Novel amphiphilic heterograft copolymers consisting of phosphoester functionalized PEG (phosPEG) and PCL (phosPCL) were synthesized by the ring-opening polymerization via "grafting through'' method. The heterograft structure and thermal properties of these copolymers with various compositions were characterized by (1)H-NMR, (31)P NMR, size exclusion chromatography (SEC), and differential scanning calorimetry (DSC) in detail. These amphiphilic copolymers could self-assemble into micellar structures in aqueous solution, and their critical micellization concentrations (CMC) were determined to be 0.69-1.25 mg/L by fluorescence technique. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements show that these heterograft copolymer micelles are spherical in shape with the particle size ranging from 20 to 60 nm, which has potential in biomedical application. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 365-374, 2012