Fluoropolymers and membranes prepared from such polymers, in particular for fuel cells and method for synthesizing such polymers

1. Fluoropolymer comprising at least one repeat unit of formula (I):

wherein:

groups X and Y, identical or different, represent H or F;

group Z represents F or a perfluorinated alkyl group;

group W represents CH 2 O or CO 2 ;

group Ar represents a divalent group including at least one optionally substituted carbon-containing aromatic cycle;

group Q represents a single bond, (CF 2 ) n with 1≦n≦10, or C 2 F 4 OC 2 F 4 ;

group G represents a cation exchanging group,

said fluoropolymer having ion exchange capacity not less than 0.5 meq/g polymer.

2. Fluoropolymer as claimed in claim 1 , further comprising at least one repeat unit of formula (II) or perfluorovinyl ether:

wherein groups R 3 , R 4 , R 5 and R 6 , identical or different, represent H, Cl, I, an optionally substituted alkyl group, or a fluorinated group containing a fluorine atom or an alkyl group substituted by at least one fluorine atom, at least one of said groups R 3 , R 4 , R 5 and R 6 representing a fluorinated group.

3. Fluoropolymer as claimed in claim 1 , wherein the fluoropolymer is a copolymer or a terpolymer.

4. Membrane containing at least one polymer as claimed in claim 1 .

5. Fuel cell system containing at least one membrane as claimed in claim 4 .

6. Fluoropolymer as claimed in claim 1 wherein the cation exchanging group is selected among:

SO 2 R 1 , with R 1 representing OH, F or Cl; or

P(═O) (OR 2 ) 2 with R 2 representing H or an optionally substituted alkyl group; or

CO 2 H.

7. Fluoropolymer as claimed in claim 1 , wherein the monomer of formula (II′) comprises a perfluoroalkyl vinyl ether (PAVE), a periluoroalkoxyalkyl vinyl ether (PAAVE), or combinations thereof.

8. Fluoropolymer as claimed in claim 1 , wherein the group Q represents C 2 F 4 OC 2 F 4 .

9. Method for synthesizing a fluoropolymer including the following steps:

a) homopolymerizing a monomer of formula (I′):

thereby forming a carboxysubstituted homopolymer; or

b) polymerizing at least one monomer of formula (I′) or perfluorovinyl ether:

with at least one monomer of formula (II′):

thereby forming a carboxysubstituted copolymer;

wherein, in formulas (I′) and (II′):

groups X and Y, identical or different, represent H or F;

group Z represents F or a perfluorinated alkyl group;

groups R 3 , R 4 , R 5 and R 6 , identical or different, represent H, Cl, I, an optionally substituted alkyl group, or a fluorinated group containing a fluorine atom or an alkyl group substituted by at least one fluorine atom, at least one of said groups R 3 , R 4 , R 5 and R 6 representing a fluorinated group;

c) reducing the carboxylic acid function in said homopolymer or said copolymer into an alcohol function;

d) grafting, using the Mitsunobu reaction, onto said alcohol function obtained in step c), of an aromatic monomer with an ionic function, of formula (III′):

wherein:

group Ar represents a divalent group comprising at least one optionally substituted carbon-containing aromatic cycle;

group Q represents a single bond, (CF 2 ) n with 1≦n≦10, or C 2 F 4 OC 2 F 4 ;

and

group G represents a cation exchanging group,

or

e) esterifying the carboxylic acid function of said homopolymer obtained in step a) or said copolymer obtained in step b) using the aromatic monomer with an ionic function of formula (III′), and

f) optionally fluorinating the product of step d) or step e).

10. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein the monomer of formula (I′) is fluorinated acrylic or methacrylic acid.

11. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein the monomer of formula (II′) is vinylidene fluoride (VDF) and/or hexafluoropropene (HFP).

12. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein step a) is carried out in the presence of a polymerisation initiator.

13. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein said reducing of the carboxylic acid function into an alcohol function in step b) is carried out at 90° C., in the presence of a solvent composed of tetrahydrofuran and the LiAlH 4 catalyst.

14. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein the aromatic monomer of formula (III′) having an ionic function comprises para phenol sulphonic acid.

15. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein said fluorinating in step d) is carried out in the presence of SF 4 or diluted HF.

16. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein the cation exchanging group is selected among:

SO 2 R 1 , with R 1 representing OH, F or Cl; or

P(═O) (OR 2 ) 2 with R 2 representing H or an optionally substituted alkyl group; or

CO 2 H.

17. Method for synthesizing a fluoropolymer as claimed in claim 10 , wherein the monomer of formula (I′) comprises trifluoromethylacrylic acid (TFMAA).

18. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein the monomer of formula (II′) comprises a perfluoroalkyl vinyl ether (PAVE), a perfluoroalkoxyalkyl vinyl ether (PAAVE), or combinations thereof.

19. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein the group Q represents C 2 F 4 OC 2 F 4 .

20. Method for synthesizing a fluoropolymer as claimed in claim 9 , wherein said fluoropolymer comprises at least 40 mole % to 60 mole % of monomer containing said ionic function.