Styrene is copolymerized with maleic anhydride by mixing maleic anhydride with styrene under mass polymerization conditions in a ratio of styrene to maleic anhydride of at least 5:1, continuing to add maleic anhydride throughout a mass polymerization stage until about 25% to about 40% of the styrene monomer is reacted to produce a reaction mass in which the polymerized maleic anhydride is about 1% to about 10% of the total reaction mass. The polymerization is then completed in a pH-adjusted free-radical initiated suspension stage, which generates homopolymer of styrene. During the suspension stage about 10% to about 20% of the bound maleic anhydride is hydrolyzed. The acid component of the bound maleic acid may be converted back to the anhydride through the use of a vented extruder.