AbstractPoly(styrene‐alt‐maleic anhydride) was prepared by polymerization in benzene, a nonsolvent for the product, and from it partial peresters were prepared in acetone solution, with t‐butyl hydroperoxide and pyridine used as catalyst. The partial peresters were used to initiate methyl methacrylate polymerization by a thermal mechanism in homogeneous solution in acetone, and both homopolymer and graft copolymer were formed. Acrylonitrile was polymerized in aqueous solution by a redox mechanism, and here graft copolymer was produced, but there was no evidence of homopolymer. Parallel polymerizations with poly(styrene‐alt‐maleic anhydride) showed that this had no initiating activity in either system. With methyl methacrylate polymerization thermally initiated with t‐butyl hydroperoxide in the presence of poly(styrene‐alt‐maleic anhydride), grafting by transfer was shown to be substantially less than grafting produced via perester polymers.