Highly heteroselective ring-opening polymerization of rac-lactide initiated by rare-earth alkoxides bearing chiral [NNOO]-type amine-bis(phenolate) ligands

Abstract

A series of new monomeric rare-earth alkoxides (1–6) was successfully synthesized by a one-pot reaction of chiral [NNOO]-type amine-bridged bis(phenolate) ligands ((S)-L^1–4-H₂), Ln[N(SiMe₃)₂]₃ (Ln = Y, Sm, Nd) and 2-propanol in a 1 : 1 : 1 molar ratio in THF at room temperature. These complexes were well characterized with X-ray crystallography, elemental analyses, IR spectra and NMR spectra in the case of yttrium complexes. Furthermore, complexes 1–6 were employed as efficient initiators for the stereoselective ring-opening polymerization of rac-LA in THF at 20 °C to provide heterotactically enriched polymers (P_r = 0.80–0.95). All of these polymerizations proceeded in a controlled manner, giving polymers with good controlled molecular weights and narrow molecular weight distributions (PDI = 1.03–1.12). The nature of the substituents of the ligands had a profound impact on the reactivity of rare-earth alkoxides but a slight impact on the stereoselectivity. Also, the reactivity and heterotacticity were strongly dependent on the ionic radii of rare-earth metals and polymerization media. The heterotactic and atactic polymers were obtained, respectively, by changing the reaction solvent.