Chemical speciation of copper complexes used in atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) is currently one of the most widespread methods in the field of metal-catalyzed living radical polymerization, which allows the production of polymers with well defined features such as high molecular weight, low polydispersity and precise architecture. Although several transition metals have displayed catalytic properties towards the common organic halides used as initiators, copper complexes with nitrogen ligands are the most used catalysts thanks to their low cost and easy handling. Despite the fundamental role Cu complexes play in ATRP, very little attention has been devoted to the identification of the species really present in the reaction medium. Studies on the catalytic system are crucial not only for understanding the reaction mechanism but also for further developments of ATRP as well as its industrial applications. However, due to the paucity of data on the stability constants of copper complexes in organic media, the possibility that complex equilibria may play an important role in the activation mechanism has not been considered yet. Indeed, the precise nature of the activator copper complex has never been clarified. Therefore, detailed analysis of the speciation of copper under the usual ATRP conditions is highly desired to appropriately address this fundamental aspect of the process. Herein, we report the results of a study on the characterization of a ternary system composed of copper, a polydentate nitrogen ligand and a halide ion in a solvent such as acetonitrile that strongly stabilizes the monovalent state. To this end, potentiometric, spectrophotometric and cyclic voltammetry measurements were carried out for determining the stability constants of Cu(I) and Cu(II) complexes with the ligands pentamethyldiethylenetriamine (PMDETA) and tris(2-dimethylaminoethyl)amine (Me6TREN). These two aliphatic polyamines are among the most widely used ligands in ATRP thanks to their commercial availability and the high activity of their copper complexes.