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

Abstract

Abstract 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(SiHMe 2 ) 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.