Method of purifying dianhydrides

1. A method of purifying a dianhydride, said method comprising:

(a) preparing a first mixture comprising water, at least one inorganic acid, and at least one dianhydride;

(b) heating said first mixture until substantially all of said dianhydride is converted to a tetraacid comprised in a second mixture;

(c) filtering at least a portion of said second mixture to provide a solid tetraacid and a filtrate; and

(d) heating the tetraacid provided by step (c) in a solvent with concurrent distillation of water to provide a third mixture comprising a purified dianhydride and a solvent.

2. A method according to claim 1 wherein said first mixture further comprises a diluent selected from the group consisting of alcohols, ketones, amides, aromatic solvents, and ethers.

3. A method according to claim 2 wherein said diluent comprises orthodichlorobenzene.

4. A method according to claim 1 wherein said dianhydride is selected from the group consisting of dianhydrides having structure I

wherein Q is a bond, an oxygen atom, a sulfur atom, a sulfinyl group, a sulfonyl group, a selenium atom, a hexafluoroisopropylidene group, a C 1 - C 30 aliphatic radical, a C 3 -C 30 cycloaliphatic radical, or a C 3 - C 30 aromatic radical, a carbonyl group; R 1 is a halogen atom, a nitro group, a cyano group or hydrogen, a C 1 -C 12 aliphatic radical, C 3 -C 12 cycloaliphatic radical, or a C 3 -C 12 aromatic radical; b is independently at each occurrence an integer from 0 to 3.

5. A method according to claim 4 wherein said dianhydride is selected from the group consisting oxydiphthalic anhydride; thiodiphthalic anhydride; sulfinyldiphthalic anhydride; sulfonyldiphthalic anhydride; carbonyldiphthalic anhydride; bisphenol A bisphthalic anhydride, hexafluoroisopropylidene bisphthalic anhydride; and biphenyldianhydride.

6. A method according to claim 1 wherein the dianhydride contains a water-soluble impurity and said water-soluble impurity is selected from the group consisting of quaternary ammonium salts, quaternary phosphonium salts, hexalkylguanidinium salts, and mixtures thereof.

7. A method according to claim 6 wherein hexalkylguanidinium salt is hexaethylguanidinium chloride.

8. A method according to claim 1 wherein said inorganic acid comprises at least one acid selected from the group consisting of carbonic acid, boric acid, phosphoric acid, phosphorous acid, hydrochloric acid, sulfuric acid, sulfurous acid, hydrobromic acid.

9. A method according to claim 1 wherein said inorganic acid comprises at least one acid selected from the group consisting of carbonic acid, and phosphoric acid.

10. A method according to claim 1 wherein said inorganic acid consists essentially of carbonic acid.

11. A method according to claim 1 wherein said inorganic acid consists essentially of phosphoric acid.

12. A method according to claim 1 wherein said inorganic acid consists essentially of hydrochloric acid.

13. A method according to claim 1 wherein said heating comprises heating at a temperature in a range between about 60° C. and about 160° C.

14. A method according to claim 1 wherein said heating is carried out at supratmospheric pressure.

15. A method according to claim 1 wherein said solvent is orthodichlorobenzene.

16. A method of preparing a purified oxydiphthalic anhydride, said method comprising:

(a) preparing a first mixture comprising water, at least one inorganic acid, and oxydiphthalic anhydride, said oxydiphthalic anhydride comprising at least one impurity which is soluble in aqueous acid;

(b) heating said first mixture until substantially all of said anhydride is converted to 3, 3′,4,4′-oxydiphthalic acid comprised in a second mixture;

(c) filtering at least a portion of said second mixture to provide a solid 3, 3 ′,4, 4′-oxydiphthalic acid and a filtrate; and

(d) heating the 3, 3′,4,4′-oxydiphthalic acid provided by step (c) in a solvent with concurrent distillation of water to provide a third mixture of a purified oxydiphthalic anhydride and a solvent.

17. A method according to claim 16 wherein said impurity is selected from the group consisting of quatemary ammonium salts, quaternary phosphonium salts, hexalkylguanidinium salts, and mixtures thereof.

18. A method according to claim 16 wherein said impurity is hexaethylguanidinium chloride.

19. A method according to claim 16 wherein said inorganic acid comprises at least one acid selected from the group consisting of carbonic acid, boric acid, phosphoric acid, phosphorous acid, hydrochloric acid, sulfuric acid, sulfurous acid, hydrobromic acid.

20. A method according to claim 16 wherein said solvent is orthodichlorobenzene.

21. A method of preparing a purified oxydiphthalic anhydride, said method comprising:

(a) preparing a first mixture comprising water, phosphoric acid, and oxydiphthalic anhydride, said anhydride comprising at least one impurity, said impurity being soluble in aqueous acid;

(b) heating said first mixture at temperature in a range between about 60° C. and about 160° C. and a pressure in a range between about 1 and about 4 atmospheres until substantially all of said anhydride is converted to 3, 3′,4, 4′-oxydiphthalic acid comprised in a second mixture;

(c) filtering at least a portion of said second mixture to provide a solid 3, 3′,4, 4′-oxydiphthalic acid and a filtrate, said filtrate comprising at least a portion of said impurity; and heating the 3, 3′,4,4′-oxydiphthalic acid provided by step (c) in orthodichlorobenzene with concurrent distillation of water to provide a third mixture comprising a purified oxydiphthalic anhydride and orthodichlorobenzene.