The mechanism of the ring-opening Polymerization of rac-beta-butyrolactone Using yttrium complexes supported by dianionic aminoalkoxybis(phenolate) ligands as initiators has been investigated by NMR and shown to Occur via a coordination-insertion pathway. The microstructure of the resulting syndiotactic-enriched poly(3-hydroxybutyrate)s (PHBs, P-r up to 0.94) has been studied by C-13 NMR spectroscopy, enabling a detailed assignment of resonances at the diad and triad levels. On this basis, a statistical Bernoullian analysis has been performed which evidenced that syndioselectivity originates from a chain-end control. Some thermal properties of these PHBs have been studied by WAXI) and thermoanalytical techniques and shown to be markedly affected by the syndiotacticity degree. This is especially the case for the melting temperature which raises up to 183 degrees C for P-r = 0.94, a temperature higher than that of pure isotactic PHB (ca. 180 degrees C).