This patent describes the purification of oxydiphthalic anhydride (ODPA; useful as a monomer for polyetherimides engineering thermoplastics) after it is formed by consecutive phase-transfer catalysis reactions shown as one step in the diagram. These PTC displacement reactions require high temperature and require thermally stable phase-transfer catalysts, such as hexaethyl guanidinium chloride (HEG Cl) and tetraphenyl phosphonium bromide (TPPB). The reaction also requires azeotropic drying for both reactivity to minimize side reactions.
Two different procedures were used to synthesize ODPA. ODPA is a solid that is precipitated during workup, then washed with a solvent for purification. The suitability of a particular solvent was based on the solubility of the reactants, byproducts, reaction intermediates and catalyst. The goal was to dissolve the phase-transfer catalyst and the impurities while not dissolving the desired ODPA product.
After measuring solubility of the reactants, byproducts, reaction intermediates and catalyst in 10 different solvents, seven solvents were screened for the washing step of the ODPA solid in the workup. Methanol was clearly shown to be the best solvent to clearly achieve simultaneous high yield (no loss of product during workup) and high purity (separate the phase-transfer catalyst and impurities from the product).
Tetraphenyl phosphonium bromide was found to be an effective catalyst for the PTC reactions. However, its removal from the ODPA during the washing step was not complete and ODPA product purity was only 96.5% to 97.8% after three washes of the solvents screened (isolated yields were 15%-85%). In contrast, when using HEG Cl, removal of HEG Cl from the ODPA by three washes with methanol gave non-detectable residual HEG Cl in the product with overall product purity at 99.48% and 91% isolated yield. HEG Cl appears to be the best catalyst for this application with the well chosen workup.
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