Polyanhydrides .8. Hydrolytic degradation of aromatic poly(ester-anhydride)s

The hydrolytic degradation of three aromatic poly(ester-anhydride)s, PEA's, was studied at 25 degrees C in buffer solutions at pH 6 and pH 7 over a period of 16 and 14 months, respectively. Two PEA's (1 and 2a) possessed an ester/anhydride mole ratio of 75:25. The loss of weight, the film thickness, elemental analyses, IR spectra, GPC curves and SEM micrographs were recorded. It was found that the degradation of the molecular weights is much faster than the loss of weight and the decrease of the film thickness. During the first 12 months the hydrolysis mainly involved the anhydride groups. No significant difference was observed between pH 6 and pH 7. The third PEA (2b) only contained 5% of anhydride groups. It showed little degradation at pH 6 or pH 7 over a period of 12 month. A rapid degradation involving ester groups occurred at pH 4 or pH 9. All results together clearly indicate that, in addition to an erosion of the surface, a penetration of water into the hydrophobic PEA's occurs, followed by a more or less random hydrolysis of anhydride groups.