Lanthanum–Lithium Synergistic Catalysis for the Synthesis of Highly Isotactic Cyclic and Linear Poly(mandelic acid)

The ring-opening polymerization (ROP) of O-carboxyanhydrides (OCAs) efficiently produces diverse poly(α-hydroxyalkanoic acid) (PAHA) with functional groups that are difficult to achieve via lactone polymerization. Poly(mandelic acid) (PMA), synthesized from manOCA (OCA derived from mandelic acid), is a promising polymeric material because of its high glass transition (Tg) and thermal decomposition temperatures (Td). However, the synthesis of highly isotactic PMA remains challenging due to the inherent tendency to undergo epimerization during polymerization. Herein, the simple bimetallic rare-earth metal amide complex [(Me3Si)2N]3La(μ-Cl)Li(THF)3 was developed as an efficient catalyst to catalyze the ROP of L-manOCA. This catalyst effectively suppressed epimerization with high activity to afford highly isotactic PMA. The remarkable activity and isomerization selectivity were attributed to the synergistic effect between the rare-earth metal and lithium atoms in [(Me3Si)2N]3La(μ-Cl)Li(THF)3, as confirmed by density functional theory (DFT) calculations. Furthermore, the catalyst enabled precise control over polymer topology (cyclic vs linear) by modulating BnOH addition and reaction temperature. This catalyst also exhibited remarkable polymerization activity toward OCA derivatives with high isotactic selectivity. These findings lay the foundation for the development of efficient catalysts for the synthesis of highly isotactic PAHA polymers from OCA monomers.