according to the present invention , oxy - diphthalic anhydrides can be prepared by employing a carbonate salt other than potassium carbonate and a solvent in the reaction . the shape of the carbonate salt may be powdery or granular , and preferably , the use of the granular carbonate salt leads to more effective reaction . for example , it has been found that the product can be obtained with the reaction ratio of 90 % when granular sodium carbonate called typically “ soda ash ” is employed . the carbonate salt which can be employed is selected from the group consisting of lithium carbonate , sodium carbonate , magnesium carbonate and calcium carbonate , and preferably sodium carbonate . where the reaction is carried out in a solvent employing potassium carbonate , it can not provide the predetermined reaction ratio because the reaction cannot be completed . the shape of the carbonate salt , which can be employed in the method of the present invention , may be powdery or granular , and the preference is the granular carbonate salt having the diameter of preferably 50 - 750 μm and more preferably 150 - 750 μm . from the view point of the yield , the molar ratio of the carbonate salt to the halo - phthalic anhydride is advantageously in the vicinity of about 0 . 5 , and the preferred molar ratio is 0 . 5 - 0 . 4 . the reaction temperature , at which the method of the present invention is carried out , is generally within the range of 170 - 260 ° c . the employable catalyst is a phosphonium salt or a sulfonium salt which is a phase transfer catalyst capable of tolerating at the reaction temperature . such a catalyst may includes , for example , triphenylphosphine , tetraphenylphosphonium bromide , tetrabutylphosphonium chloride , triethylsulfonium iodide , triphenylsulfonium bromide , etc . the amount of the catalyst is 0 . 1 - 10 % by weight , preferably 0 . 3 - 5 % by weight , based on the halo - phthalic anhydride . further , the use of an alkali - metal iodide or an alkali - metal fluoride as a co - catalyst allows the reactivity to heighten , and further there is also a merit that the product is little colored . the employable alkali - metal iodide includes lii , nai , ki , etc ., and the employable alkali - metal fluoride includes lif , naf , kf , etc . the amount of the co - catalyst to be employed is 0 . 05 - 5 % by weight , preferably 0 . 1 - 2 % by weight , based on the halo - phthalic anhydride . a solvent is employed in the reaction . the solvent to be employed must have a boiling point near the reaction temperature ( 170 - 260 ° c .) and not participate in the reaction . for example , a polychlorobenzene such as 1 , 2 - dichlorobenzene and 1 , 2 , 4 - trichlorobenzene , a polychlorotoluene such as 1 , 2 - dichlorotoluene can be employed . a dehydrated aprotic polar solvent , e . g ., sulfolane can also be employed . the amount to be employed is 50 - 600 % by weight , preferably 100 - 200 % by weight , based on the halo - phthalic anhydride . in order to isolate oxy - diphthalic anhydrides , the reaction product is subjected to hot filtering , followed by cooling the resulting filtrate to obtain oxy - diphthalic anhydrides . the filtrate may be concentrated if desired to allow the yield to increase . when sulfolane is employed , after allowing the filtrate to disperse in water , the precipitated oxy - diphthalic acid is subjected to the solid - liquid separation . the resulting oxy - diphthalic acid is ring - closed by heating or employing acetic anhydride to obtain oxy - diphthalic anhydrides . according to the method of the present invention , since the reaction is carried out in the presence of a solvent , it is hard to produce resin components and to color in comparison with the neat reaction , and therefore the method can save time and labor for decoloration and purification . accordingly , oxy - diphthalic anhydrides , which are preferable also for the electronic material use where the high purity is required , can be obtained . 90 . 8 g ( 0 . 4 mol ) of 4 - bromo - phthalic anhydride and 150 g of 1 , 2 - dichlorotoluene were charged in a reactor to heat at 210 ° c . then , 21 . 2 g ( 0 . 20 mol ) of granular sodium carbonate having the diameter of 200 - 500 μm , 4 . 5 g ( 0 . 011 mol ) of tetraphenylphosphonium bromide and 2 . 1 g ( 0 . 013 mol ) of potassium iodide were charged in the reactor intermittently over 90 minutes , followed by aging for 3 hours after the charge . high performance liquid chromatography test showed 90 % of the reaction ratio ( yield ). the reaction product was cooled to 150 ° c ., followed by hot filtering . after cooling the filtrate , 4 - oxy - diphthalic anhydride was isolated ( amount : 50 . 9 g ( yield : 82 . 0 %)). acetic acid is employable to wash if further purification of the product is needed . 73 . 0 g ( 0 . 40 mol ) of 3 - chloro - phthalic anhydride and 200 g of 1 , 2 , 4 - trichlorobenzene were charged in a reactor to heat at 220 ° c . then , 14 . 8 g ( 0 . 20 mol ) of granular lithium carbonate having the diameter of 200 - 500 μm , 3 . 5 g ( 0 . 015 mol ) of triethylsulfonium iodide and 1 . 8 g ( 0 . 031 mol ) of potassium fluoride were charged intermittently over 90 minutes , followed by aging for 4 hours after the charge . high performance liquid chromatography test showed 88 % of the yield . the reaction product was cooled to 150 ° c ., followed by hot filtering . after cooling the filtrate , 3 - oxy - diphthalic anhydride was isolated ( amount : 49 . 6 g is ( yield : 79 . 9 %)). acetic acid is employable to wash if further purification of the product is needed . 90 . 8 g ( 0 . 40 mol ) of 4 - bromo - phthalic anhydride and 150 g of dehydrated sulfolane were charged in a reactor to heat at 200 ° c . then , 21 . 2 g ( 0 . 20 mol ) of granular sodium carbonate having the diameter of 200 - 500 μm , 4 . 5 g ( 0 . 017 mol ) of triphenylphosphine and 2 . 1 g ( 0 . 013 mol ) of potassium iodide were charged intermittently over 90 minutes , followed by aging for 4 hours after the charge . high performance liquid chromatography test showed 85 % of the yield . the reaction product was cooled to 150 ° c ., followed by hot filtering . the filtrate was subjected to cooling to charge in 500 ml of 5 % aqueous acetic acid solution , followed by stirring with heat for 2 hours at 90 ° c . in order to allow oxy - diphthalic anhydrides to ring - open to obtain a tetracarboxylic acid . after cooling , the product was subjected to the solid - liquid separation , followed by allowing the resulting oxy - diphthalic acid to ring - close by heating in 20 g of 1 , 2 - dichlorobenzene for 3 hours at 220 ° c . when the ring - closure of the oxy - diphthalic acid has been completed , the state of the system changed from white turbid to the state between transparent and slightly turbid . after hot filtering the product , the filtrate was cooled , followed by isolating 4 - oxy - diphthalic anhydride ( amount : 49 . 0 g ( yield : 79 . 0 %)). acetic acid is employable to wash if further purification of the product is needed . the reaction was carried out under the same conditions as in example 1 , except that the granular potassium carbonate having the diameter of 200 - 500 μm was employed in place of granular sodium carbonate . the yield of 4 - oxy - diphthalic anhydride was 37 . 1 %. thus , in comparison with the reaction in which potassium carbonate was employed in the solvent , the method of the present invention provided oxy - diphthalic anhydrides of the product with the higher yield and the simpler reaction .