Hydrogen isotope exchange as a means of labeling lactides

Abstract

Lactic acid-containing polymers obtained by the ring opening polymerization of lactides are now commercially available as bioresorbable polymers, or as devices that can be assimilated by the human body, or by living species present in the outdoor environment. Studying the mechanisms of lactide polymerization or polylactide degradation and monitoring the fate of the degradation compounds produced from the polymers when they degrade in complex media is thus very important and basically feasible via the labeling of polymer chains. Therefore, the possibility of exchanging some of the protons borne by lactide monomer molecules by deuterium or tritium was investigated. The substitution reactions involving deuterium were conducted and optimized using the High Temperature Solid State Catalytic Isotopic Exchange (HSCIE) method and applied to solid DL-lactide in the presence of various catalysts. The most active and stable catalyst was palladium on calcium carbonate. The analyses of the resulting deuterated compounds by 1H NMR and mass spectrometry showed that substitution involved mainly the tertiary proton and that the reaction temperature and flask volume were critical factors. The experimental conditions that led to convenient hydrogen-deuterium exchange yields were then applied to hydrogen-tritium exchange. Highly radioactive DL-lactide having a specific activity of 150 GBq/mmol. (4.05 Ci/mmol.) was successfully synthesized. Copyright © 1998 John Wiley & Sons, Ltd.