Homoleptic Magnesium Complexes of [NNN]‐Tridentate Pyrrolylaldiminate Ligands as Dual Efficient Catalysts for the Production and Open‐/Closed‐Loop Chemical Recycling of Polylactide

Nine homoleptic magnesium complexes of [NNN]-tridentate pyrrolylaldiminate ligands were synthesized and exploited as dual efficient catalysts for the production and chemical recycling of polylactide. All complexes were highly active catalysts for the polymerization of rac-lactide in solution and under solvent-free conditions. Furthermore, all complexes demonstrated the ability to promote PLA methanolysis into methyl lactate (Me-La) under mild conditions. This is the first report of a homogenous magnesium catalyst capable of depolymerizing PLLA to provide high-purity L-LA monomer.

Abstract

A series of well-defined homoleptic magnesium complexes 1–9 bearing [NNN]-tridentate pyrrolylaldiminate ligands were prepared and fully characterized by NMR spectroscopic techniques. Their application to the ring-opening polymerization (ROP) of rac-lactide (rac-LA) to produce polylactide was investigated under solution and bulk conditions. The polymerizations initiated by all complexes in the presence of benzyl alcohol proceeded to completion within 5 min at 70 °C. The polymerizations were controlled and living, as evidenced by the obtained predetermined molar mass PLAs with narrow dispersities. Kinetic investigations revealed an unexpected zero-order rate dependence on monomer concentration. The NMR reaction studies revealed that the polymerizations proceeded via an insertion-coordination mechanism in the presence or absence of benzyl alcohol. All complexes demonstrated the ability to facilitate PLA methanolysis into methyl lactate (Me-La) under mild conditions at 50 °C, achieving complete PLA consumption (X _int = 100%) within 24 h and affording Me-La in excellent yield and selectivity (Y _Me-La = 83–99%, S _Me-La = 83–99%). The depolymerization of poly(L-lactide) (PLLA) and commercial PLLAs to L-LA monomer was demonstrated utilizing the complex 1/glycerol ethoxylate (GEO) catalyst system at 160 °C under reduced pressure, resulting in high-purity L-LA.

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