Photochemical Sciences Ph.D. Dissertations

Transformations of Siloxane-Based Materials Toward a Reuse and Recycling Loop: Catalytic Methods and Photochemistry

Date of Award

2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Joseph Furgal (Advisor)

Second Advisor

Laura Landry-Meyer (Other)

Third Advisor

Pavel Anzenbacher (Committee Member)

Fourth Advisor

Malcolm Forbes (Committee Member)

Abstract

Siloxane polymers are an important industrial commodity due to their chemical, mechanical, and thermal stability and low toxicity. However, studies to recycle siloxanes are still in their infancy. Traditional methods to synthesize polysiloxanes (silicones) involve high capital-intensive and environmental costs using carbothermal reduction. These reactions release excessive global warming gases such as CO2. Therefore, being able to reuse and recycle siloxanes is highly sought after. The first chapter of this dissertation provides a theoretical review of ways to controllably rupture the siloxane bond into known products. Hydrolysis, catalytic depolymerization, thermal depolymerization, and radical abstractions have been the main approaches to imbue the scission of the siloxane bond, resulting in a slew of products. including new polymers, cyclics, or monomeric silanes. This chapter also details the ways in which photodynamic functional groups can be incorporated into siloxanes to impart alternative reuse strategies over complete depolymerization and repolymerization methodologies. Chapter two describes the establishment of an efficient catalytic room-temperature technique for the depolymerization of silicone polymers, elastomers, and resins in the presence of catalytic fluoride in high swell solvents. The products of these reactions give well defined cyclic siloxane units (D4, D5, D6) as verified by GCMS and 29Si NMR. Silicone-rich systems result in the best conversions and the highest quantity of identifiable cyclics, while complex commercial systems resulted in complicated products alongside discernable cyclics. The products are also repolymerized from this process to reform silicones via acid, base, and fluoride catalysis, opening an avenue to large scale use. The third chapter details the expansion of the fluoride catalyzed siloxane depolymerization processes to a photochemically driven method. This approach allows for depolymerization under UV irradiation in a controlled manner under fluoride conditions established in chapter two. Therefore, the depolymerization can be triggered without adding many reagents. Chapter four explores reversible bond breaking and making strategies for polymer self-healing. Herein we developed a hexaarylbiimidazole (HABI) photodynamic bond cleavage system for incorporation into siloxane-based polymers by functionalizing them with alkoxysilanes for sol-gel based incorporation. Chapter five will provide a set of overall conclusions and future perspectives regarding the impact of this work.

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