Sustainable Polyesters and Polycarbonates via ROP and ROCOP Catalyzed by Alkyl In(III) Complexes Containing Chiral Amino Alcohol Ligand.

Abstract

A new series of optically pure dimeric In(III) complexes (1-3) incorporating bidentate chiral amino alcohol ligands were synthesized and comprehensively characterized using multiple analytical techniques. Single-crystal x-ray diffraction confirmed that complexes (S,S)-1 and (R,R)-1 adopt distorted square-pyramidal geometries at the indium centers, whereas complex 2 contains one pentacoordinate and one tetracoordinated indium site, although spectroscopic studies indicated the presence of monomeric In(III) species in solution. These complexes displayed excellent catalytic activity in the ring-opening polymerization (ROP) of rac-lactide (rac-LA), affording isotactic-enriched polylactide (PLA) (Pm = 0.60-0.65), with stereoregularity influenced by the ligand architecture. The ROP is proposed to proceed via an alkyl-initiated coordination-insertion mechanism. In the ROCOP of cyclohexene oxide (CHO) with CO2, complexes 1-3 also exhibited catalytic activity (TOF ≈ 453 h- 1), producing isotactic-enriched poly(cyclohexene carbonate) with moderate number-average molecular weights (Mn ≈ 85.3 Kg·mol- 1) and narrow dispersities (Đ ≈ 1.10-1.31). Kinetic and mechanistic investigations revealed that ROCOP is initiated by the alkyl group bound to the In(III) center and proceeds via a coordination-insertion pathway.

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