Kinetic investigation of the atom transfer radical polymerization of methyl acrylate

The kinetics of atom transfer radical polymerization (ATRP) of methyl acryl ate (MA) is discussed. MA polymerizations were carried out under homogeneou s conditions using CuBr/4,4'-di(5-nonyl)-2,2'-bipyridine (dNbpy) as the cat alyst. Plots of ln([M]o/[M]) vs time and molecular weight evolution vs conv ersion showed a linear dependence, indicating a constant number of propagat ing species throughout the polymerization as well as a negligible contribut ion of termination or transfer reactions. The dependence of the rate of pol ymerization on the concentrations of initiator, catalyst, Cu(II), and tempe rature are presented. The equilibrium constant between active and dormant s pecies, K-eq = 1.2 x 10(-9) at 90 degrees C is much smaller than in the ATR P 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 stru cture between two copper centers, a Cu(I) species with a complex counterani on that cannot bridge was used, Cu(CH3CN)(4)+PF6- (CuPF6). The resulting ap parent rate constant of polymerization under homogeneous conditions was app roximately 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 result s are discussed.