To provide: a block copolymer that exhibits excellent proton conductivity even under low-humidification conditions, exhibits excellent mechanical strength and chemical stability, and when used in a polymer electrolyte fuel cell, allows high output and exce...

There is described a random or block copolymer comprising at least two different units of general formula IV and/or V, wherein unit IV is of general formula €ƒ€ƒ€ƒand unit V is of general formula said copolymer including a first repeat unit select...