The availability of catalytic/reducing sites at metallic Cu( )(0)sources during supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) is regulated by the components of the polymerization mixture, including ligand (L), alkyl halide initiator (R-X), and Cu-II-based deactivator. Their contributions were analyzed by quantifying the dissolution of Cu species within a quartz crystal microbalance with dissipation (QCM-D), subjecting a Cu-0-coated sensor to different polymerization mixtures. The control of catalyst diffusion from Cu-0 was subsequently exploited to fabricate structured polymer brushes with diverse compositions, when ATRP was performed from surface-immobilized initiators in the presence of a Cu-0 plate, placed at a determined distance (d) from the substrate. Surface-initiated ATRP in the presence of Cu-0 (Cu-0-SI-ATRP) is compatible with a broad variety of monomers, including oligo(ethylene glycol) acrylate (OEGA), methyl acrylate (MA), and acrylamide (AAm). The kinetics of brush growth is finely tuned by the independent variation of d, polymerization time, and concentration of added deactivator. Modulation of these parameters allowed us to generate homopolymer and multiblock copolymer brush gradients featuring a variety of morphologies and controlled interfacial properties, with unprecedented spatial resolution over the brush structure.