A series of densely grafted star- and comb-shaped molecular brushes composed of polystyrene, poly(tertbutyl acrylate), and poly(N-iscopropylacrylamide) were prepared by atom transfer radical polymerization (ATRP) using either cyclotriphosphazenes or polyphosphazenes as initiators. The initiators were prepared by the introduction of a free hydroxyl group into the side chains of a phosphazene cyclic trimer and polymer, followed by esterification with 2-bromopropionyl bromide. The grafting conditions were optimized for various monomers. The kinetics of the reaction were first order with respect to the monomer concentration in both cyclotriphosphazene and polyphosphazene systems. The molecular weights of the resulting polmers were estimated by gel permeation chromatography (GPC). The side chains of the brush polymers were cleaved from the backbone and analyzed by GPC to confirm the synthesis Of well defined polymer brushes. Brushes based on poly(tert-butyl acrylate) were subjected to hydrolysis to yield negatively charged brushes. In addition, the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) brush polymers was measured by both dynamic light scattering (DLS) and differential scanning calorimetry (DSC), exhibiting a sharp phase transition at 33 degrees C. Furthermore, star- and comb block copolymers with a hard polystyrene core and a soft poly(tert-butyl acrylate) shell were also synthesized.