This article is the second part of a mechanistic study regarding the role of Cu(0) and CuBr during the Cu(0)-mediated polymerization in organic and aqueous media with the aim of offering a better understanding of the mechanism. In this contribution, we focus on disproportionation and comproportionation studies in aqueous and organic/aqueous media in the presence of either Cu(0) generated in situ and Cu(0) wire. The solvent composition, the nature of the monomer and the ligand concentration dramatically affect the thermodynamic and kinetic equilibria while changing the sequence of the reagent addition caused significant variations not only on the disproportionation equilibrium but also on the dispersities of the products obtained. This was attributed to different complexation reactions between the monomer, the solvent, the ligand, and the copper species. Reagent feeding experiments with low concentrations of CuBr were also conducted in an attempt to mimic the role of Cu(0) as a potential supplemental activator, further assessing the contributions of Cu(0) and CuBr on the polymerization rate and control over the molecular weight distributions in the presence of either disproportionating (Me6TREN) and nondisproportionating (TPMA) ligands. Crucially, the exploitation of stoichiometric amounts of Cu(0) and CuBr relative to CuBr2 allowed for a direct comparison between the SET-LRP and atom transfer living radical polymerization (ATRP) protocols, revealing very different contributions of the two catalysts depending on the conditions employed.