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Goonesinghe, Chatura

University of British Columbia

Over the years, numerous neutral indium catalysts have been studied for polymerization reactions. However, recent research conducted by Mehrkhodavandi and co-workers has shed light on the potential of cationic indium species in catalytic applications. In a comprehensive study, the impact of carefully designed hemilabile ligands on the stability, reactivity, and catalytic behavior of indium complexes is explored. Specifically, a family of cationic alkyl indium complexes supported by hemi-salen type ligands with hemilabile heterocyclic pendant donor arms are investigated. The stability and stable lifespan of these complexes directly correlate with the affinity of the pendant donor group to the indium center. The reactivity of the complexes exhibits the opposite trend in the cationic ring-opening polymerization of epoxides. Notably, the most stable complex, bearing a pyridyl donor arm, displays the remarkable ability to polymerize racemic lactide without an external initiator. The same air-stable cationic alkyl indium complex with the pyridyl pendant arm is employed to selectively produce high-molecular-weight cyclic poly(lactide) (c-PLA). This complex enables the reproducible synthesis of c-PLA with low dispersity (Đ ∼ 1.30) and molecular weights reaching up to 416,000 g mol-¹. Importantly, the complex remains active even after prolonged exposure to high-humidity air. Furthermore, it could form high-molecular-weight c-PLLA, c-PDLA, and their stereocomplex without inducing epimerization. The polymerization occurs through a cooperative Lewis-pair-based coordination–insertion mechanism, involving the coordination of monomers to the cationic indium center and the ring-opening of lactide facilitated by the pyridine donor group. Finally, these complexes are used to delve into the mechanism of the selective synthesis of spiro-orthoesters, a unique pre-sequenced monomer capable of producing perfectly alternating poly(ether-alt-ester). Structure function investigations and computational studies were conducted to elucidate the reaction mechanism. These findings reveal that this reaction follows Michaelis-Menten type saturation kinetics, which can be attributed to the low Lewis acidity of the indium catalysts, providing a basis for their exceptional selectivity.

Text

2023-09-01

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/85786

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/