Mono‐Ligated Magnesium Bis‐Ketoiminate Complexes: Highly Active Catalysts for the Polymerization of Cyclic Esters

Poly(caprolactone) (PCL) is a very useful biodegradable material with wide applications in human implants and drug delivery systems, while the low catalytic efficiency limits the synthesis of PCL with a high molecular weight. In this study, we develop a single‐component metal catalyst system, where two mono‐ligated magnesium complexes [L 1 Mg] and [L 2 Mg] with hexadentate bis‐ketoiminate ligands have been synthesized and fully characterized by NMR spectroscopy, MALDI‐TOF mass spectroscopy, elemental analyses, and single‐crystal X‐ray diffraction analysis. Complexes [L 1 Mg] and [L 2 Mg] can act as remarkable catalysts in the ring‐opening polymerization (ROP) of ε ‐caprolactone ( ε ‐CL) without additional co‐catalysts under melt and solution conditions, yielding high molecular weight polymers of up to 267 kg/mol in 0.1 mol% loading at 110°C within 6 min. Catalytic mechanism and reaction kinetics have also been investigated in detail in this work. Moreover, these two mononuclear magnesium complexes are also active in the rac ‐lactide ( rac ‐LA) polymerization in the absence of co‐catalysts under melt conditions.