Method of purifying dianhydrides, the dianhydrides formed thereby, and polyetherimides formed therefrom

A method for purifying an oxydiphthalic anhydride having structure I, the method comprising steps (a) to (f):

• (a) providing a first mixture comprising at least one oxydiphthalic anhydride, at least one solvent, at least one catalyst, and at least one inorganic salt selected from the group consisting of alkali metal halide salts, alkaline earth metal halide salts, and mixtures thereof, the oxydiphthalic anhydride being present in the first mixture in an amount of 10 to 30 percent by weight based on total weight of the first mixture;
• (b) diluting the first mixture with at least one solvent, to provide a second mixture having a solids content of 10 to 30 percent based on total weight of the second mixture;
• (c) filtering the second mixture at a temperature below the crystallization point temperature of the oxydiphthalic anhydride and washing the solid with solvent to provide a mother liquor, a wash liquor, and a third mixture of the oxydiphthalic anhydride and salts;
• (d) hydrolyzing by adding (1) a water-soluble acid having a pKa less than or equal to the pKa of oxydiphthalic tetraacid and (2) water to the third mixture, forming a fourth mixture and heating the fourth mixture;
• wherein the fourth mixture is cooled to provide a solid-liquid mixture;
• filtering the solid-liquid mixture to provide a mother liquor and solid component;
• and washing the solid component with water to provide wash liquor and a fifth mixture of oxydiphthalic tetraacid and water;
• and
• (e) ring closing the oxydiphthalic tetraacid by heating the fifth mixture under a temperature and pressure sufficient to convert the oxydiphthalic tetraacid to the oxydiphthalic anhydride, forming a sixth mixture;
• and
• (f) filtering the sixth mixture to obtain substantially pure oxydiphthalic anhydride.