Method of preparing polyester carbonates

A process for the preparation of a copolyester containing both carboxylate and carbonate groups, said copolyester having reduced coloration, comprising concurrently reacting, in the presence of a catalytic amount of a tertiary amine and an inorganic base, a carbonyl halide;

• a difunctional carboxylic acid;
• and an excess of a molar equivalent, based on the carboxylic acid, of a dihydroxy compound;
• which reaction is carried out in an aqueous medium comprising a water immiscible solvent wherein said inorganic base is present in an amount sufficient to maintain the reaction medium at a pH between about 4.5 and 8.5 until the carboxylic acid is consumed and thereafter increasing the amount of said inorganic base to a level sufficient to raise the pH of said reaction medium to between about 9 and 11.5 until the completion of the reaction.