Isoselective Polymerization of rac-Lactide by Highly Active Sequential {ONNN} Magnesium Complexes

Area of Science:

• Coordination Chemistry
• Polymerization Catalysis
• Organometallic Chemistry

Background:

• Lactide (LA) polymerization is crucial for producing biodegradable polylactides.
• Developing efficient and stereoselective catalysts for LA polymerization remains a key challenge.
• Magnesium complexes offer tunable properties for catalytic applications.

Purpose of the Study:

• To synthesize and characterize novel magnesium complexes with {ONNN}-type ligands.
• To evaluate the catalytic performance of these complexes in the ring-opening polymerization of racemic lactide.
• To investigate the influence of ligand structure on catalyst activity and stereoselectivity.

Main Methods:

• Synthesis of {ONNN}-type monoanionic ligands with varying core structures (bipyrrolidine, diaminoethane).
• Formation of magnesium complexes with these ligands, with varying counterions (Cl, HMDS).
• Ring-opening polymerization (ROP) of racemic lactide using the synthesized magnesium complexes, monitoring activity and stereoselectivity.

Main Results:

• All {ONNN}Mg-X complexes demonstrated high activity and isoselectivity in rac-LA polymerization.
• Complexes with chiral bipyrrolidine ligands showed the highest activity (5000 equiv. rac-LA consumed in 5 min at RT) and isoselectivity (P_m = 0.91).
• These chiral complexes also exhibited a living polymerization character, with meso-bipyrrolidine and diaminoethane ligands showing slightly reduced performance.

Conclusions:

• {ONNN}-type magnesium complexes are highly effective catalysts for the ring-opening polymerization of racemic lactide.
• Ligand design, particularly the use of chiral bipyrrolidine cores, significantly enhances catalyst activity and stereoselectivity.
• These findings contribute to the development of advanced catalysts for controlled synthesis of polylactides.