In the recent years, the biodegradable and compostable aliphatic polyester poly(lactic acid) (PLA) has attracted substantial global applications as resorbable seams, drug delivery agents, in the biomedical engineering industry, food packaging industry and various others.1,2 The viable route to prepare PLA with controlled molecular weight and polydispersity is through the ring-opening polymerization (ROP) of lactide generally mediated by metal catalysts.
3,4 The physical orientation of the methyl groups from the lactide
monomer in the PLA chain (tacticity) determines the physical properties of PLA.5 Considerable research has progressed over the years in determining appropriate catalysts that results in the control of tacticity in a unique manner, in addition to imparting living characteristics to the ROP. During the ROP, the enantiomeric site control mechanism (ESM) is prevalent when the ligand is chiral and results in the formation of isotactic PLA from racemic lactide (rac-LA) and syndiotactic PLA from meso lactide (mesoLA).6 However, the chain end control mechanism (CEM) during the ROP of (rac-LA) resulted in the formation of heterotactic PLA and very recently it was reported that catalysts containing bulky ligands around the metal resulted in the formation of isotactic PLA.7 Our recent research results have proved the utility of CEM in producing isotactic PLA from rac-LA.8,9
Based on the understanding from our previous work,10,11 we devised a high yield synthetic protocol for the synthesis of an unsymmetrical bidentate iminophenolate ligand system with furan-2-ylmethanimine and thiophen-2-ylmethanimine side arms. This novel ligand system was utilized towards the synthesis of Al(III),
Mg(II) and Zn(II) compounds. The correlation of tacticity of PLA derived from rac-LA with the symmetry of the catalyst was explicitly realized from this study. During the course of our findings, we realized the need to revisit the methodology of calculating accurately the tacticity of PLA.12
Our continued interest in the chemistry of CO2 sequestration prompted us to explore the utility of these compounds as catalysts towards the ring-opening copolymerization (ROCOP) of CO2 with epoxides. Our compounds were found to be very active towards the ROCOP producing isotactic polycarbonates in high yield and turn over frequency (TOF). These compounds produced polycarbonates with high molecular weight and relatively narrow polydispersities.
References:
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