Model studies of alkyl halide activation by Cu-0 and comproportionation between Cu-II/L and Cu-0 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 a-bromophenylacetate (EBPA) were used, the rate coefficients of activation by Cu-0, k(a0)(app), were the same as the rate coefficients of comproportionation, k(comp)(app) indicating that desorption of the newly formed Cu-I species from the Cu-0 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 Cu-0 is slower than desorption of Cu-I from the surface. Under the same conditions (solvent, ligands) rates of activation by Cu-0 are significantly slower than those by Cu-I. 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 Cu-0 acts as a supplemental activator of alkyl halides and also as a reducing agent for Cu-II through comproportionation, consistent with the supplemental activator and reducing agent (SARA) ATRP mechanism, as opposed to single electron transfer living radical polymerization (SET-LRP).