Browsing by Author "Chen, Eugene Y.-X., advisor"
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Item Embargo Catalyzed chemical synthesis of designer poly(3-hydroxyalkanoate)s: tuning function, microstructure, and architecture of biodegradable polymers(Colorado State University. Libraries, 2022) Westlie, Andrea Hope, author; Chen, Eugene Y.-X., advisor; Miyake, Garret, committee member; Levinger, Nancy, committee member; Herrera-Alonso, Margarita, committee memberThis dissertation describes the development of a chemocatalytic route towards biodegradable poly(hydroxyalkanoate)s (PHAs) based on the ring-opening polymerization of eight-membered cyclic diolide, 8DL, by discrete yttrium complexes. This chemocatalytic platform has transformed the brittle, poly(3-hydroxybutyrate) (P3HB) to high performance, "designer" PHAs through the use of molecular catalysts and the development of a precision polymerization methodology. There continues to be a pressing need for biodegradable polymers in applications where material recovery is unlikely or impossible or where environmental leakage of the plastic waste is highly likely. PHAs are truly biodegradable polyesters that can degrade in ambient conditions such as aerobic soil and marine environments and these polyesters are laden with tunability enabled by their chirality, composition, and architecture. A major challenge in implementing PHAs is to achieve truly tunable thermomechanical properties for any application, coupled with desirable processing conditions at scale. A critical literature review overviews the decades-long history of various chemocatalytic routes towards PHAs with either controlled tacticity or composition. To demonstrate the scope of our chemocatalytic platform, extensive study of homo- and copolymerization of three 8DLR (R = Me, Et, Bu) has been performed. Judicious choice of catalyst to match the steric bulk of the monomer results in high activity and high stereoselectivity ROP of these uncommon PHA homopolymers and allows for highly precise random copolymers of rac-8DLMe with targeted compositions ranging from 5 ~ 40 % incorporation of 8DLR (R = Et, Bu). Moving from aliphatic to aromatic substituents allowed for the synthesis of unnatural and previously unknown PHA with a glass transition (Tg) above room temperature (RT). Aliphatic-aromatic copolymers with designed architecture as random or block copolymers could be synthesized as well. And finally, recently we have designed and synthesized discrete PHA triblock copolymers towards achieving thermoplastic elastomer materials. Overall, this work has used fundamental investigation into a stereoselective, coordination-insertion polymerization mechanism and the resulting structure-property relationships to design higher-performance PHAs that are, in some cases, competitive with commodity polyolefins. This work serves as a platform for further development of PHAs using this chemocatalytic route towards new topologies, compositions, and functions.Item Open Access Development of biomass-derived furanic monomers for biorenewable polyesters and polyurethanes(Colorado State University. Libraries, 2019) Wilson, Jedediah Forrest, author; Chen, Eugene Y.-X., advisor; Reynolds, Melissa M., committee member; Ackerson, Chris J., committee member; Radford, Donald W., committee member; Crans, Debbie C., committee memberDevelopment of Biomass-Derived Furanic Monomers for Biorenewable Polyesters and Polyurethanes This dissertation describes the development of difuranic diol monomers through the N-heterocyclic carbene (NHC) catalyzed cross-coupling of the biomass-derived platform chemicals, 5-hydroxymethylfurfural (HMF) and furfural (FF), and their subsequent utilization in the synthesis of renewable polyesters and polyurethanes with tunable thermal and mechanical properties through the use of soft and rigid co-monomers. The resulting polymers can undergo reversible cross-linking with bis-maleimide cross-linkers through the thermally reversible Diels-Alder reaction involving both the internal and pendent furan rings. The ability to construct a thermally reversible cross-linked network, coupled with formation of a significant amount (up to 34%) of stable carbonaceous materials when heating the polymers to 700 °C, demonstrates some promising features of this class of new difuranic polymers. To address the need to enhance the molecular weight of the current furan-based polymers produced by the step-growth polycondensation process, alternative monomer structures have been designed to adopt the chain-growth mechanism. The first such alternative monomer belongs to a class of furan-derived lactones as candidates for ring-opening polymerization (ROP), which have been shown to produce