The synthesis of polyphosphazene-co-poly(ethylene oxide) block copolymers with well-defined molecular weights has been accomplished via end-functionalized polymer intermediates. Mono- and diamine-terminated poly(ethylene oxides) (PEO = (CH2CH2O)(n)-CH2CH2-) were used to produce polymerbased phosphoranimines, CH3O-(CH2CH2O)(n)-CH2CH2-NH(CF3CH2O)(2)P=NSiMe3 and Me3SiN=P(OCH2-CF3)NH-(CH2CH2O)(n)-CH2CH2-NH(CF3CH2O)(2)P NSiMe3. These were then converted to macroinitiators via reaction with PCl5 to produce CH3O-PEO-NH[(CF3CH2O)(2)P=NPCl3](+)[PCl6](-) and [Cl3PN=P(OCH2-CF3)NH-PEO-NH[(CF3CH2O)(2)P=NPCl3](2+) [PCl6](2)(-), respectively. These macroinitiators were used to polymerize Cl3P=NSiMe3 in a living manner to produce diblock copolymers of polyphosphazenes with poly(ethylene oxide) or triblock copolymers with poly(ethylene oxide) blocks flanked by polyphosphazene components. In addition, the monophosphoranimine-terminated PEO was employed as a terminator in the living, cationic polymerization of Cl3P=NSiMe3 to produce triblock copolymers with a polyphosphazene block flanked by two poly(ethylene oxide) blocks. Polymers for use as solid-ionic conduction media or hydrogels were produced by nucleophilic replacement of the chlorine atoms along the polyphosphazene segments by methoxyethoxyethoxy units. The ionic conductivities of these materials, after complexation with varying ratios of lithium triflate, ranged from 7.6 x 10(-6) to 1.0 x 10(-4) S cm(-1) for a temperature range of 20 - 80 degreesC.