The copolymer includes divalent units represented by formula —[CF2—CF2]—, divalent units represented by formula: (I), and one or more divalent units independently represented by formula: (II) When Z is hydrogen, the copolymer has an alpha transition temperature of up to 100 ?C. The copolymer has an —SO3Z equivalent weight in a range from 300 to 1400, and a variation of the copolymer in which —SO3Z is replaced with —SO2F has a melt flow index of up to 80 grams per ten minutes measured at a temperature of 265° C. and at a support weight of 5 kg. A catalyst ink or polymer electrolyte membrane including the copolymer are also provided.