Method for preparing copolyestercarbonates

A method of preparing block copolyestercarbonates wherein al one dihydroxy-substituted aromatic hydrocarbon moiety and at least one aromatic diacid chloride are reacted under interfacial conditions to give a hydroxy-terminated polyester intermediate. The dihydroxy-substituted aromatic compound is used in about 10 mole to about 125 mole percent excess relative to the diacid chloride. Enhanced control of hydroxy-terminated polyester intermediate molecular weight is achieved by limiting the amount of water present to provide a final salt level of greater than 30 percent. The final salt level is a theoretical value but is readily calculable. The hydroxy-terminated polyester intermediate is then converted to a block copolyestercarbonate by reaction with a carbonate precursor such as phosgene.