A thermoplastic sheet containing a polymer composition including a polymer formed by polymerizing a mixture including 40% to 90% of styrenic monomers; 5% to 45% of maleate-type monomers; 0.1% to 25% by weight of elastomeric polymers with Mn greater than 12,000; and 0.1% to 10% by weight of low molecular weight polymers that include one or more monomers according to the formula CH.sub.2.dbd.CR.sup.3R.sup.2, where R.sup.3 is H or a C.sub.1-C.sub.3 alkyl group and R.sup.2 is a C.sub.1-C.sub.22 linear, branched or cyclic alkyl or alkenyl groups. The low molecular weight polymer has Mn of from 400 to 12,000 and can optionally include functional groups. The 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.