A copolymer having tetrafluoroethylene units, hexafluoropropylene units, and units independently represented by formula in a range from 0.001 to 2 mole percent, based on the total amount of the copolymer. In these units, a is 0 or 1, each b is independently from 1 to 4, c is 0 to 4, d is 0 or 1, and e is 1 to 6. In the —SO2X groups, X is independently —F, —NH2, —OH, or —OZ, wherein Z is independently a metallic cation or a quaternary ammonium cation. The copolymer has a melt flow index in a range from 20 grams per 10 minutes to 40 grams per 10 minutes. The copolymer can be extruded to make articles, such as insulated cables. A method of making the copolymer is also disclosed.