Curable fluoropolymers containing bromine groups having improved compression set

A cured fluoroelastomer composition comprising the reaction product of:

• a) a copolymer comprising tetrafluoroethylene and propylene having an average of two or more bromine atoms including one or more terminal bromine atom(s) per polymer chain;
• b) a peroxide curing agent;
• and
• c) optionally, a crosslinking co-agent;
• wherein the composition has a compression set value lower than a comparative copolymer having a similar comonomer ratio and having an average of less than two bromine atoms per polymer chain.

A method of preparing a fluoropolymer composition comprising:

• a) combining
• i) at least one monomer of the general formula CF2═CF—Rf wherein Rf is fluorine or a C1-C8 perfluoroalkyl,
• ii) at least about 10 mole percent of at least one hydrogen-containing C2-C9 olefin, based on the total moles of said CF2═CFRf and said olefin, iii) a brominated salt,
• iv) optionally CX2═CX—R, wherein CX2═CX—R is chemically distinct from the CF2═CF—Rf and the hydrogen-containing C2-C9 olefin, and wherein each X is independently H, F, or Cl and R is a halogen or a C1-Cx alkyl or alkenyl group that may include one or more ether linkage(s), and
• v) optionally a bromine-containing cure site monomer;
• and
• vi) an initiator;
• and
• b) polymerizing the combination
• wherein said fluoropolymer has an average of two or more bromine atoms including more than one terminal bromine atom(s) per fluoropolymer chain and wherein the fluoropolymer is substantially free of residue from a chain transfer agent.