The heat-resistant copolymer of N-phenylmaleimide (NPMI)styrene (St)maleic anhydride (MAH) was synthesized in xylene at 125 degrees C with di-tert-butyl diperoxyterephthalate as an initiator. The characteristics of the copolymer were analyzed by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy (1H-NMR and 13C-NMR), gel permeation chromatography, and elemental analysis. The 13C-NMR results show that the copolymer possessed random sequence distribution; this was also supported by the differential scanning calorimetry experiment, in which a single glass-transition temperature (Tg) of 202.3 degrees C was observed. The thermal stability and degradation mechanism of the copolymer were investigated by thermogravimetric analysis. Using the Kissinger equation and Ozawa equation, we proved a nucleation controlling mechanism with an apparent activation energy of 144 kJ/mol. Blends of acrylonitrilebutadienestyrene with the NPMIStMAH copolymer with various contents were prepared with a twin-screw extruder processes. The mechanical and thermal properties of the materials, such as the tensile and flexural strength, Tg's, and Vicat softening temperatures, were all enhanced with the addition of the modifier, whereas the melt flow index decreased. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012