Styrenic resin composition comprising a rubber modified styrene maleic anhydride copolymer and polybutene. The resin is prepared by several methods including adding polybutene into the reactor, or adding polybutene to the syrup exiting the reactor and prior to entering the devolatilizer, or compounding polybutene into the polymer in an extruder after the polymer exits the devolatilizer. The polybutene ranges from 0.1 to 8% by weight and has a number average molecular weight from 900 to 2500. The rubber ranges from 4% to 20% by weight and has a particle size from 0.1 micron to 11 microns. The resin can be extruded into sheet and thermoformed into an article or can be coextruded to produce a laminated article, which may be a container for packaged foods that can be heated in microwave ovens and which container has improved toughness, elongation, and heat distortion resistance properties.