The replacement of polyacrylonitrile by polymethacrylonitrile is expected to provide an improvement of the electrochemical stability in reduction of the plasticized polymer electrolytes. In addition to the investigation on the polymer electrolyte, a comparative study of polymethacrylonitrile and polyacrylonitrile, performed through their respective model repeat unit, i.e. trimethylacetonitrile and isobutyronitrile confirms the stability improvement obtained by replacing polyacrylonitrile tertiary hydrogen by a methyl in polymethacrylonitrile.