Cu(0) was prepared via disproportionation of Cu(I)Br in the presence of Me-6-TREN in various solvents in a glove box. The resulting nanopowders were used as mimics of "nascent" Cu(0) catalyst in the single-electron transfer living radical polymerization (SET-LRP) of methyl acrylate (MA), providing faster polymerization than any commercial Cu(0) powder, Cu(0) wire, or Cu(I)Br and achieving 80% conversion in only 5 min reaction time. Despite the high rate, a living polymerization was observed with linear evolution of molecular weight, narrow polydispersity, no induction period, and high retention of chain-end functionality. In addition to providing an unprecedentedly fast, yet controlled LRP of MA, these studies suggest that the very small "nascent" Cu(0) species formed via disproportionation in SET-LRP are the most active catalysts. Thus, when bulk Cu(0) powder or wire may be the most abundant catalyst and dictates the overall kinetics, any Cu(0) produced via disproportionation will be rapidly consumed and contributes to the overall catalytic cycle. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 403-409, 2010