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

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)-L1-4 -H-2), Ln[N(SiMe3)(2)](3) (Ln = Y, Sm, Nd) and 2-propanol in a 1: 1 :1 molar ratio in THE 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 THE at 20 degrees 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.

Automated summary

A series of new monomeric rare-earth alkoxides were successfully synthesized and characterized. These complexes served as efficient initiators for the stereoselective ring-opening polymerization of rac-LA, resulting in polymers with high heterotacticity. The substituents of the ligands had a significant impact on the reactivity of the alkoxides, while the stereoselectivity was minimally affected. The reactivity and heterotacticity were influenced by the ionic radii of the rare-earth metals and the polymerization media.