A graft copolymer produced by polymerizing 2-hydroxyethyl methacrylate, or a mixture of ethylenically unsaturated monomers which contains as a major constituent thereof 2-hydroxyethyl methacrylate and contains not more than 20 weight % of at least one ethylenically unsaturated monomer other than 2-hydroxyethyl methacrylate, by using a polymeric polymerization initiator having a number average molecular weight of from about 1,000 to about 1,000,000 produced by copolymerizing (A) 40 to 60 mole % of an N-vinyllactam and (B) 60 to 40 mole % of at least one peroxy-fumarate, said peroxy fumarate (B) being represented by the following formula:
- ##STR14## wherein R1 is a linear alkyl, branched alkyl or cycloalkyl group containing not more than 18 carbon atoms, or an aromatic hydrocarbon group containing 6 to 18 carbon atoms;
- and R2 is a linear alkyl, branched alkyl or cycloalkyl group containing not more than 13 carbon atoms, or a phenyl group.