The effect of TMEDA on the kinetics of the anionic polymerization of methyl methacrylate in tetrahydrofuran using lithium as counterion

Abstract

The kinetics of the anionic polymerization of methyl methacrylate in presence of N,N,N', -tetramethylethylenediamine (TMEDA) in THF are investigated using 1,1-diphenyl-hexyllithium as initiator in a temperature range between -20°C and 0°C in a flow-tube reactor. The rate constants of propagation determined in the presence of TMEDA are compared to those obtained in the absence of a chelating agent. For propagation, the reaction order with respect to active centers is found to be 0.5 in both cases which indicates that the chelation of the lithium cation does not effectively perturb the aggregation of the enolate ion pair. Both the rate constants of propagation via non-aggregated ion pairs, k_±, and the equilibrium constants of aggregation, K_A, are not significantly affected by TMEDA. The rate constants of termination in absence and presence of TMEDA are also comparable. Thus, it is concluded that TMEDA is not a better ligand for lithium ester enolates than THF. Even the addition of a cyclic tertiary polyamine ("crown amine") such as 1,4,8,11-tetramethyltetraazacyclotetradecane, does not lead to a significant change in the propagation rate constant, however, no termination is observed at -20°C. Extrapolation of the observed rate constants at two temperatures gives estimates for the equilibrium constants of aggregation, K_A, and the corresponding enthalpy and entropy. It is shown, that aggregation of PMMA-Li in THF is an endothermic, entropy-driven process (ΔH_A; ΔS_A > 0), similar to the dimer-tetramer equilibrium of model ester enolates.