Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. Activation of Alkyl Halides by Cu0

The rate coefficients for activation (k a0 app) of two alkyl halides, methyl 2-bromopropionate, MBrP, and Br-capped poly(methyl acrylate), by Cu0 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 Cu0 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 × 10–4 cm s–1 with Me6TREN as the ligand in DMSO at 25 °C while the activation rate coefficient of Br-capped poly(methyl acrylate) by Cu0 was slightly lower, k a0 app = 1.0 × 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 CuIBr/Me6TREN (k a1 app = 3.2 × 102 M–1 s–1) is similar to the activation rate by 2 km Cu0 wire with diameter of 0.25 mm in 7 mL of DMSO. Thus, under typical conditions, conducted in the presence of ca. 1 cm Cu0 wire, alkyl halides are predominantly activated by CuI species. Consequently, Cu0 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 Cu0 and instantaneous disproportionation of CuI.