THE AQUEOUS PHASE IN THE INTERFACIAL SYNTHESIS OF POLYCARBONATES .1. IONIC EQUILIBRIA AND EXPERIMENTAL SOLUBILITIES IN THE BPA-NAOH-H2O SYSTEM

The BPA-NaOH-H2O system for polycarbonate synthesis is analyzed by solving the simultaneous chemical equilibria, material balances, and ionic charge balance to yield the aqueous-phase composition under arbitrary 2,2-bis(4-hydroxyphenyl)propane (BPA) loadings and pH levels. The composition is represented in composition maps as undissociated BPA, BPA monoanion, and BPA dianion concentrations and by the existence and quantity of solid BPA and its disodium salt hexahydrate. The calculations require only a knowledge of the pK's of BPA, BPA's aqueous solubility at low pH, and the maximum ionic product for the disodium salt. At 40-degrees-C the measured BPA solubility in aqueous NaOH solutions increases from 2.72 x 10(-3) mol/L at low pH to a maximum of 1.2 mol/L at pH 11.2. At higher pH levels, BPA solubility decreases owing to the precipitation of Na2BPA.6H2O. An ideal solution model gave pK1 = 9.59 and pK2 = 10.2 for the dissociation of the first and second proton, respectively.