Novel metal complexes with a single catalytic site and less transesterification seem to provide alternative efficient synthetic approaches to preparing new biodegradable and biologically responsive materials with well-defined structures. In this study, we rationally designed a new category of aluminum metal complexes bearing a bulky Salen ligand and diverse steroidal alkoxy moieties to synthesize novel biodegradable aliphatic polyesters end-capped with steroidal building blocks. At first, three new aluminum metal complexes (9-11) were synthesized with good yields of 80-90%, bearing cholesterol and diosgenin derivatives as functional alkoxy moieties. By means of nuclear magnetic resonance (NMR) spectrometry, matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS), and Fourier transform infrared spectrometry, the molecular structures of 9-11 were characterized. Furthermore, new biodegradable aliphatic polyesters, poly(epsilon-caprolactone) and poly(delta-valerolactone) end-capped with diverse steroidal moieties, were synthesized through the ring-opening polymerization of epsilon-caprolactone and delta-valerolactone catalyzed by these new metal complexes under 100 degrees C in toluene, and they were also characterized by gel permeation chromatography, NMR, MALDI-FTMS, differential scanning calorimetry, and thermogravimetric analysis. Very narrow molecular weight distributions were revealed for these new polymer products, and their thermal crystallization and stability strongly depended on the degree of polymerization of the polyester building blocks and the distinct steroidal moieties. Because of the nature of the steroidal moieties, these biodegradable polymers may pave a path to new possibilities as potential biomaterials. (c) 2006 Wiley Periodicals, Inc.