Fluorinated elastic copolymer and process for its production, and crosslinked rubber article

1. A fluorinated elastic copolymer,

which comprises repeating units (a) of tetrafluoroethylene, repeating units (b) of a fluorinated monomer having one polymerizable unsaturated bond, provided that tetrafluoroethylene is excluded, and repeating units (c) of a fluorinated monomer having at least two polymerizable unsaturated bonds,

wherein the ratio (molar ratio) of the repeating units (a) to the repeating units (b) is (a)/(b)=63.4/36.6 to 80/20, and the proportion of the repeating units (c) of the total amount of the repeating units (a) and the repeating units (b) is from 0.01 to 1 mol %,

wherein the fluorinated elastic copolymer comprises iodine atoms, bromine atoms or both iodine and bromine atoms at molecular terminals thereof,

wherein the fluorinated monomer having one polymerizable unsaturated bond is a perfluoro(alkyl vinyl ether) represented by the formula CF 2 ═CFOR f1 , where R f1 is a C 1-3 perfluoroalkyl group which may have an etheric oxygen atom, and

wherein the fluorinated monomer having at least two polymerizable unsaturated bonds is a monomer represented by the formula R 14 ((O) a CF═CF 2 ) b , where a is 0 or 1, b is an integer of from 2 to 6, and R 14 is a C 1-25 b-valent perfluoro saturated hydrocarbon group which may have an etheric oxygen atom.

2. The fluorinated elastic copolymer according to claim 1 , wherein the fluorinated monomer having at least two polymerizable unsaturated bonds is at least one member selected from the group consisting of CF 2 ═CFO(CF 2 ) c OCF═CF 2 , CF 2 ═CFO[(CF 2 )O d ] f (CFX 1 CF 2 O) c CF═CF 2 , and CF 2 ═CFO(CF 2 CFX 2 O) g [(CF 2 ) h O] k (CFX 3 CF 2 O) i CF═CF 2 ,

wherein:

c is an integer of from 1 to 10,

d is an integer of from 1 to 10, f is 0 or an integer of from 1 to 5, e is an integer of from 1 to 5, and X 1 is F or CF 3 ,

g is 0 or an integer of from 1 to 5, h is 0 or an integer of from 1 to 10, k is 0 or 1, and

i is an integer of from 1 to 5, and each of X 2 and X 3 which are independent of each other, is F or CF 3 .

3. The fluorinated elastic copolymer according to claim 1 , wherein the fluorinated monomer having at least two polymerizable unsaturated bonds is CF 2 ═CFO(CF 2 ) 4 OCF═CF 2 .

4. The fluorinated elastic copolymer according to claim 1 , wherein the proportion of the repeating units (c) is from 0.05 to 0.5 mol %.

5. The fluorinated elastic copolymer according to claim 1 , wherein the proportion of the repeating units (c) is from 0.05 to 0.3 mol %.

6. The fluorinated elastic copolymer according to claim 1 , wherein the fluorinated elastic copolymer of the present invention preferably has a storage elastic modulus G′ of from 50 to 700 kPa, as measured at a temperature of 100° C. with an amplitude of 0.5 degree at a frequency of 50 times/min.

7. The fluorinated elastic copolymer according to claim 1 , wherein the fluorinated elastic copolymer of the present invention preferably has a storage elastic modulus G′ of from 100 to 650 kPa, as measured at a temperature of 100° C. with an amplitude of 0.5 degree at a frequency of 50 times/min.

8. The fluorinated elastic copolymer according to claim 1 , wherein the fluorinated elastic copolymer comprises bromine atoms or both iodine and bromine atoms at molecular terminals thereof.

9. The fluorinated elastic copolymer according to claim 1 , wherein the fluorinated elastic copolymer comprises bromine atoms at molecular terminals thereof.

10. The fluorinated elastic copolymer according to claim 1 , wherein the perfluoro(alkyl vinyl ether) represented by the formula CF 2 ═CFOR f1 is perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), or perfluoro(propyl vinyl ether).

11. The fluorinated elastic copolymer according to claim 1 , wherein the fluorinated monomer having at least two polymerizable unsaturated bonds is a monomer represented by the formula CF 2 ═CFO(CF 2 ) c OCF═CF 2 , where c is an integer of from 1 to 10.

12. A process for producing a fluorinated elastic copolymer according to claim 1 , which comprises reacting tetrafluoroethylene, a fluorinated monomer having one polymerizable unsaturated bond, provided that tetrafluoroethylene is excluded, and a fluorinated monomer having at least two polymerizable unsaturated bonds to radical copolymerization in the presence of a radical polymerization initiator and a chain transfer agent having an iodine atom, a bromine atom, or both iodine and bromine atoms, to obtain the fluorinated elastic copolymer.

13. The process for producing a fluorinated elastic copolymer according to claim 12 , wherein the chain transfer agent is at least one member selected from the group consisting of a compound represented by the formula R f5 I 2 and a compound represented by the formula R f6 IBr,

wherein

R f5 is a polyfluoroalkylene group which may have an etheric oxygen atom, and

R f6 is a polyfluoroalkylene group which may have an etheric oxygen atom.

14. The process for producing a fluorinated elastic copolymer according to claim 12 , wherein the chain transfer agent is 1,4-diiodoperfluorobutane, 1,6-diiodoperfluorohexane or a mixture thereof.

15. The process for producing a fluorinated elastic copolymer according claim 12 , wherein the radical copolymerization is carried out by emulsion polymerization.

16. A crosslinked rubber article obtained by crosslinking the fluorinated elastic copolymer as defined in claim 1 .