Novel biodegradable amphiphilic polymers composed of hydrophobic polyphosphate grafted with well-defined hydrophilic poly [2-methacryloyloxyethyl phosphorylcholine (MPC)] were newly synthesized. 2-Isopropyl-2-oxo-1,3,2-dioxaphospholane and 2-(2-oxo-1,3,2-dioxaphosphoroyloxyethyl-2-bromoisobutylate) (OPBB) were copolymerized by ring-opening polymerization using triisobutylaluminum as an initiator. Polyphosphates (PIBr) whose molecular weights were 3.4 x 10(4) g/mol with 3.0 units of OPBB and 3.7 x 10(4) g/mol with 10.5 units of OPBB were obtained. MPC was grafted from the OPBB sites in PIBr via atom transfer radical polymerization (ATRP). The molecular weights of the graft copolymers [PIBr-g-poly(MPC) (PMPC)] and the amphiphilicity of the graft copolymers were well controllable with polymerization time. The solution properties of various amphiphilic PIBr-g-PMPCs were investigated by the methods of surface tension measurement, light scattering, and fluorescence probe. PIBr-g-PMPC showed high surface activity. The critical micelle concentration (cmc) of the surface tension of the PIBr-g-PMPCs increased with an increase in the molecular weight and density of the PMPC chain. Above the cmc of PIBr-g-PMPC with 3.0 units of PMPC per PIBr, the polymer associated and formed nanosize-hydrogels (nanogels) (R-g = 26.2 nm) observed by a light scattering method and a fluorescence probe method. Degradation of the PIBr-g-PMPC was also investigated under various pH conditions. In a basic condition, the rate of hydrolysis of the graft copolymer was quite large. The polyphosphate of PIBr-g-PMPC completely degraded after only 3 days. PIBr-g-PMPCs were novel biodegradable amphiphilic polymers.