Solvent effects on the activation of atom transfer radical polymerization

Atom transfer radical polymerization is an important method of synthesis of polymeric materials with controllable molecular weights, narrow molecular weight distributions, and well-defined functionalities, compositions and architectures. The process is catalyzed by a transition metal complex, which reversibly reacts with an initiator RX to give the propagating radical R● and the metal complex at a higher oxidation state. The most widely used initiators and catalysts are activated alkyl halides and copper complexes with amine ligands, respectively. The success of the process relies on the establishment of a fast equilibrium with a small equilibrium constant, KATRP, so that the concentration of the active propagating radicals is very small and bimolecular termination reactions are hampered. Both KATRP and the activation rate constant, kact, are influenced by the redox properties of the catalyst, which remarkably depends on the reaction conditions, especially the nature of the solvent. In this communication we wish to discuss the results of a study aimed to rationalize the role of the solvent on the activation step of ATRP. We investigated the effect of selected 10 solvents on the standard reduction potentials of ClCuIIL+, BrCuIIL+ and CuIIL2+, where L = Me6tren is one the most active ligands used in copper-catalyzed ATRP. The kinetics of the activation reaction with a model alkyl halide initiator were also examined in a series of solvents. The results were analyzed through non-linear correlations with solvent parameters, which evidenced that the solvent interacts with the metal complex, especially Cu(II), thereby significantly affecting the activation/deactivation reaction.