In accordance with prior practice, the desired cyclic esters were prepared by first condensing hydroxy acids, typically .alpha.-hydroxy acids, to an oligomeric prepolymer of relatively high molecular weight. The prepolymer was then depolymerized at high temperature and low pressure in a heated, evacuated reactor to a crude cyclic ester. Extensive purification processes were required to obtain cyclic esters of requisite purity to be sufficient to synthesize polymers of desired molecular weight.
The production of a cyclic ester from an oligomeric .alpha.-hydroxy acid prepolymer is sometimes referred to as a back-biting reaction since it involves the step-wise removal of cyclic dimer esters from the tail ends of the prepolymer to form the cyclic ester as illustrated below with reference to a lactic acid oligomer. ##STR2##
U.S. Pat. No. 4,727,163 to Bellis is directed to a process which includes first making a prepolymer comprising a block polymer that includes a thermally stable polyether core with an .alpha.-hydroxy acid or its ester polymerized onto the core. Upon heating under vacuum conditions, the chain ends of the .alpha.-hydroxy acids are thermally degraded to form a cyclic ester which can be condensed under vacuum.
U.S. Pat. No. 4,835,293 to Bhatia is directed to a back-biting process which includes the use of an inert gas sweep which permits the process to be operated at or above atmospheric pressure. The inert gas intimately contacts the prepolymer, which is in the liquid phase, so as to create a large interfacial area between the prepolymer and the inert gas to vaporize the cyclic ester and sweep the vapors out of the reactor for subsequent recovery and purification.
As illustrated above, and as discussed in the referenced Bellis and Bhatia patents, back-biting depolymerization of an e-hydroxy acid can result in the production of a cyclic ester. However, the back-biting reaction is typically a slow one, and is conducted as a batch operation which extends over significant time and which results in an undesirable high molecular weight byproduct heel which must be disposed of; moreover, the cyclic ester product must be separated from noxious discolored pyrolysis products.