Method of preparing maleic anhydride terpolymers

A method for polymerizing a terpolymer, comprising adding monomers consisting essentially of a monovinylic aromatic monomer, acrylonitrile and maleic anhydride to a reactor, the maleic anhydride being added to the reactor in a molten state, with the molten maleic anhydride being free of solvents and diluents, and the feed of the monomers to the reactor being controlled to provide a terpolymer consisting essentially of from about 60 to about 95 weight percent of the monovinylic aromatic monomer, from about 5 to about 35 weight percent acrylonitrile and up to about 30 weight percent maleic anhydride;

• and conducting the polymerization reaction in bulk in the absence of solvents and diluents, the polymerization reaction being initiated thermally in the absence of a chemical initiator.