Thermally stable polymer electrolytes based oil ionic liquids were prepared and analyzed, Mono-functional ionic liquid monomers, ionic liquid cross-linkers, and ethylimidazolium-type ionic liquid salts were mixed and polymerized. The ionic liquid-type cross-linkers were effective to prepare thermally stable polymer films. In particular, the copolymerization of cross-linker and ethylimidazolium-type ionic liquid monomers were used to make polymer electrolytes with high ionic conductivities. The copolymerization in ethymethylimidazolium bis(trifluoromethanesulfonyl) imide gave a transparent film showing no thermal degradation Lip to 400 degrees C. (c) 2005 Elsevier Ltd. All rights reserved.