A kinetic study was conducted to examine the effect of varying the ratio of ligand to transition metal in a Cu(I)Br/NN,N',N",N"-pentamethyldiethylenetriamine (PMDETA) catalyst system for atom transfer radical polymerization (ATRP) of n-butyl acrylate (nBA) using methyl 2-bromopropionate as the initiator. Experimental molecular weights were higher than theoretical when low molecular weight polymers were targeted at low ratios of [PMDETA](0),/[Cu(I)Br](0), (< 1), indicating inefficient initiation/deactivation. A downward curvature in the plot of M. versus conversion was observed at high monomer conversion when targeting high molecular weight polymers. This deviation became more significant when an excess of ligand was used, indicating a contribution of chain transfer to ligand. The maximum rate of polymerization was obtained at [PMDETA](0)/[Cu(I)Br](0) approximate to 0.5 for bulk ATRP of nBA; however for polymerization in the presence of 10 vol% DMF, the maximum appeared at the ratio approximate to 1:1. Addition of acetone or DMF improved solubility of Cu(II) complex, which consequently improved the level of control over the polymerization at low ratios of [PMDETA](0)/[Cu(I)Br](0), but also reduced the reaction rate. The polymerization rate increased with temperature, but at the expense of increased polydispersities. (C) 2004 Wiley Periodicals, Inc.