In vitro degradation of glycineh-lactic acid copolymers

The in vitro degradation of glycine-DL-lactic acid copolymers was studied as a function of the composition. These polydepsipep-tides were prepared by ring-opening copolymerization of 6-methyl-2,5-morpho-linedione and DL-lactidc. The degradation of discs of the copolymers was performed in a phosphate buffer at pH 7.4 and 37°C. The decrease in molecular weight and weight was determined until complete weight loss had occurred. Poly(DL-lactide) was used as a reference material. All (co)-polymers show an immediate decrease in molecular weight, whereas the weight re-mains almost unchanged during a longer period of time. Decrease in weight started earlier as the glycine content of the co-polymer increased. The lactic acid content of the residual material increased during the weight loss showing a higher solubil-ity of polymer fragments with a relatively high content of glycine residues. From the hydrolysis constants it was concluded that the degradation was best described by hy-drolysis of ester bonds via a bulk erosion process, autocatalyzed by the generated carboxylic acid end groups. The rate con-stants varied from 4-7 x lo- ' (day-') for all (co)polymers. All (co)polymers show an increase in the molecular weight distri-bution upon weight loss