A composition for use as a polymer electrolyte, wherein said composition includes one or more polar materials and one or more polyesters of formula III, ##STR00001## wherein each unit A may be identical or different and is of the structure IV, ##STR00002## wherein each unit B may be identical or different and is of the structure V, ##STR00003## wherein R and R.sup.1 are each, independently, hydrogen, optionally substituted hydrocarbyl or an inert functional group; a process for preparing said composition; the use of said composition as a polymer electrolyte in coulometers, displays, smart windows, cells or batteries; and a cell and/or battery having said composition.