UBC Theses and Dissertations
Effects of ligand tuning on dinuclear indium catalysts for the polymerization of lactide Osten, Kimberly Marie
We are interested in the biodegradable polymer poly(lactic acid) (PLA) formed from the ring-opening polymerization of lactide. Our promising results on the polymerization of racemic lactide to form isotactically enriched PLA by a dinuclear indium catalyst bearing a chiral diaminophenolate ligand prompted us to investigate several ligand modifications in order to establish detailed structure-activity relationships within these complexes. Modifications to the terminal amine substituents, the central amine donors and the phenolate substituents of our tridentate ligands were undertaken. The factors affecting the stereoselectivity and activity of these indium catalysts were investigated in detail. Finally, pentadentate dinucleating ligands were used to synthesize dinuclear indium complexes with the goal of producing more stereoselective and/or active catalysts. Modifications to our tridentate ligand system led to complications in their coordination to indium, possibly due to flexibility of the ligands leading to aggregation. It was found that bulkier substituents on the terminal amine position of these ligands led to a lowering of the isoselectivity of the resulting indium complexes due to dissociation of the dimers during the polymerization of lactide. Changing the central amine donors from secondary to tertiary amines led to a profound decrease in polymerization rate. The contributions of intramolecular hydrogen bonding in these dimers on their resulting polymerization activity was explored. However, the nature of the amine, not hydrogen bonding, was found to be the determining factor in their activity towards lactide polymerization. Increasing the steric bulk of the phenolate substituents was found to influence the structure of indium dichloride complexes made with these ligands in solution and the solid state. However, these modifications were found to have only minor impact on the lactide polymerization activity and stereoselectivity of the related dinculear indium ethoxide complexes. A family of pentadentate proligands was utilized for the formation of dinuclear indium ethoxide complexes for the polymerization of racemic lactide. However, only one dinuclear indium ethoxide complex could be isolated cleanly. It was found to have low activity in the polymerization of racemic lactide, requiring weeks to reach full conversion. However, the complex was highly stereoselective producing over 90% heterotactic PLA.
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