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, degradab le polymers for use in orthopedic applications. These polymers are non-toxi c, biocompatible, and exhibit good bone apposition when in contact with har d tissue. Tyrosine-derived polycarbonates were designed to incorporate two hydrolytically labile bonds in each repeat unit, a carbonate bond that conn ects the monomer units and an ester bond connecting a pendent chain. The re lative hydrolysis rate of the two bands will determine the type of degradat ion 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 p olymer. Results obtained from the use of these model compounds suggested th at the backbone carbonate bond is hydrolyzed at a faster rate than the pend ent chain ester bond. Increasing the length of the alkyl pendent chain lowe red the hydrolysis rates of both hydrolyzable linkages, possibly by hinderi ng the access of water molecules to those sites. The hydrolysis rates of bo th linkages were pH dependent with the lowest rate at pH about 5. The resul ts from this study can be used to explain the degradation behavior of the c orresponding polycarbonates as well as their degradation mechanisms. This i nformation is essential when evaluating the utility of tyrosine-derived pol ycarbonates as degradable medical implant materials. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.