The kinetics of atom transfer radical polymerization (ATRP) of methyl acrylate (MA) is discussed. MA polymerizations were carried out under homogeneous conditions using CuBr/4,4＇-di(5-nonyl)-2,2＇-bipyridine (dNbpy) as the catalyst. Plots of ln([M]o/[M]) vs time and molecular weight evolution vs conversion showed a linear dependence, indicating a constant number of propagating species throughout the polymerization as well as a negligible contribution of termination or transfer reactions. The dependence of the rate of polymerization on the concentrations of initiator, catalyst, Cu(II), and temperature are presented. The equilibrium constant between active and dormant species, K-eq = 1.2 x 10(-9) at 90 degrees C is much smaller than in the ATRP of styrene or methyl methacrylate. On the basis of an Arrhenius plot, the apparent enthalpy of activation for homogeneous ATRP of MA Delta H double dagger(app) = 27.5 kcal/mol corresponds to an enthalpy of equilibrium Delta H-eq(0) = 23 kcal/mol, which is much higher than in the ATRP of styrene or methyl methacrylate. Because halides can potentially create a bridged structure between two copper centers, a Cu(I) species with a complex counteranion that cannot bridge was used, Cu(CH3CN)(4)+PF6- (CuPF6). The resulting apparent rate constant of polymerization under homogeneous conditions was approximately 40 times that of the corresponding CuBr/dNbpy catalyzed reaction. The apparent enthalpy of activation for the CuPF6/dNbpy system was Delta H double dagger(app) = 10.3 kcal/mol, in contrast to the larger value found for the CuBr/dNbpy system. Possible reasons behind these unexpected results are discussed.