Atom transfer radical bulk copolymerization of styrene (St) and methyl methacrylate (MMA) was performed in the presence of CuCl/PMDETA as a catalyst system and trichloromethyl-terminated poly(vinyl acetate) telomer as a macroinitiator at 90 degrees C. The overall monomer conversion was followed gravimetrically and the cumulative average copolymer composition at moderate to high conversion was determined by H-1 NMR spectroscopy. Reactivity ratios of St and MMA were calculated by the extended Kelen-Tudos (KT) and Mao-Huglin (MH) methods to be r(St) = 0.605 +/- 0.058, r(MMA) = 0.429 +/- 10.042 and r(St) = 0.602 +/- 0.043, r(MMA) = 0.430 +/- 0.032, respectively, which are in good agreement with those reported for the conventional free-radical copolymerization of St and MMA. The 95% joint confidence limit was used to evaluate accuracy of the estimated reactivity ratios. Results showed that in the controlled/living radical polymerization systems such as ATRP, more reliable reactivity ratios are obtained when copolymer composition at moderate to high conversion is used. Good agreement between the theoretical and experimental composition drifts in the comonomer mixture and copolymer as a function of the overall monomer conversion was observed, indicating the accuracy of reactivity ratios calculated by copolymer composition at the moderate to high conversion. Instantaneous copolymer composition curve and number-average sequence length of comonomers in the copolymer indicated that the copolymerization system tends to produce a random copolymer. (c) 2008 Elsevier Ltd. All rights reserved.