Novel polymer electrolytes based on network polymers with free chain ends have been prepared, and the effects of the free chain ends on the thermal and mechanical properties, the ionic conductivity, and the charge-transfer resistance at the lithium electrode interface have been explored in detail. Terminal hydroxyl groups of poly(ethylene oxide-co-propylene oxide) triol (MW 7940) were partly methylated, and the residual hydroxyl groups were esterifited by acrylic acid. The resulting macromonomers were cross-linked by photoirradiation in the presence of an electrolyte salt to produce the network polymer electrolytes. The free chain ends, caused by the methylation, were proved to function as an immobile internal plasticizer, as demonstrated by the decreases in the glass transition temperature and the elastic modulus with increasing number of free chain ends; however, creep-free mechanical strength was maintained due to the crosslinked structure. The introduction of free chain ends not only increased the bulk ionic conductivity but also reduced the charge-transfer resistance. As a result, network polymer electrolytes having a large number of the free chain ends exhibited an ionic conductivity of 10(-3) S cm(-1) and a charge-transfer resistance of 20 Omega cm(2) at 80 degrees C when lithium bis(trifluoromethylsulfonyl)imide was used as an electrolyte salt.