high molecular weight polyesters because they follow the chain-growth mechanism. Two synthetic routes have been explored to produce such lactone monomers, and their polymerization behavior has been subsequently examined. The second such alternative is centered on a multifunctional furan acrylate monomer, methacrylate furan aldehyde (MFA). The studies tested a hypothesis that auto-tandem or cascading reaction involving the aldehyde functionality in MFA would undergo a benzoin condensation, then the consequent diacrylate would have the appropriate functionality for NHC catalyzed tail-to-tail coupling resulting in a proton transfer polymerization (HTP). It was found that the benzoin condensation was successful but an oxidation occurred at the α-hydroxy of the furoin diacrylate resulting in a highly electrophilic diketone furil diacrylate. Exploration of the coupling mechanism suggests that the enolate acts as a base catalyzing the oxidation. Through careful analysis of the adducts formed when the NHC was reacted with the furil diacrylate showed that the NHC had strong affinity for the diketone moiety thus blocking the HTP pathway. Overall, this work added significantly to our understanding of furans as monomers, NHC catalysis in furan monomer synthesis as well as polymerizations, and enhanced our ability to control thermal and mechanical properties of furan containing polymers.Item Open Access Development of Lewis pair methodologies for advanced polymer synthesis and application(Colorado State University. Libraries, 2022) McGraw, Michael Lawrence, author; Chen, Eugene Y.-X., advisor; Bailey, Travis S., committee member; Rappe, Anthony K., committee member; Linden, James C., committee memberThis dissertation describes the development of new chemistries related to Lewis pair polymerization methodology with emphasis on chemical selectivity and control. Selectivity is defined as the ability to promote desirable chemistry while simultaneously discouraging unwanted chemistry. Control is defined as the ability to target specific and highly sophisticated products from the outset and the ability to reliably achieve that desired product as a consequence of predictable reaction behavior. The themes contained herein relate to catalysis, mechanism elucidation and application, polymer synthesis, and green chemistry. Chapter 1 introduces LPP and includes sections from my published perspective article Lewis Pair Polymerization: Perspective on a Ten-Year Journey. This chapter includes a brief history of the technique, as well as mechanistic fundamentals and key features of the method. Chapters 2 & 3 describes the LPP of the challenging biorenewable monomer methyl crotonate. Chapter 4 describes the invention of the compounded sequence control (CSC) LPP method. Chapter 5 discusses the application of CSC to the synthesis of more advanced cyclic diblock structures using by utilizing a unique sorbate-based initiation system. Chapter 6 details the mechanism by which the sorbate-based initiation system effects cyclization to produce cyclic polymers with spatial and temporal control. Finally, Chapter 7 offers some conclusions and future directions.Item Open Access Lewis acid-mediated controlled anionic polymerization at high temperature(Colorado State University. Libraries, 2011) Bornhoft, Laura O'Neill, author; Chen, Eugene Y.-X., advisor; Shores, Matthew P., committee member; Kipper, Matthew J., committee memberLewis acid-mediated controlled anionic polymerization under industrially desired conditions (ambient or higher temperature) is described in this thesis. The central theme focuses on the utilization of Lewis acids, primarily trialkylaluminum tris(pentafluorophenyl)alane and isoelectronic silylium cation, in three different anionic polymerization systems to mediate, or act as catalyst, for the controlled anionic polymerization of (meth)acrylic monomers or the anionic coordination ring opening polymerization of bio-derived cyclic monomers ɛ-caprolactone and meso-lactide. The first system uses potassium hydride as an anionic initiator, activated with aluminum Lewis acids of varied acidity, sterics and equivalents, for the controlled polymerization of methyl methacrylate (MMA) and block copolymerization of MMA with other alkyl methacrylates such as n-butyl methacrylate and 2-ethylhexyl methacrylate. The second system involves the synthesis of nonpolar-polar block copolymer of poly(styrene-b-methacrylate) in aliphatic solvents at high temperature. Aluminum Lewis acids or non-polymerizable monomers such as N,N-dimethyl methacrylamide are added between blocks to control the propagation crossover from the styrenic block to the methacrylate block. The third system utilizes silylium ions to catalyze anionic polymerization, focusing on the anionic coordination polymerization of cyclic esters and the development of new dinuclear catalysts exhibiting potential to new polymerization pathways that could promote faster polymerization under dilute conditions and enhanced polymerization stereochemical control.Item Open Access Organopolymerization of multifunctional γ-butyrolactones(Colorado State University. Libraries, 2018) Tang, Jing, author; Chen, Eugene Y.