Chemical Reactivity Under Confinement: High-Pressure Synthesis of Carbon Nanothreads and Polymer-Based Catalysts
Restricted (Penn State Only)
- Author:
- Huss, Steven
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 24, 2023
- Committee Members:
- Ramesh Giri, Major Field Member
Elizabeth Elacqua, Chair & Dissertation Advisor
Raymond Schaak, Major Field Member
Philip Bevilacqua, Program Head/Chair
Enrique Gomez, Outside Unit & Field Member - Keywords:
- Nanothreads
Polymers
Single-Chain Polymer Nanoparticles
Photoredox
Catalysis
Electron Transfer
Energy Transfer
High-Pressure - Abstract:
- Confining molecules in close proximity through encapsulation, porous materials, or high pressures can lead to intriguing chemistry, resulting in the formation of novel products, high product selectivity, and/or enhanced reaction rates that would otherwise be challenging to achieve under normal conditions. In this context, I have investigated two aspects of this concept: high pressures and confined polymer environments. By subjecting aromatic molecules to high pressures in their solid state, they are tightly confined in proximity. This high-pressure environment can trigger reactions and/or polymerization of the molecules’ sp2 carbons, resulting in the formation of new sp3 carbon–carbon bonds. This effort is aimed towards the formation of well-ordered one-dimensional structures. Furthermore, I synthesized copolymers featuring photoredox-active cocatalytic groups. These copolymers facilitate the close proximity of cocatalysts through two distinct methods, enabling efficient interactions that would otherwise be restricted by diffusion limitations within the solvent. Carbon's pervasiveness in our daily lives stems from its remarkable ability to form diverse compounds and materials. Its strong covalent bonds with itself and other atoms, as well as its ability to exist in various orbital hybridizations allows for tailored structures with a wide range of properties, from mechanical strength to electrical conductivity. This versatility extends to organic molecules, plastics, electronics, and even distinct allotropes like diamond and graphene, etc. In 2015, our group reported on the synthesis of an atypical one-dimensional sp3 carbon nanostructure, termed nanothreads, through the high-pressure solid-state polymerization of molecular benzene. My initial research broadened the synthetic toolbox by including larger ring systems to create nanothreads. We investigated the high-pressure polymerization of various polycyclic aromatic hydrocarbons (PAHs) and characterized the resulting polymeric products. These PAH-derived nanothreads are predicted to possess improved mechanical properties due to increased number of carbon-carbon bonds in their cross-section. Experimental and computational results support the formation of PAH-derived nanothreads, improving our understanding of high-pressure polymerization and leading to a class of nanothreads with potentially superior mechanical properties. Nanothread syntheses typically require very high pressures, ranging from 18 to 25 GPa. However, it's well-known that these pressures pose challenges in scaling up production using equipment like the Paris-Edinburgh Press or Multi-Anvil Press. In light of this, we aimed to develop techniques to lower the necessary reaction pressure for creating nanothreads. To achieve this, we explored the use of less aromatic precursors. As these reactions typically involve the breaking of aromaticity of the precursor for polymerization to proceed, using a less aromatic precursor may reduce the activation barrier, and thus pressure for polymerization. Furan was chosen because of its lower aromatic character compared to benzene. Furan reacted at a reduced pressure compared to benzene and led to successful nanothread formation. We also investigated derivatives of furan, as well as other precursors, to understand functional group tolerance under high-pressure conditions and unlock new classes of nanothreads. Many organic syntheses are substantially accelerated or made possible in the first place by exposure to UV radiation. While light-mediated ring-opening metathesis polymerization, cationic polymerization, ring-opening polymerization, and radical polymerization have all been recently demonstrated to afford access to controlled 1D sp3- and sp2-rich polymer sequences, photochemical methods that access sp3-enriched nanothreads are limited and underexplored. To address this, we developed a method that utilizes broadband UV light to produce nanothreads from pyridine and furan precursors at high pressures. This method may further allow for production of nanothreads at lower pressures and realize new nanothread structures that the traditional thermal-mediated method may not allow. Cooperative catalysis enables synthetic transformations that are not feasible using monocatalytic systems. These reactions are often governed by diffusion and necessitate the collision of cocatalysts. Leveraging the constrained environment of a single-chain polymer nanoparticle, we developed a confined dual-catalytic polymer nanoreactor that enforces catalyst colocalization to enhance reactivity in a fully homogeneous system. Previously, we established the efficacy of this catalytic system, which features triarylpyrylium-based pendants and styrylpyrene as an electron relay catalyst and crosslinking group. This compartmentalized system facilitated the efficient light-activated catalysis of [2+2] cycloaddition reactions involving styrenic molecules. Building on this foundation, we broadened the scope of reactions. Notably, this system demonstrated its ability in catalyzing the oxidation of benzylic alcohols and the amidation of aryl aldehydes. Remarkably, in each instance, the single-chain nanoparticle system surpassed the performance of the small molecule catalyst counterparts, validating the effectiveness of the confined environment. The last research effort involved the synthesis of a copolymer bearing Ru(II) polypyridyl moieties and pyrene pendant groups, serving as cocatalysts for the C–H arylation of (hetero)aryl bromides with radical trapping reagents. While single-chain polymer nanoparticles rely on the crosslinking of a single polymer chain with itself, this polymer system relies on the aggregation of multiple polymer chains into a larger structure through the pi-pi interactions of pyrene. This system highlights the potential for tailored catalytic copolymers but also emphasizes the importance of intermolecular forces in designing polymer architectures for catalysis. Overall, the results described herein highlight the synthesis of a diverse array of PAH-derived nanothreads, a critical initial step towards potentially achieving nanothreads with superior strength. This work also presents design strategies and synthesis techniques for producing nanothreads under reduced pressures, as well as sheds some light on the use of various precursors/functionalities for nanothread synthesis. In the realm of cooperative catalysis, this thesis work expands on the interplay of photoredox catalysis and polymer chemistry with a future goal of designing a high-fidelity polymer catalyst.