This article is the first in a series of papers, describing reversible-deactivation radical polymerization (RDRP) in the presence of metallic copper. The aim of these papers is to determine the proportions and roles of Cu-0, (CuBr)-Br-I/L, and (CuBr2)-Br-II/L, and the overall reaction mechanism. This paper is focused on the comproportionation and disproportionation equilibrium between Cu-0, (CuBr)-Br-I/L and (CuBr2)-Br-II/L in dimethyl sulfoxide (DMSO) for various surface areas of Cu-0 and different ligand concentrations, in both the absence and presence of methyl acrylate (MA). Comproportionation dominated disproportionation when there was enough ligand present in the reaction medium to stabilize all soluble copper species. The relative amount of Cu-I at comproportionation/disproportionation equilibrium increased with ligand concentration. Cu-I represents approximately 99.95% of all soluble Cu species in MA/DMSO = 2/1 (v/v) at the ratio [Me6TREN](0) :[(CuBr2)-Br-II]0 = 6:1. Under typical polymerization conditions, there is essentially no disproportionation, since the ratio [Me6TREN]:[Cu-II] is very large, starting from infinity and decreasing down to 6.7, for similar to 3% terminated chains under the initial conditions [MA](0) :[MBrP](0):[Me6TREN](0) = 222:1:0.1, in 33.3% (v/v) DMSO, with excess Cu-0. The kinetics of comproportionation and disproportionation were both slow, requiring hours to reach equilibrium The apparent rate coefficients for comproportionation and disproportionation were calculated as k(comp)(app) = 9.0 X 10(-4) cm s(-1) and k(disp)(app) = 2.0 X 10(-5) cm s(-1) in DMSO, as well as 3.5 x 10(-3) cm s(-1) and 3.1 X 10(-6) cm s(-1) in MA/DMSO = 2/1 (v/v), respectively. The results of this study invalidate the assumption of instantaneous and complete disproportionation, proposed in single electron transfer living radical polymerization (SET-LRP). These findings agree with Cu-0 acting as a supplemental activator and reducing agent in atom transfer radical polymerization (SARA ATRP).