Temperature-Dependent Stereoselectivity Switch in the Lithium-Catalyzed ROP of rac-Lactide: Interplay of a Flexible Cation−π Interaction, Ligand Fluxionality, and Donor-Arm Architecture

Abstract Precise control over the stereochemistry of polylactide (PLA) is crucial for tailoring its properties, yet achieving a temperature-driven switch between heteroselectivity and isoselectivity in the ring-opening polymerization (ROP) of racemic lactide (rac-LA) remains challenging. Herein, we report a rare example of a temperature-driven stereoselectivity switch: complex 1 affords heterotactic PLA (Pr = 0.79) at room temperature but produces isotactic PLA (Pm = 0.78) at −50 °C. Combined VT-NMR, DOSY, single-crystal X-ray diffraction, kinetic experiments, control experiments, and DFT studies reveal that the ligand backbone is inherently flexible, endowing this series of lithium complexes with fluxionality in solution, and that a flexible cation−π interaction in complex 1 is involved in its temperature-dependent conformational/coordination reorganization. Comparative VT-NMR studies of complexes 1, 4, 6, and 7 further reveal the structural origin of this switching. The trityl-containing complexes 1, 6, and 7 each exist as two distinguishable species in solution, but their temperature responses depend markedly on the side arm length: the species ratio remains essentially unchanged for complexes 6 (shortest) and 7 (longest), whereas it changes nonlinearly for complex 1 (intermediate). The tert-butyl analogue 4 shows no distinguishable species and only peak broadening upon cooling. These results indicate that the switching arises from the synergy among the intrinsic ligand fluxionality, the flexible cation−π interaction, and the donor-arm structure, which are appropriately matched only in complex 1. This work provides a strategy for designing switchable stereoselective catalysts by exploiting this synergy.