Influence of Ligand Design and Non-Covalent Interactions on the Isoselective Ring-Opening Polymerization of rac-β-Butyrolactone Using Salan and Salalen Rare-Earth Metal Catalysts

We herein report the influence of the ligand framework on the isoselective ring-opening polymerization (ROP) of rac-β-butyrolactone using highly active in situ generated salan [ONNO]^H2 and salalen [ONNO]^H rare-earth metal complexes. The stereochemistry was found to be highly dependent on the ortho-substituents, enabling the synthesis of either isotactic poly(3-hydroxybutyrate) (PHB) (up to P_m = 0.92) or syndiotactic PHB (up to P_r = 0.91). To obtain further information on the mechanism of the isoselective ROP, which exhibits an enantiomorphic site control (ESC), a new hybrid Y[ONNO]^H(N(SiHMe₂)₂)(THF) complex was isolated and characterized via NMR, DFT, and mass-spectrometry experiments. The impact of non-covalent interactions (NCIs) was demonstrated by the addition of NCI inhibitors. Kinetic studies revealed a secondary kinetic isotope effect (SKIE) of 1.14, indicating that the pre-coordination of the monomer plays a significant role in the mechanism. These findings provide a foundation for the future design of catalysts for isoselective ROP.