Atmospheric pressure process for preparing pure cyclic esters

An improved process for making a highly pure cyclic ester of the form ##STR2## wherein R.sub.1 and R.sub.2 can be independently hydrogen or an aliphatic hydrocarbon having from I to about 6 carbon atoms by cracking a prepolymer comprising a polymer of an .alpha.-hydroxy acid or its ester or a block copolymer of an .alpha.-hydroxy acid or its ester on a thermally stable polyether wherein the improvement comprises cracking the prepolymer at a suitable temperature and at a pressure at or above atmospheric pressure while maintaining a sufficiently large flow of an inert gas, the inert gas being in intimate contact with the liquid prepolymer so as to create a large interfacial area between the prepolymer and the inert gas, to evolve the cyclic ester vapors.