Group 4 complexes 1-3 [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)] supported by two phenolate bidentate ligands (t-BuOS-H=4,6-di-tert-butyl-2-phenylsulfanylphenol) promote the well-controlled ring opening polymerization of rac--butyrolactone. In presence of isopropanol, low dispersities and molecular weights proportional to the equivalents of isopropanol are achieved. Moreover, the zirconium complex is effective in the copolymerization of rac--butyrolactone with rac-lactide. The C nuclear magnetic resonance analysis revealed that the obtained copolymers have a tapered diblock microstructure consisting of an initial block composed of lactide sequences and a terminal block composed of butyrolactone sequences. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3132-3139