The mechanism of surface-initiated atom transfer polymerization (SI-ATRP) of methacrylates in confined volumes is systematically investigated by finely tuning the distance between a grafting surface and an inert plane by means of nanosized patterns and micrometer thick foils. The 0 polymers were synthesized from monolayers of photocleavable initiators, which allow the analysis of detached brushes by size exclusion chromatography (SEC). Compared to brushes synthesized under "open" polymerization mixtures, nearly a 4-fold increase in brush molar mass was recorded When SI-ATRP was performed within highly confined reaction volumes. Correlating the SI-ATRP of methyl methacrylate (MMA), with and without "sacrificial" initiator to that of lauryl methacrylate (LMA) and analyzing the brush growth rates within differently confined Volumes, we demonstrate faster grafting kinetics with increasing confinement due to the progressive hindering of Cu-II-based deactivators from the brush propagating, front. This effect is especially noticeable when viscous polymerization mixtures are generated and enables the synthesis: of several hundred nanometer thick brushes within relatively short polymerization times. The faster rates of confined SI-ATRP can be additionally used to fabricate, in. one pot, precisely structured brush gradients, when volume confinement is continuously varied across a single substrate by spatially tuning the vertical distance between the grafting and the confining surfaces.