Polyether copolymer and polymer solid electrolyte

A polyether copolymer having repeating structural units comprising 5 to 95% by mol of a monomer of the formula (I), 95 to 5% by mol of a monomer of the formula (II) and 0 to 15% by mol of a monomer of the formula (III) or (IV) as a crosslinking component;

• the copolymer having a weight-average molecular weight within the range from 10.sup.3 to 10.sup.7 :
• ##STR15## wherein R.sup.1 of the formula (I) is a group selected from the group consisting of an alkyl group having 1 to 12 carbon atoms, an alkenyl group having 2 to 8 carbon atoms, a cycloalkyl group having 3 to 8 carbon atoms, an aryl group having 6 to 14 carbon atoms, an aralkyl group having 7 to 12 carbon atoms and a tetrahydropyranyl group;
• a polymerization degree n of an oxypropylene unit as the side chain portion is from 1 to 12;
• each of a substituent R.sup.2 of the formula (III) and substituent R.sup.3 of the formula (IV) is a substituent having an ethylenically unsaturated group, or a substituent having a reactive silicon group, or a substituent having an epoxy group at the terminal represented by the formula (V):
• ##STR16## in which a group A represents a divalent organic moiety comprising atoms selected from the group consisting of carbon, oxygen and hydrogen atoms.