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 (CuL) react with an alkyl halide initiator (RX) in the atom transfer step to generate the higher oxidation state halido complex CuLX and the radical R. However, CuL present in the reaction also has the potential to react with the liberated radicals to generate the organometallic species CuLR (where R is formally a carbanion). The reversible association of radical and CuL 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.

In a possibly biomimetic fashion, formally copper­(III)–oxygen complexes LCu­(III)–OH (1) and LCu­(III)–OOCm (2) (L2– = N,N′-bis­(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Cm = α,α-dimethylbenzyl) have been shown to activate X–H bonds (X = C, O). Herein, we demonstrate similar X–H bond activation by a formally Cu­(III) complex supported by the same dicarboxamido ligand, LCu­(III)–O2CAr1 (3, Ar1 = meta-chlorophenyl), and we compare its reactivity to that of 1 and 2. Kinetic measurements revealed a second order reaction with distinct differences in the rates: 1 reacts the fastest in the presence of O–H or C–H based substrates, followed by 3, which is followed by (unreactive) 2. The difference in reactivity is attributed to both a varying oxidizing ability of the studied complexes and to a variation in X–H bond functionalization mechanisms, which in these cases are characterized as either a hydrogen-atom transfer (HAT) or a concerted proton-coupled electron transfer (cPCET). Select theoretical tools have been employed to distinguish these two cases, both of which generally focus on whether the electron (e–) and proton (H+) travel “together” as a true H atom, (HAT), or whether the H+ and e– are transferred in concert, but travel between different donor/acceptor centers (cPCET). In this work, we reveal that both mechanisms are active for X–H bond activation by 1–3, with interesting variations as a function of substrate and copper functionality.

The direct and quantitative reduction of the air-stable Cu(ii)Br2/Me6TREN to Cu(0) by NaBH4 represents a new method for the aqueous single electron transfer-living radical polymerization (SET-LRP) of water soluble polymers.

The pathway of atom transfer radical polymerisation (ATRP) is influenced by the nature of the alkyl bromide initiator (RBr) to the extent that reactions between the radical R˙ and the original copper(i) catalyst can divert the reaction toward different products.

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