Kinetic investigation of the activation of alkyl halides of relevance to controlled radical polymerization by copper(I)-amine complexes

Atom transfer radical polymerization (ATRP) is one of the most widely used methods of controlled radical polymerization for the synthesis of a vast range of polymeric materials with controlled molecular weights and well-defined architectures. The process is catalyzed by transition metal complexes, mainly copper complexes with polydentate amine ligands. One of the most important parameters in ATRP is the rate constant (kact) of the activation reaction between the metal catalyst and the initiator or dormant species, PnX. Often activated alkyl halides are used as initiators and mimics of the dormant species. The kinetics of the activation reaction of a wide range of alkyl halides by several copper(I) complexes has been studied by techniques such as NMR, UV-Vis, HPLC and gas chromatography. Since these techniques are suitable for slow reactions, only processes with very low rate constants have been examined. In addition, in most studies the rate constants were determined in the presence of halide ions, X-, without examining the role of such ions in the process. In this study we used electrochemical methods for the determination of kact of the reaction between [CuIMe6TREN]+ (Me6TREN = tris(2-dimethylaminoethyl)amine with some alkyl halides both in the absence and presence of halide ions in acetonitrile. The reactions were investigated in a wide range of temperatures, which allowed determination of the activation parameters of the process. The kinetics of the reactions has been analyzed by voltammery with rotating disc electrode by monitoring the decrease of the limiting current for the oxidation of Cu(I), either under pseudo-first-order conditions or under second-order conditions, depending on the value of kact. A remarkable dependence of kact on the concentration of X- was found for each alkyl halide. This finding, together with the activation parameters and copper speciation data obtained in a separate study by our research group, has shed light on the mechanism of RX activation by copper(I) complexes.