Fluorocopolymer

1. A perfluoroelastomer which comprises repeating units (a) based on tetrafluoroethylene, repeating units (b) based on perfluoro(methyl vinyl ether) and repeating units (c) based on perfluoro(propyl vinyl ether) in a molar ratio of (a)/(b)/(c) of (40-70)/(3-57)/(3-57).

2. The perfluoroelastomer according to claim 1 , which further comprises, in addition to the above repeating units (a),(b) and (c), repeating units (d) based on a monomer represented by the formula CF 2 ═CFOR f X (wherein R f is a C 1-10 perfluoroalkylene group or a C 1-10 perfluoroalkylene group containing an etheric oxygen atom, and X is Cl, Br, I or CN) in a molar ratio of (d)/((a)+(b)+(c)) of from 0.01 to 10.

3. The perfluoroelastomer according to claim 2 , wherein the monomer represented by the formula CF 2 ═CFOR f X is CF 2 ═CFOCF 2 CF 2 BR.

4. The perfluoroelastomer according to any one of claims 1 to 3 , which has a glass transition temperature of at most −1° C.

5. The perfluoroelastomer of claim 1 , wherein said perfluoroelastomer has a Mooney viscosity of 20 to 150.

6. The perfluoroelastomer of claim 1 , which has a glass transition temperature of at most −4° C.

7. The perfluoroelastomer of claim 1 , wherein tetrafluoroethylene, perfluoro(methyl vinyl ether) and perfluoro(propyl vinyl ether) are subjected to radical copolymerization in the presence of a radical polymerization initiator.

8. The perfluoroelastomer of claim 1 , wherein tetrafluoroethylene, perfluoro(methyl vinyl ether) and perfluoro(propyl vinyl ether) are subjected to radical copolymerization in the presence of a radical polymerization initiator and a chain transfer agent.

9. The perfluoroelastomer of claim 8 , wherein said chain transfer agent is at least one selected from the group consisting of an alcohol, a chlorofluorohydrocarbon, a hydrocarbon, RI 2 , where R is a C 1-16 saturated polyfluoroalkylene group, RIBr where R is a C 1-16 saturated polyfluoroalkylene group or a mercaptan.

10. The perfluoroelastomer of claim 9 , wherein said chain transfer agent is RI 2 in an amount of 0.01 to 5 mol % based on the total number of moles of monomers.

11. The perfluoroelastomer of claim 1 , wherein tetrafluoroethylene, perfluoro(methyl vinyl ether) and perfluoro(propyl vinyl ether) are subjected to radical copolymerization in the presence of a radical polymerization initiator, an aqueous medium and an emulsifier.

12. The perfluoroelastomer of claim 11 , wherein said emulsifier is an anionic emulsifier.

13. The perfluoroelastomer of claim 11 , wherein said emulsifier is used in an amount of 0.01 to 15 parts by mass per 100 parts by mass of said aqueous medium.

14. A method for producing the perfluoroelastomer as defined in claim 1 , which comprises radical copolymerization of tetrafluoroethylene, perfluoro(methyl vinyl ether) and perfluoro(propyl vinyl ether) in the presence of a radical polymerization initiator.

15. The method for producing the perfluoroelastomer according to claim 14 , wherein the radical copolymerization is carried out in the presence of a monomer represented by the formula CF 2 ═CFOR f X (wherein R f is a C 1-10 perfluoroalkylene group or a C 1-10 perfluoroalkylene group containing an etheric oxygen atom, and X is Cl, Br, I or CN).

16. The method for producing the perfluoroelastomer according to claim 14 or 15 , wherein the radical copolymerization is carried out in the presence of a compound represented by the formula RI 2 (wherein R is a C 1-16 polyfluoroalkylene group) as a chain transfer agent.

17. The method for producing the perfluoroelastomer according to claim 16 , wherein the chain transfer agent is 1,4 -diiodoperfluorobutane.

18. The method for producing the perfluoroelastomer according to any one of claims 14 or 15 , wherein the radical copolymerization is emulsion polymerization.

19. A crosslinked rubber formed by crosslinking the perfluoroelastomer as defined in any one of claims 1 to 3 by an organic peroxide.

20. The crosslinked rubber of claim 19 , wherein said fluoroelastomer is crosslinked in the presence of a crosslinking aid.