A styrenic copolymer composition comprising 55% to 94% by weight of one or more styrenic monomers; 2% to 25% by weight of one or more maleate-type monomers; and 4% to 20% by weight of an elastomer composition comprising a first a first high molecular weight elastomer polymer having a number average molecular weight of greater than 12,000, a particle size ranging from about 0.05 micron to about 1.0 micron, and ranging from 50 to 99% by weight based on the weight of the elastomer composition, and a second high molecular weight elastomer polymer having a number average molecular weight of greater than 12,000, a particle size ranging from about 0.50 to about 3.0 microns, and ranging from 1.0% to 50% by weight based on the weight of the elastomer composition. The styrenic copolymer composition optionally further comprises from about 0.1 to about 8.0% by weight. The styrenic copolymer composition is formed via polymerizing methods. A thermoplastic sheet is made by extruding the polymer melt composition to provide a thermoplastic sheet. The thermoplastic sheets can be thermoformed into containers suitable for use in microwave heating of food.