Amphilic triblock copolymers with varying ratios of hydrophilic poly[bis (methoxyethoxyethoxy)phosphazene] (MEEP) and relatively hydrophobic poly(propylene glycol) (PPG) blocks were synthesized via the controlled cationic-induced living polymerization of a phosphoranimine (Cl3P=NSiMe3) at ambient temperature. A PPG block can function as either a classical hydrophobic block or a less hydrophobic component by varying the nature of a phosphazene block. The aqueous phase behavior of MEEP-PPG-MEEP block copolymers was investigated using fluorescence techniques, TEM, and dynamic light scattering (DLS). The critical micelle concentrations (cmcs) of MEEP-PPG-MEEP block copolymers were determined to be in the range of 3.7-16.8 mg/L. The mean diameters of MEEP-PPG-MEEP polymeric micelles, measured by DLS, were between 31 and 44 rim. The equilibrium constants of pyrene in these micelles ranged from 4.7 x 10(4) to 9.6 x 10(4). (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 692-699, 2009