Copolymers of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (PDD) were synthesized in carbon dioxide at low temperatures. Bis(perfluoro-2-N-propoxypropionyl) peroxide was used as initiator. A range of copolymers with various compositions and different molecular weights were prepared in yields as high as 74%. The glass transition temperature of the copolymers increased with increasing PDD content and ranged from 67 to 334degreesC for the PDD homopolymer. The phase behavior was found to depend on the feed composition. A comparison of a fully fluorinated commercially available product with a sample synthesized in carbon dioxide (without post-fluorination procedures) to match the composition and the molecular weight of the commercial sample showed no significant difference using IR spectroscopy, NMR spectroscopy, and differential scanning calorimetry, indicating similar microstructures. A simple method to determine the copolymer composition based on infrared spectroscopy was developed.