Date of Award

January 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Guodong Du

Abstract

In recent years, significant efforts have been made regarding the development and synthesis of degradable and thermally stable polymers from renewable feedstock due to the sustainability concerns caused by petroleum-based chemicals and materials. The present dissertation describes the synthesis, structural studies, degradation, and thermal stabilities of various poly(silylether)s prepared from biomass-based chemicals.

An air-stable manganese salen nitrido complex [MnVN(salen-3,5-tBu2)] (Mn-1) was found to be an effective catalyst for the synthesis of poly(silylether)s via dehydrogenative coupling and hydrosilylation reactions. The catalytic activity of the manganese catalyst was initially examined in the protection of hydroxyl and carboxylic groups of alcohols and carboxylic acids with hydrosilanes via dehydrogenative coupling reactions. Under an inert atmosphere, hydroxyl and carboxylic acid functional groups were successfully protected to generate silyl ethers and silyl esters, respectively. A wide variety of functional groups such as chloro, nitro, methoxy, carbonyl, and carbon–carbon multiple bonds were tolerated in the reaction.

Thereafter, the Mn-1 complex was employed as the catalyst to synthesize a series of poly(silylether)s via step-growth polymerization from diol, dicarbonyl, and hydroxy carbonyl substrates including aliphatic and aromatic backbones. Due to the notable dual activity (dehydrogenative coupling and hydrosilylation) of the manganese complex, unsymmetrical substrates with alcohol and carbonyl functional groups produced poly(silylether)s with multiple silicon connectivity in the polymer backbone. Driven by concern for the environmental sustainability, we then directed our studies to synthesize hydrolytically degradable and thermally stable poly(silylether)s from renewable feedstock, such as 1,4:3,6-dianhydrohexitols (isosorbide and isomannide), 5-hydroxymethylfurfural and its derivatives 2,5-bis(hydroxymethyl)furan, 2,5-diformylfuran, and 5,5′-[oxybis(methylene)]di(2-furaldehyde). Thermal analysis demonstrated that isosorbide and isomannide based poly(silylether)s displayed high thermal stability with thermal decomposition temperatures (T-50%) up to 510 °C and glass transition temperatures up to 120 °C. Structure-property analysis suggested that steric hindrance of substrates plays a vital role in determining the thermal properties of these polymers.

In consideration of the high catalytic activity of Mn-1 in Si-H bond activation, the same catalytic system was employed to activate the B-H bond in hydroboration of carbonyl compounds. The reactions were performed at room temperature with using 0.02-0.2 mol % of catalyst and proceeded rapidly (99 % conversion in 5 min). Several synthetically important functional groups were successfully tolerated and achieved chemoselective hydroboration of aldehydes over ketones. Impressively, TOF of these reactions was observed as 5700 h-1 under these conditions. Mechanistic investigations indicate that a reduced manganese species, Mn-H, acts as the active catalyst.

Download

Share

COinS