Abstract: Disclosed are: a polymer electrolyte which comprises, as the main component, a block/graft copolymer comprising, as constituent components, polymer blocks (A), (B) and (C) which cause phase-separation from one another, wherein the polymer block (A) comprises a vinyl compound unit as the main repeating unit and has an ion-conductive group, the polymer block (B) comprises a vinyl compound unit capable of forming a flexible phase as the main repeating unit and forms a flexible phase, and the polymer block (C) comprises a styrene derivative unit carrying an alicyclic hydrocarbon group having a polycyclic structure as the main repeating unit and forms a restrained phase; a membrane; a membrane-electrode assembly; and a solid polymer fuel cell.

Abstract: The present invention provide a copolymer which exhibit high electricity generation characteristics when being used as a polyelectrolyte for fuel cell, and also provide a copolymer comprising an aromatic unit substantially free from an ion-exchange group, and an aliphatic unit having an ion-exchange group and a main chain consisting of an aliphatic repeating unit.