Degradable High Molecular Weight Polymers and Copolymers from Lactic Acid

Published May 1993

The newest families of degradable polymers are polyesters based on lactic acid. These polylactides are produced by polymerizing lactide, the cyclic dimer formed by self-esterification of lactic acid. They depolymerize by hydrolysis, and the resulting lactic acid is decomposed by common bacteria. High molecular-weight polylactides are strong enough to be used for injection molding and packaging applications. In addition, they are biocompatible and are useful for sutures, bone plates, and similar medical devices. They maintain strength for a desired period and decompose harmlessly in the body. Polylactides are quite distinct from degradable polymers made directly from lactic acid, which have a low molecular weight and limited strength.

At present, polylactides are still expensive medical specialties. The high price of polylactides prevents their use in a wider variety of applications. However, newly disclosed processes for making lactide promise to make lactide and polylactides available at much lower cost. Lactide is made by the depolymerization of lactic acid oligomer. Until now, depolymerization has been a slow process conducted under vacuum. A series of recent patents by Du Pont describes a rapid depolymerization process conducted at atmospheric pressure. A stannous octoate catalyst is used, and a stripping gas assists in removing lactide as it is formed. Du Pont has also described procedures for efficient lactide recovery and purification. High-purity lactide is required for making high molecular weight polylactide. This review describes each process step (with several variations) and includes a speculative process flow diagram and cost estimate. It also summarizes announced plans for commercialization and the status of the supply of lactic acid raw material.