Organo-Copper(II) Complexes as Products of Radical Atom Transfer

Copper complexes bearing polyamine chelate ligands are among the most widely used and highly active catalysts for atom transfer radical polymerization (ATRP). Copper(I) complexes of these ligands (Cu^IL) react with an alkyl halide initiator (RX) in the atom transfer step to generate the higher oxidation state halido complex Cu^IILX and the radical R^•. However, Cu^IL present in the reaction also has the potential to react with the liberated radicals to generate the organometallic species Cu^IILR (where R is formally a carbanion). The reversible association of radical and Cu^IL would facilitate the operation of an alternate, competitive controlled radical polymerization pathway known as organometallic-mediated radical polymerization (OMRP). Recently this possibility has been proposed to occur for a number of different copper catalysts under ATRP conditions, but unequivocal evidence of this organometallic adduct is lacking. Herein we provide direct observation of this species, including an optical spectrum for two of the most commonly used copper catalysts. Furthermore, using cyclic voltammetry coupled to simulations, we are able to determine each of the key thermodynamic and kinetic steps involved in both the atom transfer and radical transfer pathways to assess the impact of ligand, solvent, and initiator on these.