Model Studies of Alkyl Halide Activation and Comproportionation Relevant to RDRP in the Presence of Cu0

Model studies of alkyl halide activation by Cu0 and comproportionation between CuII/L and Cu0 in the presence of tris[2-(dimethylamino)methyl]amine (Me6TREN), tris(2-pyridylmethyl)amine (TPMA) and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) as ligands were conducted and quantified in dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and acetonitrile (MeCN). When more active alkyl halides such as ethyl α-bromophenylacetate (EBPA) were used, the rate coefficients of activation by Cu0, ka0app, were the same as the rate coefficients of comproportionation, kcompapp indicating that desorption of the newly formed CuI species from the Cu0 surface could be a rate-determining step. When less active alkyl halides, such as methyl 2-bromopropionate (MBrP) were used, the activation was 2.5 orders of magnitude slower than for EBPA and slower than comproportionation. This indicates that atom transfer between alkyl halide and Cu0 is slower than desorption of CuI from the surface. Under the same conditions (solvent, ligands) rates of activation by Cu0 are significantly slower than those by CuI. Rate coefficients of comproportionation in MeCN were 10 and 5 times higher than in DMSO and DMF, respectively, and much larger than those of disproportionation. These results indicate that Cu0 acts as a supplemental activator of alkyl halides and also as a reducing agent for CuII through comproportionation, consistent with the supplemental activator and reducing agent (SARA) ATRP mechanism, as opposed to single electron transfer living radical polymerization (SET-LRP).