Thermal bulk copolymerization of styrene (St) and maleic anhydride (MAH) has been carried out at 110-130 degrees C and up to around 55 wt % conversion in a stirred tank reactor with an anchor impeller to prepare the random copolymer of St-MAH (R-SMA). A series of experiments in semicontinuous monomer adding process were done to investigate the effects of operating condition on monomer conversion, copolymer composition, and its uniformity. It has been shown that a random copolymer with constant composition can be obtained by semicontinuous copolymerization. A reactor model was developed to simulate the copolymerization processes. The numerical method in which the gel effect on the copolymerization is incorporated has exhibited excellent agreement between the model calculation and the experimental data. However, when using the assumption that(1) k(22) = 0; (2) k(21)[M-1] >> k(12)[M-2]; and(3) (R-1/2h(t))(1/2) is a constant, an analytical solution to the model was found to be available also.