We present a study of the initiation step in copper(0)-mediated atom transfer radical polymerization (also known as SET-LRP or SARA ATRP), in which a nitroxide free radical, N-tert-butyl-N[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1), is used to trap radicals derived from ethyl 2-bromoisobutyrate (EBiB), copper metal and pentamethyl diethylene triamine (PMDETA) ligand. A kinetic model was developed relating the rate of conversion of EBiB to the rate constants of activation by copper metal (k(20) and comproportionation of Cu-0 and Cu-II (k(comp)). This allowed the determination of these parameters in several solvents, including dimethyl sulfoxide (DMSO), imethylformamide (DMF), ethanol (EtOH), and acetonitrile (MeCN), in which significant levels of comproportionation were observed, leading to autoacceleration of the reaction. A similar treatment of copper-mediated polymerization kinetics predicts 1/2 order dependence on initiator concentration and copper surface area, in accordance with previously published experimental data.