Mechanism and kinetics of activation of alkyl halides by Cu(I) complexes used as catalysts in atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) is one of the most used methods of controlled/living radical polymerization for the synthesis of a vast range of well-defined, low-polydispersity polymeric materials. The process is initiated by a reversible reaction between a transition metal complex (mainly Cu(I) with an amine ligand, CuIL+) and an activated alkyl halide to produce the propagating radical. The initiation reaction is considered to involve a transfer of a halogen atom from the alkyl halide to the metal center. A fundamental question which, however, has not been adequately addressed is on the nature of the active Cu(I) species, since the metal ion exists under ATRP conditions as a multiplicity of species. A second important issue on the activation reaction regards determination of the activation rate constants. Several methods, mainly limited to the study of slow reactions, have previously been proposed to measure kact. Herein, we first examine the mechanism of the activation reaction with the aim of unambiguously identifying the active Cu(I) species. We investigated the kinetics of activation of RX by CuIL+ (L = Me6TREN) in CH3CN both in the absence and presence of X-. It is found that although the system CuI/L/X- is mainly composed of CuIL+, XCuIL and CuIX2-, only CuIL+ is an active catalyst reacting with RX. Next, we describe electrochemical methods for the determination of kact both for slow and fast reactions and apply them for the measurement of kact for a variety of activated alkyl halides and Cu(I) complexes.