Degradable polyesters through chain linking for packaging and biomedical applications.

The major route to convert lactic acid to high-molecular-weight polymers is ring-opening polymerization of lactide. We have investigated alternative synthesis routes based on oligomerization and chain linking to produce high-molecular-weight thermoplastic degradable polymers cost-effectively. Chain linking also offers new possibilities to prepare degradable polyesters for biomedical applications by extending the range of polymer properties achievable. In this paper, we briefly review different chain linking techniques used in our laboratory. Typically, lactic acid prepolymers with molecular weights of around 3,000-15,000 g x mol(-1) have been prepared by direct polycondensation. Hydroxyl terminated oligomers have been chain linked by using diisocyanate coupling agents, preferably 1,4-butane diisocyanate, forming poly(ester-urethanes). Poly(ester-amides) have been prepared by using 2,2'-bis(2-oxazoline) as coupling agent for carboxylic acid telechelic oligomers. Chain linking by end functionalization has been used in the preparation of poly(ester-anhydrides). In addition, a variety of crosslinked degradable polymers and copolymers have been synthesized through different crosslinking routes, by using methacrylic, itaconic or maleic double bonds or triethoxysilane moieties. A biodegradation test and ecotoxicological evaluation of the degradation products were carried out in addition to hydrolysis tests. Lactic acid based chain linked polymers were biodegradable and the degradation products were harmless. In hydrolysis tests, enzymatic degradation was pronounced in the chain linked poly(epsilon-caprolactone).