Process for preparing graft copolymers

A process for preparing a graft copolymer comprising grafting an ethylenically unsaturated monomer onto a halogen-containing polymer dispersed in a polar solvent in the presence of 0.1 to 50 parts by weight, per 100 parts by weight of the halogen-containing polymer used, of a reducing agent and 0.1 to 50 parts by weight, per 100 parts by weight of the halogen-containing polymer used, of a phase transfer catalyst selected from the group consisting of onium compounds and ionophore compounds, whereby a polymerization system is formed by the reaction of the halogen-containing polymer with the reducing agent and the phase transfer catalyst so that free radicals are generated on the halogen-containing polymer thereby to polymerize said monomer by the free radicals so formed to be grafted onto the halogen-containing polymer, said polymerization being conducted at a temperature of from about 0.degree. to about 150.degree. C., the ratio of the weight of the halogen-containing polymer to the weight of the ethylenically unsaturated monomer being in the range of 0.05-20.