Synthesis of aliphatic polyesters has been studied intensively due to their biocompatible and biodegradable properties and their applications in medical and agricultural fields. Among them, polyhydroxybutyrate (PHB), which is a naturally occurring polymer, is synthesized by a variety of bacteria and algae. However, this polymer has poor mechanical properties due to its brittleness. Herein is presented a practical route to a highly syndiotactic PHB by way of a one-pot reaction. We report the results of a comprehensive investigation of the newly discovered stereoselective and controlled polymerization of racemic β-butyrolactone (rac-BBL) using an initiator prepared in situ from yttrium(III) isopropoxide Y(OiPr)3, and a bisphenoxide ligand. DFT studies on the alkoxide catalyst reveal that the R and S monomers are almost equivalent for the first ring-opening reaction. The selectivity of the next insertion was scrutinized, demonstrating that the polymerization process is predicted to be stationary (back-side insertion).