Method for making polymers of alpha-hydroxy acids

A method for preparing polymers of alpha-hydroxy acids by catalyzed ring opening of cyclic diesters selected from the group consisting of lactide, glycolide and mixtures thereof and comprising up to 20 weight percent of a monomer selected from the group consisting of ##STR2## where n is 4 or 5, h, i, k and m are independently 1 or 2, each R is independently selected from hydrogen or hydrocarbyl containing up to 12 carbon atoms comprising the steps of:

• a. forming a liquid phase dispersion of the cyclic diesters, monomer and ring-opening catalyst selected from the group consisting of tin compounds, yttrium compounds, compounds of lanthanum series rare earth metals having atomic numbers 57-71, titanates, zirconates, zinc compounds, antimony compounds and mixtures thereof, wherein the mole ratio of monomer to catalyst is 500-20,000:1 and the mole ratio of organic acid to catalyst is less than 6:1;
• and
• b. maintaining (1) the temperature of the dispersion at 50-250 C., (2) the mole ratio of cyclic ester to catalyst at 500-20,000:1 and (3) the mole ratio of organic acid to catalyst in the dispersion below 6:1 to effect polymerization of the monomers to a conversion of at least 95% at a high rate.