A series of group 4 metal complexes 1-4 (1 = (t-BuOS)(2)Ti(O-i-Pr)(2); 2 = (t-BuOS)(2)Zr(O-t-Bu)(2); 3 = (t-BuOS)(2)Hf(O-t-Bu)(2); 4 = ((OS)-O-Cum)(2)Zr(O-t-Bu)(2)) supported by two phenolate bidentate ligands (1-Bu0S H = 4,6-di-tert-butyl-2-phenylsulfanylphenol and (OS)-O-Cum-H = 4,6-dicumyl-2phenylsulfanylphenol) were synthesized by the reaction of appropriate metal tetra(alkoxide)s with 2 equiv of the ligands. The X-ray structure of 2 revealed that the two ligands were kappa(2)-chelated to the metal center with two phenoxy groups in trans positions and the two thioether moieties in cis positions. VT NMR analysis revealed a fast inversion of configuration at the metal center. Complexes 1-4 promoted the ring-opening polymerization (ROP) of lactide and e-caprolactone. Complex 4 exhibited the highest activity with a pseudo-first-order rate constant of 0.061 +/- 0.003 min(-1) at 100 degrees C, which compares favorably with those reported for the most active group 4 complexes. Polymerizations were well-controlled, giving predictable molecular weights and narrow molecular weight distributions. Essentially atactic PLA were obtained in the ROP of rac-lactide. A DFT study on ROP promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive metal center. In presence of isopropanol, lower PDI indexes and molecular weights of the PLAs proportional to the equivalents of isopropanol suggested that adequate conditions for effective "immortal" polymerizations were achieved. More interestingly, these catalysts promoted the copolymerization of L-lactide and epsilon-caprolactone. The microstructure disclosed by C-13 NMR analysis and the thermal behavior exhibited in DSC studies indicated a random distribution of the two monomer along the polymer chain.