-X., advisor; Szamel, Grzegorz, committee member; Miyake, Garret M., committee member; Bailey, Travis S., committee member; Belfiore, Laurence A., committee memberThe complexity of polymerizations increases drastically as the functionality of monomers increases, which brings about challenges for elucidation of polymerization mechanisms, establishing control of the polymerization, and characterization of the resulting polymer structures. On the other hand, the increased multifunctionality in monomers and polymers offers new opportunities to create polymers with unique structures and interesting properties. The research described in this dissertation demonstrates both challenges and advantages that multifunctionality brings into the polymerization and polymer structures. The first successful polymerization of the naturally occurring, OH-containing, tri-functional monomer Tulipalin B (βHMBL) was achieved by utilizing N-heterocyclic carbene and phosphazene superbase catalysts. Owing to its presence of both the reactive exocyclic double bond and hydroxyl group, the resulting P βHMBL is a branched vinyl–ether lactone copolymer structure with six different types of substructural units. The results reveal multiple types of reaction pathways and their mechanistic crossovers involved in the polymerization, including conjugate Michael and oxa-Michael additions, proton transfer processes, as well as ene-type dehydration reactions, enabled by proton transfer. The reactions of other less complicated multifunctional γ-butyrolactone-based monomers under same conditions was also studied to help uncover the polymerization mechanism, including the polymerization of bifunctional (endocyclic double bond, lactone ring) dihydrofuran-2(3H)-one (FO), 3-methylfuran-2(5H)-one (3-MFO), and 5-methylfuran-2(5H)-one (5-MFO), as well as trifunctional (endocyclic or exocyclic double bond, lactone ring, hydroxyl group) 3-(hydroxymethyl) furan-2(5H) one (3-HMFO). The polymerization of the parent FO leads to a vinyl-addition polymer, while the predominant trimerization and dimerization are observed in the reaction involving the two methyl substituted derivatives, 3-MFO and 5-MFO. The polymerization of trifunctional 3-HMFO gives a poly(vinyl–ether lactone) copolymer structure, via two different types of base activation mechanisms and a combination of Michael and ox-Michael additions and proton transfer processes. This thesis work also investigates how different initiation and termination chain ends of poly(γ butyrolactone) (PγBL) affect the materials properties, including thermal stability, thermal transitions, thermal recyclability, hydrolytic degradation, and dynamic mechanical behavior. Four different chain end-capped polymers with similar molecular weights have been synthesized. The termination chain end showed a large effect on polymer decomposition temperature and hydrolytic degradation. Overall, by chain-end capping, linear PγBL behaves much like cyclic PγBL in those properties sensitive to the chain ends.Item Open Access Reactivity and selectivity in the polymerization of multifunctional acrylic monomers by chiral zirconocenium catalysts(Colorado State University. Libraries, 2017) Peña, Fernando Vidal, author; Chen, Eugene Y.-X., advisor; Finke, Richard G., committee member; Strauss, Steven, committee member; Fisher, Ellen, committee member; Wang, David, committee memberDescribed in this dissertation are the results of investigating the reactivity and selectivity in the polymerization of multifunctional acrylic monomers by chiral cationic zirconocenium catalysts. The unprecedented precision polymer synthesis method developed in this work-the polymerization of polar divinyl monomers that is not only living but also simultaneously chemoselective and stereoselective-has enabled the synthesis of well-defined highly stereoregular functionalized polymers bearing reactive C=C bonds on every chiral repeat unit. Thus, under ambient conditions, chiral ansa-ziroconocenium catalysts of the appropriate symmetry (C2- vs CS-ligated) have afforded highly isotactic and highly syndiotactic double-bond-carrying polymers, respectively, with controlled molecular weights and narrow dispersities. The enantiomorphic-site controlled, conjugate-addition coordination