A copolymer having tetrafluoroethylene units and second polymerized monomer units in a range from 0.2 to 1 percent by weight, based on the total weight 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, n is independently from 1 to 6, and z is 0, 1, or 2. The copolymer has a melt flow index in a range from 0.02 grams per 10 minutes to 19.4 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.