The free-radical graft copolymerization of maleic anhydride (MAH) onto polypropylene (PP) with the assistance of styrene (St) in supercritical carbon dioxide (CO2) was studied. The effects of the St concentration and initiator concentration on the functionality degree of the grafted PP in supercritical CO2 were investigated. The addition of St drastically increased the MAH functionality degree, which reached a maximum when the molar ratio of MAH and St was 1:1. St, an electron-donating monomer, could interact with MAH through charge-transfer complexes to form the St-MAH copolymer (SMA), which could then react with PP macroradicals to produce branches by termination between radicals. There was SMA in the grafting reaction system characterized by Fourier transform infrared and differential scanning calorimetry. Furthermore, the highest MAH functionality degree was obtained when the concentration of 2,2'-azobisisobutyronitrile (AIBN) was 0.6 wt % based on PP. The effects of the temperature and pressure of supercritical CO2 on the functionality degree of the grafted PP were analyzed. An increase in the temperature accelerated the decomposition rate constant of AIBN, thereby promoting the grafting reaction. In addition, an increase in the temperature increased the diffusion of monomers and radicals in the disperse reaction system of supercritical CO2 The highest degree of functionality was found at 80degreesC. Also, the functionality degree of grafted PP decreased with an increase in the pressure of supercritical CO2 within the experimental range. The morphologies of pure PP and grafted PP were significantly different under polarizing optical microscopy. The PP spherulites were about 38 mum in size, and the grafted PP spherulites were significantly reduced because of heterogeneous nucleation. (C) 2003 Wiley Periodicals, Inc.