polymerization mechanism is responsible for the observed high degree of control over the polymerization characteristics, chemoselectivity and stereochemistry. Soft-material applications of such stereoregular and reactive ene-bearing polymers have also explored, including chemical post-functionalization to functional materials, photocuring to elastic films, and molecular recognition to robust crystalline supramolecular stereocomplexes and helical C60 inclusion complexes. A class of important biorenewable monomers containing one methacrylic C=C and two ester groups, namely itaconic esters and anhydride, have been examined, for the first time, for their polymerizability towards both neutral and cationic bridged ansa-zirconocenes and nonbridged zirconocenes. This investigation has yielded a fundamental understanding of the reactivity of the zirconocene complexes towards such monomers, specifically the fundamental chain initiation and propagation steps, structures of resting metallocyclic chelates, as well as kinetic and thermodynamic intermediates.Item Open Access Redesigning organic catalysts and polymers for recycling towards sustainable catalysis and materials(Colorado State University. Libraries, 2021) Cywar, Robin Marcelle, author; Chen, Eugene Y.-X., advisor; Beckham, Gregg T., advisor; Reynolds, Melissa M., committee member; Borch, Thomas, committee member; Peebles, Christie A. M., committee memberThis dissertation describes the development of catalysts and polymers designed for a sustainable, circular materials economy in which end-of-life is considered during the design stage through properties of inherent recyclability. Products that are recyclable-by-design contrast with those of the current, linear economy, which has led to global environmental crises: greenhouse gas emissions due to finite fossil fuel consumption, contributing to climate change, and tremendous accumulations of plastic waste in landfills and the environment. A major challenge associated with the development of circular lifecycle products, including both catalysts and polymers (plastics, in particular), is achieving performance properties competitive with those of their incumbents, which this work aimed to address. A critical literature review provides an overview of materials derived from renewable, biomass feedstocks (referred to as bio-based polymers) which exhibit performance-advantaged properties relative to petroleum-based polymers. Bio-based chemicals and materials are considered to have a circular carbon lifecycle (carbon-neutral), but those with a circular lifecycle (i.e., recyclable or biodegradable) are given special emphasis. To increase the circularity of all aspects of production of bio-based chemicals and polymers, a polymer-supported organocatalyst has been explored for the coupling of biomass-based furaldehyde platform chemicals; these products can be used for bio-fuels or polymers after further transformations. The developed thermally activated N-heterocyclic carbene (NHC) organocatalyst can be recycled for furfural coupling in excellent yield simply by controlling the temperature, demonstrating promising features for improved circularity in catalyzed chemical processes and ultimately, waste reduction. The discovery of acid-base interactions between the catalyst and hydroxylated substrates has enhanced the understanding of NHC catalysis and contributed to improved design principles for these catalysts. To address plastic waste accumulation by designing for recyclability, polyester and polyamide materials with full chemical recyclability to monomers have been demonstrated from lactone and lactam monomers, respectively. In each case, study of polymerization activity through chemical catalysis revealed fundamental information about the thermodynamic (de)polymerizability of the novel systems, including selectivity considerations, and enabled synthesis of robust structural models for thermomechanical characterization. Overall, understanding the resultant structure-property relationships informs further development of materials with full chemical recyclability and attractive materials properties, including copolymer formulations and design of new monomers to explore for addressing tradeoffs between (de)polymerization activity and material properties.Item Open Access Synthesis of stereochemically controlled functionalized vinyl polymers and polyesters by metallocene-based catalysts(Colorado State University. Libraries, 2009) Ning, Yalan, author; Chen, Eugene Y.