Synthesis and Function of Cyclic Poly(Lactic Acid) and Oligo(Lactic Acid)

Synthesis methods of cyclic polyesters and oligoesters are reviewed with a focus on cyclic poly(lactic acid) and oligo(lactic acid) (OLA). Two approaches are generally used to obtain cyclic polyesters: 1) the extraction of cyclic polyesters as by-products contained in the polymerized product obtained via direct polycondensation and 2) synthesis from an ester monomer with an appropriate catalyst. Among the cyclic polyesters obtained with these approaches, compounds with a particular degree of polymerization tend to express specific function, such as including alkali metal ions interacting with the carbonyl group toward the inside of the ring in an organic solvent. This inclusion behavior may affect the transportation of ions within an organism leading to effects such as the antitumor activity of cyclic oligo(lactic acid). Keywords: Cyclic oligo(lactic acid), cyclic poly(lactic acid), cyclic polyester, cyclic oligoester, direct polycondensation, extraction, inclusion complex.