PRODUCTION OF GRAFT POLYMER

PROBLEM TO BE SOLVED: To produce a graft polymer having a controlled structure in an industrially advantageous manner by radical-polymerizing a redical-polymerizable monomer with a polymer having mercapto groups branching from the molecular chain. SOLUTION: This polymer is produced by (1) radical-polymerizing a monomer mixture essentially consisting of a radical-polymerizable ethylene compound having an acylthio group in the molecule to obtain a polymer (a) having acylthio groups branching from the molecular chain, (2)removing the protective groups from polymer (a) to obtain a polymer (b) comprising at least one type of mercapto-containing repeating units selected from the group consisting of repeating units represented by formula I (wherein R is H or methyl; and X is a divalent hydrocarbon group), formula II (wherein X is X ), formula III (X is X ) and formula IV (wherein R is R ; and X is X ), and (3) radical- polymerizing a radical-polymerizable monomer with polymer (b).