Date of Award

Summer 8-2021

Author's School

Graduate School of Arts and Sciences

Author's Department

Chemistry

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Intellectual Gaps: Synthesis of high molecular weight, unimolecular, and sequence-defined polyviologens; spatiotemporal control over mechanical properties of materials using visible and near-IR light; on-demand, post hoc manipulation of materials

Intellectual Merit: Robust method for synthesizing higher-order polyviologens of distinct molecular weight; control the change in the mechanical properties of polymer-based, crosslinked materials using a wide range of wavelengths of light; degradation of viologen-crosslinked hydrogels

To understand fundamentally how viologens behave within a polymer chain, we needed a better and more efficient way of synthesizing these long chains of polyviologens. As simple as this process looks on paper, the synthesis is tedious due to numerous degradation pathways, the hygroscopic nature of the linker, and intractability of the components. After numerous trials, a streamlined synthesis of a 26 viologen unit chain tethered by aliphatic spacers was developed and its controlled degradation under mildly basic conditions was studied. Switching from a hydrophilic and hygroscopic linker (hexaethylene glycol from previous works) to a more hydrophobic chain greatly improved solubility in organic solvents and subsequent purification. This new methodology allowed for the investigation into how the sequence of eight different polyviologens affects the bulk properties of the individual. With an eye toward biomedical applications, this work lays the foundation for the facile synthesis of new viologen-based materials.

Next, the incorporation of the viologen-based compounds in hydrogels was investigated. First-generation syntheses of hydrogels proceed through azide-alkyne “click “ chemistry between azide-terminated polyviologens/polyethylene glycol and alkyne-functionalized crosslinkers. The second-generation hydrogel syntheses were improved by implementing new chemical functionality amenable to free-radical polymerizations, making gel synthesis operationally easier and scalable. A photoredox-based mechanism for reducing viologens to their radical cations inside the hydrogels was demonstrated using visible and near-infrared light and sacrificial reductants. The O2-reversible photoreduction results in total gel contraction; in some cases down to 15–20% the original volume, while also making the gels stronger, tougher, and stiffer.

Language

English (en)

Chair and Committee

John-Stephen Taylor

Committee Members

Jonathan C. Barnes
, Marcus Foston, 
William Tolman, 
Timothy Wencewicz

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