Abstract: Mixture of tetrafluoroethylene/hexafluoropropylene (TFE/HFP) copolymers comprising 5 to 20% by weight of hexafluoropropylene, the specific melt viscosity at 380.degree. C. of which is from 1.times.10.sup.4 to 60.times.10.sup.4 poise,(a) the melt flow ratio being at least 3.5, and(b) the zero strength time (ZST, second) at 380.degree. C. satisfying the following equation:ZST.gtoreq.1/3x.sup.2 +17x+107wherein x is (specific melt viscosity).times.10.sup.-4 having an improved extrudability, particularly an improved extrusion speed. These mixtures of copolymers contain a high specific melt viscosity portion and a low specific melt viscosity portion.

Abstract: A fluorine-containing copolymer comprising monomeric units of(a) from 95.8 to 80% by weight of tetrafluoroethylene,(b) from 4 to 14% by weight of hexafluoropropene, and(c) from 0.2 to 6% by weight of perfluorovinyl ether of the formula: ##STR1## wherein n is an integer of 1 to 4, and m is an integer of 0 to 3, and having excellent stress crack resistance when formed into a molded article.

Abstract: A process for preparing a tetrafluoroethylene/fluoro(alkyl vinyl ether) copolymer comprising polymerizing tetrafluoroethylene and fluoro(alkyl vinyl ether) in an aqueous suspension system which comprises a mixed medium of water and a fluorohydrocarbon or a chlorofluorohydrocarbon in a volume ratio of from 1:1 to 10:1 at a temperature of from 0.degree. to 25.degree. C. in the presence of a polymerization initiator of the formula:[Cl(CF.sub.2).sub.n --COO].sub.2 (I)wherein n is an integer of 1 to 10, the copolymer obtained by the process having a small number of unstable terminal groups and excellent flex life.