-X., advisorThe research presented herein studies the polymerization of functionalized alkenes (methacrylates and acrylamides) and cyclic esters (lactide and lactone) catalyzed by group 4 metallocene complexes. Key findings of this study include: (1) development of the novel diastereospecific ion-pairing polymerization effected by a catalyst system comprising a chiral C 2-zirconocene bis(ester enolate) and two equiv of lewis acid Al(C 6F5)3, producing polymers with isotactic, syndiotactic, and isotactic-b-syndiotactic multiblock microstructures, (2) development of the highly syndiospecific polymerization by C s-ligated ansa-zirconocene bis- and mono(ester enolate) complexes at room temperature, and (3) development of quantitatively isospecific polymerization of L-lactide by Cs-ligated ansa-zirconocene bis- and mono(ester enolate) complexes. The broad focus of this dissertation concerns the kinetic and mechanistic studies of the polymerization of functionalized alkenes and cyclic esters, which provided several novel and useful polymerization systems.Item Embargo Unlocking the potential of crosslinked polymers: from dynamic covalent bonds to recyclable thermosets(Colorado State University. Libraries, 2022) Clarke, Ryan William, author; Chen, Eugene Y.-X., advisor; Miyake, Garret M., committee member; Li, Yan V., committee member; Crans, Debbie C., committee memberThis dissertation describes the development of dynamic covalent chemistries and bond exchange principles in the context of crosslinked polymers demonstrating high performance properties and sustainable end-of-life avenues. Traditional crosslinked polymers, or thermosets, present a significant challenge in meeting the goals of a circular materials economy caused by a strict orthogonality between performance and recyclability. Covalent adaptable networks (CANs) are a developing class of responsive, crosslinked materials that mimic the performance qualities of static thermosets but access flow-state melt processability by triggering crosslink-bond exchange. Several impactful and fundamental advances to this topic and broader sustainable chemistry are discussed herein: demonstration of new dynamic covalent bonds, establishment of intra- and inter-domain exchange principles, orthogonal working/healing exchange conditions for classic thermoset behavior, and upcycling compatibilization of mixed-feed commodity plastics via dynamic crosslinking. The multidisciplinary knowledge developed herein spans the themes of polymer synthesis, catalysis, block copolymers, self-assembly, structure/property relationships, mechanism elucidation, sustainability, materials chemistry, and polymer physics. Chapter 1 introduces dynamic covalent bonds, sustainable thermoset strategies, and standing challenges impeding wide-scale adoption. Background is also supplied on relevant themes to the following chapters, including pathways for plastics recycling, block copolymer technology, and mechanical reprocessing. Chapter 2 describes the Lewis pair polymerization (LPP) of renewable indenone to a high Tg (> 300 oC), optically transparent bio-polymer. Special emphasis is made for the unique upcycling of polyindenone, as well as the Lewis pair polymerization mechanism and methodology which is critical for complex polymer synthesis in Chapters 3 & 4. Chapter 3 discloses the merging of A-B-A triblock copolymer self-assembled thermoplastic elastomer (TPE) structure with CAN dynamic bond exchange function to establish tethered domain-restriction as an effective strategy to lock out creep deformation during working conditions. We also define the fundamental principles governing inter- and intra-domain exchange and thermomechanical activation of newly established dynamic covalent bonds. Chapter 4 describes our compounded-sequence-control LPP technology applied to produce architecturally and topologically sophisticated cyclic and linear A-B-A-B multiblock copolymers in timely (< 10 min), scalable (~40 g), one-pot two-step processes. Structure/property relationships are meticulously investigated in solution, bulk, and film phase for differentiation of isomeric products. Chapter 5 reports a highly collaborative, multidisciplinary (molecular, material, and computational) study on bis(diazirine) crosslinker molecules with imbedded dynamic covalent bond functionality as universal crosslinking agents for upcycling commodity thermoplastics (polyethylene, etc.) to high-performance, recyclable thermosets. Demonstration of successful crosslinking by C-H insertion, improvements to material performance (tensile pulling, creep-recovery, and thermomechanical durability), and reprocessability are thoroughly discussed. Most critical, we establish the compatibilization of (otherwise immiscible) mixed plastics in dynamically crosslinked polymer blends and examine the phenomenon by molecular modeling and structural characterization. Chapter 6 offers conclusions, remaining challenges, and future opportunities.