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In order to estimate the effect of Cu(0) particle size in SET-LRP, a comparative analysis of Cu(0)/Me₆-TREN-catalyzed polymerization of MA initiated with methyl 2-bromopropionate at 25 °C was performed in DMSO, a solvent that mediates the disproportionation of Cu(I)X, and in MeCN, a solvent in which Cu(I)X does not disproportionate Cu(I)X. Decreasing the Cu(0) particle size results in a marked increase in the apparent rate constant of propagation (k_p^app). Decreasing the Cu(0) particle size from 425 to 0.05 μm (50 nm) increases the k_p^app by almost an order of magnitude. Regardless of the Cu(0) particle size used, in DMSO a perfect SET-LRP occurs with a first-order polymerization in growing species up to 100% conversion. However, in MeCN the polymerization is not first order in growing species. The results presented here demonstrate that, in addition to the disproportionation of Cu(I)X/L into Cu(0) and Cu(II …