Rational design and development of Lewis acid tethered cobalt porphyrin catalysts for stereocontrolled living radical polymerization
Date of Award
Spring 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Environmental Engineering (ENAS)
First Advisor
Zhong, Mingjiang
Abstract
Polymer tacticity is a critical microstructural parameter that profoundly determines the properties of polymeric materials. While free radical polymerization has been extensively employed for producing vinyl polymers containing a variety of pendant chemical functionalities in diverse reaction media, achieving tacticity control within this process remains a significant challenge. Despite advancements in Lewis acid (LA)-assisted stereocontrolled (living) radical polymerization, the understanding of mechanistic role is not yet fully understood, and its broader application is restricted by the high metallic LA requirements and low efficiency observed when polymerizing strongly chelating or ionic monomers.Cobalt–porphyrin complexes (Co(por)) are widely used catalysts in numerous reactions, including O2 reduction, CO2 reduction, H2 evolution, C–H functionalization, epoxidation, and living radical polymerizations. Although these reactions have typically been catalyzed by Co(por) complexes without LA addition, the introduction of LA has been found to enhance reactivity or chemo-/stereo-selectivity in similar catalytic systems. Particularly, the covalent localization of LA in close proximity to the catalytic center in other systems has been shown to significantly modulate the primary or secondary coordination sphere. This observation has spurred interest in investigating the potential advantages of LA addition in the Co(por) system. My PhD research, therefore, mainly focuses on achieving stereocontrolled radical polymerization by Lewis acid tethered cobalt porphyrin (LACoP). This thesis starts with a review of state-of-art stereocontrolled polymerization of vinyl monomers, including coordination, ionic, and radical polymerizations (Chapter 1). Subsequently, a set of LACoP complexes are designed and synthesized. Living radical polymerizations of acrylamides catalyzed by these LACoPs displayed superior meso-configurated chain-end monomer addition compared to conventional free radical and Co(por)-mediated radical polymerizations, resulting in isotacticity enrichment (Chapter 2). In chapter 3, the thermo-responsive, adhesive, and electrical properties of the resulting homopolymers with varied tacticities are evaluated. To expand the monomer scope from acrylamides to acrylates and methacrylates, Chapter 4 aims to elucidate the role of Lewis acids and polymer pendants on the stereocontrolled process by performing kinetic analyses of elementary steps and investigating the stereochemistry of the resulting products. This includes synthesizing dimeric initiators, assessing their radical formation and reversible capping rate constants in the presence of LAs, and analyzing the polymerization propagation rate constant. Notably, this series of studies on dimeric initiators provides an explanation for the challenges in controlling acrylate monomers during polymerization. In Chapter 5, the interaction between the cobalt center and different LAs is thoroughly examined. NMR spectroscopy and electrochemistry reveal that groups I and II cationic LAs (i.e., Li+, Na+, K+, Ca2+, Sr2+, and Ba2+) covalently bind to LACoP’s aza-crown ether group (LA ligand). This binding induces an anodic shift in the CoII/CoI couple of between 10 and 40 mV and affects the CoIII/CoII couple. The alteration in the electrochemical properties of the cobalt center upon LA binding not only furthers our understanding of LACoPs in stereocontrolled radical polymerization but also opens avenues for applications extended to CO2 reduction reactions and Co(por) mediated cross coupling reactions.
Recommended Citation
Zhang, Xiaowei, "Rational design and development of Lewis acid tethered cobalt porphyrin catalysts for stereocontrolled living radical polymerization" (2024). Yale Graduate School of Arts and Sciences Dissertations. 1430.
https://elischolar.library.yale.edu/gsas_dissertations/1430
