Designing Polymers Bearing Cyclic or Linear Amidines: Exploring the Physical and Chemical Responses to the Reversible Capture and Release of Carbon Dioxide

Post-combustion capture involves passing flue gases through a sorbent material to trap carbon dioxide before it enters the atmosphere. Industrial-scale carbon capture relies on aqueous amine solutions, which generate toxic byproducts, have high volatility, and require significant heat loading to release captured carbon dioxide. An alternative approach employs solid sorbents with greater stability and lower heat requirements for carbon dioxide release. Amidine-functionalized polymers have shown promise in capturing and releasing carbon dioxide reversibly at near-ambient temperatures. However, the inherent stability issues of amidines have hindered efforts to maintain high carbon dioxide capture efficiency and characterize these polymers effectively. My dissertation addresses the synthetic challenges of incorporating amidine functionality into monomers and polymers, alongside examining the physical and chemical responses during carbon dioxide capture and release. Specifically, I assessed various synthetic methods for attaching a monocyclic amidine group to acrylate- or styrene-based monomers and polymers. These studies revealed the instability of monocyclic amidines, which hydrolyze in atmospheric water, react at the vinyl site of acrylate-based monomers, and undergo alkylation reactions that compete with carbon dioxide binding. Consequently, a more stable linear amidine moiety was chosen for functionalizing styrenic polymers used to investigate reversible carbon dioxide capture and release behaviors. The resulting linear amidine-functionalized polymer exhibited significantly improved stability and solubility, facilitating ease of experimentation and thorough characterization. Notably, this polymer demonstrated exceptional carbon dioxide adsorption capacity comparable to leading porous organic polymers. Investigation into the physical and chemical dynamics of carbon dioxide binding included detailed exploration of environmental influences (e.g. moisture content, carbon dioxide concentration, inert gas and temperature swings) on capture and release behaviors, DFT calculations to elucidate binding mechanisms at the amidine motif, comprehensive hydrolysis characterization in amidine-functionalized polymers, and assessment of thermomechanical properties as a result of hydrolysis. These studies underscore how amidines respond to diverse synthetic and environmental conditions and how these chemical modifications influence the physical properties of functionalized polymers, thereby impacting their effectiveness in carbon dioxide adsorption applications.