Atom transfer radical bulk copolymerization of styrene (St) and methyl acrylate (MA) initiated with trichloromethyl-terminated poly(vinyl acetate) macroinitiator was performed in the presence of CuCl/PMDETA as a catalyst system at 90 degrees C. Linear dependence of In[M](0)/[M] versus time data along with narrow polydispersity of molecular weight distribution revealed that all the homo- and copolymerization reactions proceed according to the controlled/living characteristic. To obtain more reliable monomer reactivity ratios, the Cumulative average copolymer composition at moderate to high conversion was determined by H-1-NMR spectroscopy. Reactivity ratios of St and MA were calculated by the extended Kelen-Tudos (KT) and Mao-Huglin (MH) methods to be r(St) = 1.018 +/- 0.060, r(MA) = 0.177 +/- 0.025 and r(St) = 1.016 +/- 0.053, r(MA) = 0.179 +/- 0.023, respectively, which are in a good agreement with those reported for the conventional free-radical copolymerization of St and MA. Good agreement between the theoretical and experimental composition drifts in the comonomer mixture and copolymer as a function of the overall monomer conversion were observed, indicating that the reactivity ratios calculated by copolymer composition at the moderate to high conversion are accurate. 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. However, MA-centered triad distribution results indicate that the spontaneous gradient copolymers can also be obtained when the mole fraction of MA in the initial comonomer mixture is high enough. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2509-2521, 2009