The rate coefficients for activation (k(a0)(app)) of two alkyl halides, methyl 2-bromopropionate, MBrP, and Br-capped poly(methyl acrylate), by Cu-0 were determined under various conditions. The value of k(a0)(app) was studied in two solvents, dimethyl sulfoxide (DMSO) and acetonitrile (MeCN), and their mixtures with methyl acrylate (MA), using tris[2-(dimethylamino)ethyl]amine (Me6TREN) and tris(2-pyridylmethyl)amine (TPMA) as ligands. The experiments showed that the rate of activation increased with the surface area of Cu-0 but was typically not affected by the ratio of ligand to initiator, if a sufficient amount of ligand was present. The choice of solvent and presence of monomer/polymer had a small influence on k(a0)(app). The activation rate coefficient of MBrP was k(a0)(app). 1.8 X 10(-4) cm s(-1) with Me6TREN as the ligand in DMSO at 25 degrees C while the activation rate coefficient of Br-capped poly(methyl acrylate) by Cu was slightly lower, k(a0)(app) = 1.0 X 10(-4) cm s(-1), as measured in a polymerization of MA in MA/DMSO = 1/1 (v/v) with Me6TREN. On the basis of the measured rate coefficients, the activation rate of MBrP by 1 mM (CuBr)-Br-I/Me6TREN (k(a1)(app) = 3.2 X 10(2) M-1 s(-1)) is similar to the activation rate by 2 km Cu-0 wire with diameter of 025 mm in 7 mL of DMSO. Thus, under typical conditions, conducted in the presence of ca. 1 cm Cu-0 wire, alkyl halides are predominantly activated by Cu-I species. Consequently, Cu-0 acts as a supplemental activator and also as a reducing agent (SARA) because cornproportionation dominates disproportionation, for the polymerization of MA in DMSO. These results support the SARA ATRP mechanism rather than the proposed single electron transfer-living radical polymerization (SET-LRP) process, which requires exclusive activation by Cu-0 and instantaneous disproportionation of Cu-I.