We report on the synthesis and characterization of poly(diethylene glycol methylether methacrylate) (PDEGMA) brushes by surface-initiated atom transfer radical polymerization inside ordered cylindrical nanopores of anodic aluminum oxide with different pore radii between 20 and 185 nm. In particular, the dependence of polymerization kinetics and the degree of pore filling on the interfacial curvature were analyzed. On the basis of field emission scanning electron microscopy data and thermal gravimetric analysis (TGA), it was concluded that the polymerization rate was faster at the pore orifice compared to the pore interior and also as compared to the analogous reaction carried out on flat aluminum oxide substrates. The apparent steady-state polymerization rate near the orifice increased with decreasing pore size. Likewise, the overall apparent polymerization rate estimated from TGA data indicated stronger confinement for pores with increased curvature as well as increased mass transport limitations due to the blockage of the pore orifice. Only for pores with a diameter to length ratio of similar to 1, PDEGMA brushes were concluded to grow uniformly with constant thickness. However, because of mass transport limitations in longer pores, incomplete pore filling was observed, which leads presumably to a PDEGMA gradient brush. This study contributes to a better understanding of polymer brush-functionalized nanopores and the impact of confinement, in which the control of polymer brush thickness together with grafting density along the nanopores is key for applications of PDEGMA brushes confined inside nanopores.