Abstract
The rate coefficients for activation (ka 0 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 ka 0 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 (Me 6 TREN) 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 ka 0 app . The activation rate coefficient of MBrP was ka 0 app = 1.8 × 10 -4 cm s -1 with Me 6 TREN as the ligand in DMSO at 25 °C while the activation rate coefficient of Br-capped poly(methyl acrylate) by Cu 0 was slightly lower, ka 0 app = 1.0 × 10 -4 cm s -1 , as measured in a polymerization of MA in MA/DMSO = 1/1 (v/v) with Me 6 TREN. On the basis of the measured rate coefficients, the activation rate of MBrP by 1 mM Cu I Br/Me 6 TREN (ka 1 app = 3.2 × 10 2 M -1 s -1 ) is similar to the activation rate by 2 km Cu 0 wire with diameter of 0.25 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 comproportionation 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 . © 2013 American Chemical Society.