Method of purifying dianhydrides

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.

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.