The heterogeneous atom transfer radical polymerization of methyl methacrylate using the catalyst complex CuBr/1,1,4,7,10,10-hexamethyltriethylene tetramine (CuBr/HMTETA) was undertaken under a number of conditions to elucidate the effects of key process variables. It was found that a 100-fold reduction in metal salt concentration, to 21 ppm in solution (metal to intiator = 0.01:1), reduced polymerization rates 3-fold. PMMA produced at these low concentrations had high initiator efficiencies (I-eff = 88%) and low polydispersisties (1.1). Decreasing metal salt concentrations 1000-fold, to 2 ppm (Mt:I = 0.001:1), resulted in PMMA with controlled molecular weights (I-eff = 96%) but high polydispersities (2.1). Polymerization rates above and below a metal concentration of 210 ppm were found to be proportional to the metal salt concentration to the power of 0.05 and 0.37, respectively. Rates were found to increase with ligand concentration to the power of 0.52. Stirring was found to increase polymerization rates by 40%.