PURPOSE:To obtain a graft copolymer for use in foams excellent in flame retardancy and suitable for building materials, by graft-polymerizing a vinyl chloride-based monomer onto a styrene polymer in the presence of an aqueous medium and an oil-soluble polymerization initiator under a specified condition. CONSTITUTION:A vinyl chloride-based monomer is graft-polymerized onto a styrene polymer in the presence of an aqueous medium, an oil-soluble polymerization initiator (e.g., di-2-ethylhexyl peroxydicarbonate) and, optioinally, a dispersant (e.g., partially saponified polyvinyl acetate). Then, the purpose graft copolymer for flame-retarding foams is obtained by effecting the graft copolymerization under a condition including a vinyl chloride-based monomer/styrene polymer weight ratio controlled to below 1/1 at the start of the polymerization and, optionally, adding a vinyl chloride-based monomer during the course of the graft polymerization.
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