A copolymer having tetrafluoroethylene units and units independently represented by formula (I) in a range from 0.02 to 2 mole percent, based on the total amount of the copolymer. Rf is a linear or branched perfluoroalkyl group having from 1 to 8 carbon atoms and optionally interrupted by one or more —O— groups, each n is independently from 1 to 6, m is 0 or 1, and z is 0, 1, or 2. The copolymer has a melt flow index in a range from 20 grams per 10 minutes to 40 grams per 10 minutes and has in a range from 2 to 200 —SO2X groups per 106 carbon atoms and up to 100 unstable end groups per 106 carbon atoms. The copolymer can be extruded to make articles, such as insulated cables. A method of making the copolymer is also disclosed.