Preparation method of 4, 4 '-(hexafluoroisopropylidene) diphthalic anhydride

The invention discloses a preparation method of 4, 4 '-(hexafluoroisopropylidene) diphthalic anhydride. The method comprises the following steps of (a) accurately weighing N-alkyl phthalimide and a catalyst, stirring and raising the temperature to 90-110 DEG C, and dissolving; slowly dropwise adding hexafluoroacetone hydrate, raising the temperature to 100 to 130 DEG C, reacting for 22 to 80 hourswhile stirring, and discharging to obtain 4, 4-(hexafluoroisopropyl) bis (N-alkyl phthalimide); (b) adding alkali to hydrolyze, and acidifying to obtain hexafluorotetracid; and (c) dehydrating to obtain the 4, 4 '-(hexafluoroisopropylidene) diphthalic anhydride. According to the preparation method, the hexafluoroacetone and specific N-alkyl phthalimide directly react under the action of a specific catalyst to prepare hexafluorodianhydride, the step of oxidizing methyl in a traditional method is omitted, the use of metal oxides is avoided, the environmental pollution is small, a normal-pressure reaction is adopted, a