Copolyethers were prepared by ring-opening polymerization of two oxiranes, ethylene oxide (EO) and allyl glycidyl ether (AGE), in a high-pressure reactor vessel. The random distribution of AGE units in the copolymers prepared by an anionic route, results in a marked decrease of the crystallinity as well as of the melting point. Free-radical initiation of the solvating copolymer improves the mechanical properties while reducing still the crystallinity and melting point;The cross-linked electrolytes provide high conductivities from ambient temperature.