Heterometallic Li/Zn, Li/Al and Li/In catalysts for rac-lactide ring-opening polymerisation: "ate" or "non-ate"...

Area of Science:

• Polymer Chemistry
• Organometallic Chemistry
• Materials Science

Background:

• Ring-opening polymerization (ROP) of lactide is key for synthesizing biodegradable aliphatic polyesters.
• Bimetallic catalysts offer high activity, but the origins of cooperativity in heterometallic systems require further investigation.
• Understanding metal combinations, particularly alkali metals with transition or post-transition metals, is crucial for catalyst design.

Purpose of the Study:

• To synthesize and characterize novel homo- and heterobimetallic complexes of lithium with aluminum, zinc, and indium.
• To investigate the catalytic activity of these complexes in the ring-opening polymerization of racemic lactide.
• To elucidate the role of metal identity and properties in catalyst performance and reaction pathways.

Main Methods:

• Synthesis and single-crystal X-ray diffraction of lithium-based homo- and heterobimetallic complexes with salen ligands.
• Catalytic evaluation of synthesized complexes in racemic lactide ROP using an epoxide initiator.
• Density Functional Theory (DFT) studies to analyze structural and electronic properties influencing catalytic activity.

Main Results:

• Heterobimetallic complexes LLiZnCl, LLiAlCl2, and LLiInCl2 were successfully synthesized and characterized.
• All synthesized heterobimetallic complexes exhibited activity in lactide ROP, with LLiInCl2 being the most efficient.
• The homobimetallic complex LLi2 was found to be inactive, highlighting the importance of heterometallic cooperation.
• DFT and structural analyses suggested that oxophilicity, Lewis acidity, and electronegativity differences govern catalytic performance.

Conclusions:

• Heterobimetallic cooperation is essential for efficient lactide ROP, as demonstrated by the inactivity of the homobimetallic LLi2 complex.
• The combination of lithium with indium, zinc, or aluminum can lead to highly active catalysts for polyester synthesis.
• Metal properties such as oxophilicity and Lewis acidity, influenced by metal identity, are critical factors determining catalyst efficiency and reaction mechanisms.