Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry

Department

Chemistry

First Advisor

Matthew K. Kiesewetter

Abstract

The overarching theme of my research work involves understanding the mechanistic aspects of dually activated hydrogen-bonding catalyst systems and applying that knowledge to synthesize polymers from some of the less explored monomers. This entailed a thorough approach to some of the already hypothesized mechanisms in the polymer community and building on that with additional perspective on catalytic interactions. The other aspect of my research encompassed the application of these H-bond mediated catalysts in controlled ring-opening polymerization (ROP) of sulfur-based lactones. This allowed the growth in monomer scope using these catalysts for the first time.

H-bonding catalysis, particularly the ones involving ureas and thioureas, began about a decade or so ago. The tremendous rise in organocatalytic ring-opening polymerization has sparked a wide range of catalysts developments in the past few years. Due to their lower cost, reduced toxicity and greener approach, the field has been booming ever since its inception. The wide range of architectures in polymer production that were seemingly difficult previously were possible with great control and selectivity. Using a bifunctional catalytic species, either as one unit or two separate entities, monomer activation and chain propagation can be achieved for polymer production. The first chapter in this dissertation delineates on that growth of dual activation process in organocatalysis as a book chapter “Bifunctional and Supramolecular Organocatalysts for Polymerization” in Organic Catalysts for Polymerization. My contribution to this review work has primarily focused on Dual Catalysts, Rate Accelerated Dual Catalysis and Supramolecular Catalysts.

In the second chapter, we looked at the binding interaction that inherently is a determining factor in the dual activation process. We obtained binding constants between the cocatalytic pair of thiourea and a set of bases which allowed us to comprehend the reason behind enhanced selectivity and reaction control. Finally, we applied this phenomenon to test its feasibility with a new, very active cocatalyst pair for a wellcontrolled ROP of some common cyclic esters. I was involved in the latter part of this study where I applied our binding interaction knowledge to test via ROP using a commercial base and thiourea.

As our understanding of the activation process grew, we determined that a higher order moiety of (thio)urea may prove to be an even better choice for increased rate and selectivity in polyester synthesis. It is with this notion that we developed a tris-urea motif for the monomeric activation of lactones, described in the third chapter. Although a rate acceleration is distinctly demonstrated using such a catalytic species, the molecular weight control or living behavior in ROP was never sacrificed along the way. My part in this study was only limited to the synthesis of this tris-urea catalyst with some initial reaction condition screening.

Carrying that knowledge of catalytic interaction with monomer from the initial studies, we delved into the investigation of equilibrium process of the ROP in the fourth chapter. We observed a catalyst dependence on the overall reaction process of lactonebased ROP where a change in reactant and product interaction with the thiourea can be observed. This results in a similar Gibbs free energy difference between monomer to catalyst and polymer to catalyst. As a result, a change in monomer concentration (recoverable) can be seen at the reaction equilibrium with a change in catalyst concentration. This work was mainly performed by me, except the final recovery of the monomer at equilibrium.

After this point, the scope of monomers that can undergo this dual activation was broadened with some of the sulfur-based monomers. Since previous literature studies demonstrated poor control in ROP of such monomers with the assistance of metal-based catalysts, the use of H-bonding catalysts was deemed to be very appropriate. With that in mind, I performed the first-ever organocatalyzed ring-opening polymerization of a sulfurized lactone, ε-thionocaprolactone, shown in fifth chapter. Both reaction control and living nature allowed the possibility of copolymer production using this monomer under the same H-bonding catalysis. A range of new polymeric materials were created at the end of this study.

From that initial sulfur-based monomer, the study was extended to some of the less explored thionated monomers in sixth chapter. The same H-bonding organocatalysis was implemented here as well for a broad range of larger lactones (macrolactones). Besides validating the mechanistic aspects of these polymerizations, thermodynamics and kinetics of reaction were also evaluated. As expected for macrolactones over 10 ring sizes, entropic contribution showed dominance over enthalpy which was the case for 9-membered lactones or below. Further material characterizations are currently undergoing to shed light on future applications of these polymers. My contribution to this study involved mainly the synthesis of 8-membered lactones (ζ-heptalactone, ζ-thionoheptalactone), thiono-ethylene brassylate and optimization of reaction conditions for the polymerization of those monomers.

In the seventh chapter, I have included some of the other thionated monomer synthesis besides lactones and their preliminary ROP results. Though none of those monomers of amides and lactide functionality showed good prospect for organocatalyzed ROP, further growth in tuning the structure of the monomers may demonstrate a better way to synthesize polymers from such systems. Additionally, other applications of these sulfurbased polymers (i.e. newer copolymerizations, crosslinking ability) were reported for possible development in these materials in the future. This chapter fully encompasses all of these unfinished works that can be quite useful for a researcher to pick up at a later time.

The eighth chapter is quite different from the rest of the other chapters in this dissertation in that no organic catalysts were employed for the molecular transformation of styrene to stilbene. In fact, metal catalyst developed by Prof. Robert Grubbs was utilized for this transformation via cross-metathesis reaction. This was a manuscript for educational purpose of undergraduate laboratory setting where the ulitization of a well-known Nobel winning catalyst was used by students to form carbon-carbon bond from an olefinic motif. My input in this experiment was mainly to assist the co-authors of the manuscript to carry out the reaction properly in the undergraduate laboratory with students comprising mostly of chemistry major as well as formulate a report to aid in the writing portion of the journal publication.

Available for download on Monday, April 20, 2020

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