous Na2S2O8 solution and toluene at 55 degrees C under a nitrogen atmosphere. The initial rate of graft copolymerization was expressed as the combined terms of quaternary onium cation and peroxydisulfate anion in the aqueous phase rather than the fed concentrations of catalyst and Na2S2O8. The observed initial rate of graft copolymerization was used to analyze the graft copolymerization mechanism with a cycle phase-transfer initiation step in the heterogeneous liquid-liquid system.