The exceptional features of Cu0-mediated surface-initiated atom transfer radical polymerization (Cu0 SI-ATRP), and its potential for implementation in technologically relevant surface functionalizations are demonstrated thanks to a comprehensive understanding of its mechanism. Cu0 SI-ATRP enables the synthesis of multifunctional polymer brushes with a remarkable degree of control, over extremely large areas and without the need for inert atmosphere or deoxygenation of monomer solutions. When a polymerization mixture is placed between a flat copper plate and an ATRP-initiator-functionalized substrate, the vertical distance between these two overlaying surfaces determines the tolerance of the grafting process toward the oxygen, while the composition of the polymerization solution emerges as the critical parameter regulating polymer-grafting kinetics. At very small distances between the copper plate and the initiating surfaces, the oxygen dissolved in the solution is rapidly consumed via oxidation of the metallic substrate. In the presence of ligand, copper species diffuse to the surface-immobilized initiators and trigger a rapid growth of polymer brushes. Concurrently, the presence and concentration of added CuII regulates the generation of CuI-based activators through comproportionation with Cu0. Hence, under oxygen-tolerant conditions, the extent of comproportionation, together with the solvent-dependent rate constant of activation (kact) of ATRP are the main determinants of the growth rate of polymer brushes.