Hydrolytic degradation of tyrosine-derived polycarbonates, a class of new biomaterials. Part I: Study of model compounds

Tyrosine-derived polycarbonates have been identified as promising, degradable polymers for use in orthopedic applications. These polymers are non-toxic, biocompatible, and exhibit good bone apposition when in contact with hard tissue. Tyrosine-derived polycarbonates were designed to incorporate two hydrolytically labile bonds in each repeat unit, a carbonate bond that connects the monomer units and an ester bond connecting a pendent chain. The relative hydrolysis rate of the two bonds will determine the type of degradation products and the degradation pathway of the polymers. In order to study the degradation mechanism of these polycarbonates in more detail, a series of small model compounds were designed that mimic the repeat unit of the polymer. Results obtained from the use of these model compounds suggested that the backbone carbonate bond is hydrolyzed at a faster rate than the pendent chain ester bond. Increasing the length of the alkyl pendent chain lowered the hydrolysis rates of both hydrolyzable linkages, possibly by hindering the access of water molecules to those sites. The hydrolysis rates of both linkages were pH dependent with the lowest rate at pH about 5. The results from this study can be used to explain the degradation behavior of the corresponding polycarbonates as well as their degradation mechanisms. This information is essential when evaluating the utility of tyrosine-derived polycarbonates as degradable medical implant materials.