In vitro degradation of glycine/DL‐lactic acid copolymers

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Abstract
The in vitro degradation of glycine‐DL‐lactic acid copolymers was studied as a function of the composition. These polydepsipeptides were prepared by ring‐opening copolymerization of 6‐methyl‐2,5‐morpholinedione and DL‐lactide. 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 remains almost unchanged during a longer period of time. Decrease in weight started earlier as the glycine content of the copolymer increased. The lactic acid content of the residual material increased during the weight loss showing a higher solubility 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 hydrolysis of ester bonds via a bulk erosion process, autocatalyzed by the generated carboxylic acid end groups. The rate constants varied from 4–7 × 10−2 (day−1) for all (co)polymers. All (co)polymers show an increase in the molecular weight distribution upon weight loss.