Fluorinated copolymer and ion exchange membrane

1. A fluorinated copolymer comprising units of i) a fluorinated monomer having a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization and ii) a fluorinated olefin,

wherein the proportion of components having a common logarithm (logM) of a molecular weight M of from 2.0 to 3.5 is at most 10 mass % per 100 mass % of components having a common logarithm (logM) of a molecular weight M of at least 2.0, and

wherein the proportion of components is determined by measuring a soluble content of the fluorinated copolymer by the following extraction method:

a powdery fluorinated copolymer comprising units of a fluorinated olefin and a fluorinated monomer having a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization is mixed with CClF 2 CF 2 CClFH with a mass ten times the fluorinated copolymer; a resulting mixture is stirred at 65° C. for 5 or more hours; the mixture is cooled to room temperature, and then methanol with a mass twice the fluorinated copolymer is added to the mixture; the mixture is filtrated, and CClF 2 CF 2 CClFH and methanol are distilled off from a filtrate to obtain a soluble content.

2. The fluorinated copolymer according to claim 1 , which has an ion exchange capacity of from 0.8 to 1.3 meq/g dry resin.

3. The fluorinated copolymer according to claim 1 , wherein the fluorinated monomer having a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization is represented by formula (1):

CF 2 ═CF—(O) p —(CF 2 ) q —(CF 2 CFX) r —(O) s —(CF 2 ) t —(CF 2 CFX′) u -A  (1)

wherein

X is a fluorine atom or a trifluoromethyl group,

X′ is a fluorine atom or a trifluoromethyl group,

A is a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization,

p is 0 or 1,

q is an integer of from 0 to 12,

r is an integer of from 0 to 3,

s is 0 or 1,

t is an integer of from 0 to 12, and

u is an integer of from 0 to 3,

1≦p+s, and

1≦r+u.

4. The fluorinated copolymer according to claim 3 , wherein the functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization is at least one member selected from the group consisting of —CN, —COF, —COOR 1 , —COOM, and —CONR 2 R 3 ,

wherein R 1 is a C 1-10 alkyl group,

wherein M is an alkali metal or a quaternary ammonium salt group, and

wherein each of R 2 and R 3 is a hydrogen atom or a C 1-10 alkyl group, and R 2 and R 3 may be the same or different.

5. The fluorinated copolymer according to claim 1 , wherein the fluorinated olefin is tetrafluoroethylene.

6. The fluorinated copolymer according to claim 1 , wherein the proportion of the components having a common logarithm (logM) of a molecular weight M being from 2.0 to 3.5 is at most 5 mass %.

7. The fluorinated copolymer according to claim 1 , wherein the functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization is at least one member selected from the group consisting of —CN, —COF, —COOR 1 , —COOM, and —CONR 2 R 3 ,

wherein R 1 is a C 1-10 alkyl group,

wherein M is an alkali metal or a quaternary ammonium salt group, and

wherein each of R 2 and R 3 is a hydrogen atom or a C 1-10 alkyl group, and R 2 and R 3 may be the same or different.

8. The fluorinated copolymer according to claim 1 , wherein the fluorinated copolymer is obtained by washing with a fluorinated solvent as a washing solvent.

9. The fluorinated copolymer according to claim 8 , wherein the washing solvent is from 1 to 100 times the mass of the fluorinated copolymer.

10. The fluorinated copolymer according to claim 8 , wherein the temperature of said washing is from room temperature to 150° C.

11. An ion exchange membrane comprising the fluorinated copolymer as in claim 1 .

12. An ion exchange membrane according to claim 11 , further comprising

a layer containing a fluorinated copolymer having a sulfonic acid functional group or a functional group capable of being converted to a sulfonic acid group by hydrolysis or neutralization.

13. The ion exchange membrane according to claim 11 , which is used for electrolysis of a sodium chloride aqueous solution.

14. A fluorinated copolymer comprising units of i) a fluorinated monomer having a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization and ii) a fluorinated olefin,

wherein the proportion of components having a common logarithm (logM) of a molecular weight M of from 2.0 to 3.5 is at most 10 mass % per 100 mass % of components having a common logarithm (logM) of a molecular weight M of at least 2.0,

wherein the fluorinated monomer having a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization is represented by formula (1):

CF 2 ═CF—(O) p —(CF 2 ) q —(CF 2 CFX) r —(O) s —(CF 2 ) t —(CF 2 CFX′) u -A  (1)

wherein

X is a fluorine atom or a trifluoromethyl group,

X′ is a fluorine atom or a trifluoromethyl group,

A is a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization,

p is 1,

q is 0,

r is 1,

s is 0 or 1,

t is an integer of from 1 to 3, and

u is 0 or 1,

1≦p+s, and

1≦r+u, and

wherein the proportion of components is determined by measuring a soluble content of the fluorinated copolymer by the following extraction method:

a powdery fluorinated copolymer comprising units of a fluorinated olefin and a fluorinated monomer having a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization is mixed with CClF 2 CF 2 CClFH with a mass ten times the fluorinated copolymer; a resulting mixture is stirred at 65° C. for 5 or more hours; the mixture is cooled to room temperature, and then methanol with a mass twice the fluorinated copolymer is added to the mixture; the mixture is filtrated, and CClF 2 CF 2 CClFH and methanol are distilled off from a filtrate to obtain a soluble content.

15. The fluorinated copolymer according to claim 14 , wherein the functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization is at least one member selected from the group consisting of —CN, —COF, —COOR 1 , —COOM, and —CONR 2 R 3 ,

wherein R 1 is a C 1-10 alkyl group,

wherein M is an alkali metal or a quaternary ammonium salt group, and

wherein each of R 2 and R 3 is a hydrogen atom or a C 1-10 alkyl group, and R 2 and R 3 may be the same or different.

16. The fluorinated copolymer according to claim 14 , wherein the units of the fluorinated monomer having a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization are units of:

CF 2 ═CF—O—CF 2 CF 2 —CF 2 —COOCH 3 .

17. A process for producing a fluorinated copolymer according to claim 1 , the process comprising:

(I) obtaining a fluorinated copolymer by polymerizing a mixture comprising i) a fluorinated monomer having a carboxylic acid functional group or a functional group capable of being converted to a carboxylic acid group by hydrolysis or neutralization and ii) a fluorinated olefin,

(II) separating the fluorinated copolymer from a polymerization medium and unreacted monomer, and

(III) washing the fluorinated copolymer with a washing solvent.