The copolymerization of styrene (St) and maleic anhydride (MA) was carried out in supercritical carbon dioxide (SC CO2). It was found that St and MA are easy to copolymerize in SC CO2 and the conversion can reach 97%, and that very fine and dry powders are obtained. The products were characterized using Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). GPC data showed that the molecular weight of the copolymer reach 3.62 x 10(4) g mol(-1). Scanning electron microphotographs showed the minimum particle size of the product is about 200 nm. DSC measurements indicated that the glass transition temperature (T-g) of the copolymer increases with increasing the M-A content in the copolymer. TGA curve showed that the copolymers were decomposed at about 350 degrees C. (c) 2007 Wiley Periodicals, Inc.