Regioselective Design of Polysaccharide Ester Graft Copolymers
dc.contributor.author | Thompson, Jeffrey Eric | en |
dc.contributor.committeechair | Edgar, Kevin J. | en |
dc.contributor.committeemember | Moore, Robert Bowen | en |
dc.contributor.committeemember | Madsen, Louis A. | en |
dc.contributor.committeemember | Bortner, Michael J. | en |
dc.contributor.committeemember | Matson, John | en |
dc.contributor.department | Graduate School | en |
dc.date.accessioned | 2025-07-04T08:00:21Z | en |
dc.date.available | 2025-07-04T08:00:21Z | en |
dc.date.issued | 2025-07-03 | en |
dc.description.abstract | Polysaccharide graft (co)polymers, where a polysaccharide comprises the backbone and a separate (co)polymer comprises grafted side chains, are growing in popularity as blend compatibilizers, sustainable elastomers, self-assembling materials, and drug delivery components. This popularity is attributed to the inherent renewability, abundance, and general low toxicity of the polysaccharide backbone, coupled with the extensive functionality and range of physicochemical properties that can be imparted through attachment of a second polymeric side chain. However, polysaccharide graft (co)polymers can be exceedingly complex due to polymeric attributes of both the backbone and grafted side chains (namely, molecular weight (MW) and molar mass dispersity (Ð)). Further structural complications arise from complexities inherent to the polysaccharide backbone, as the presence of pendant functional groups on each monosaccharide result in polymer grafts (and any other substituents present) originating from up to three positions. In order to properly elucidate relationships between the structure (i.e., topology) of polysaccharide graft (co)polymers and exhibited physicochemical properties, the position of polymer grafts on each repeating unit, degree of substitution (DS) of grafted chains, and MW of grafted chains must be controlled and properly characterized. Regioselective functionalization of polysaccharides is an appealing route to produce well-defined polysaccharide graft (co)polymers. By selectively derivatizing a single functional group per monosaccharide with a moiety capable of polymer conjugation (i.e., grafting-to) or polymer initiation (i.e., grafting-from), the graft density of the ensuing graft (co)polymer can be controlled by limiting the DS at the reactive site. However, regioselective modification of polysaccharides, especially polyglucopyranoses such as amylose and cellulose, is difficult due to the similar reactivities of pendant hydroxy groups. One distinct structural feature of amylose and cellulose is the sole primary hydroxy group located at the C6 position of each monosaccharide. The increased steric accessibility and wider approach angles of the primary C6-OH compared to the more hindered secondary C2/C3-OH is a valuable structural feature that can lend itself to selective functionalization. In this work, two distinct chemistries regioselective for the C6-OH of amylose and cellulose were leveraged to produce well-defined polysaccharide derivatives and graft (co)polymers. The primary C6-OH of amylose was selectively converted to a primary alkyl bromide through treatment with N-bromosuccinimide (NBS) and triphenylphosphine (PPh3), which could subsequently be displaced by the azide anion to prepare a well-defined amylose derivative with functionality reactive for the highly efficient copper-catalyzed azide-alkyne cycloaddition (CuAAC). A series of 2,3-di-O-acetyl-6-azido-6-deoxy (2,3Ac-6N3) amyloses were prepared through efficient transformations with control over DS of acetyl (Ac) and N3 moieties. Prepared 2,3Ac-6N3 amyloses served as the backbone for a series of well-defined amylose acetate-graft-polylactide (AmAc-g-PLA) graft polymers, which show promise as compatibilizers for immiscible blends of highly renewable starch acetate (StAc) and PLA, both of which are sourced from starch feedstocks. Polysaccharide graft polymer compatibilizers (where the backbone comprises a polymer that is miscible with one blend component, and the grafted side chain comprises a polymer that is miscible with the other blend component) have shown promise in stabilizing the morphology of immiscible polymer blends, but are seldom produced with topological control allowing for determination of an ideal graft length or density for compatibilization. To address this, a series of AmAc-g-PLA graft polymers prepared by grafting-to CuAAC of alkyne-terminated PLA with 2,3Ac-6N3 amylose, permitting precise control over graft length and graft density by varying the MW of alkyne-terminated PLA (i.e., graft length) and DS(N3) of 2,3Ac-6N3 amylose (i.e., graft density). Preliminary investigations reveal that effective compatibilization is observed for AmAc-g-PLA graft polymers with low graft density, permitting favorable enthalpic interactions between the AmAc backbone and the StAc phase, and high graft length, allowing for entanglements between PLA side chains and the PLA phase. Selective protection of the C6-OH with the bulky 4-monomethoxytriphenylmethyl (MeOTr) ether is another effective regioselective modification. However, regioselective generation of polysaccharide derivatives employing MeOTr protection chemistry can be time-consuming due to the plethora of protection, functionalization, and deprotection steps. To address this, an efficient, regioselective pathway was developed to produce 2,3-di-O-acyl-6-O-MeOTr (2,3A-6MeOTr) cellulose esters through C6-OH tritylation and C2/C3-OH acylation in a sequential, one-pot manner. Subsequent treatment of 2,3A-6MeOTr cellulose esters with an acyl donor under acidic (yet mild) conditions generated a series of 2,3-di-O-A-6-O-B (2,3A-6B) mixed cellulose esters with up to 100% selectivity for C6-OH protection. Mixed cellulose esters with regioselective incorporation of initiating sites for atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization were prepared, providing an efficient route to well-defined macroinitiators for controlled radical polymerizations. Development of such functional cellulose esters is expected to be invaluable when conducting grafting-from polymerization, as regioselective incorporation of initiating sites will ensure controllable graft density. In comparison to previous routes to polysaccharide graft (co)polymers, the methods described herein offer distinct improvements in control over graft length and graft density. By developing efficient routes to regioselectively substituted polysaccharide derivatives capable of grafting-to and grafting-from polymerization chemistries, topological features most important to optimizing the physicochemical properties of such biobased materials can be identified. The expansion of synthetic strategies described herein will be invaluable to both polymer and polysaccharide chemists aiming to develop a sustainable future by leveraging the fascinating and complex family of polysaccharide graft (co)polymers. | en |
dc.description.abstractgeneral | Polysaccharides comprise long chains of repeating sugar units connected through strong chemical bonds that are among the most abundant natural materials found on Earth. Polysaccharides, such as cellulose (the structural reinforcing component found in plant cell walls) and amylose (the hot water-soluble component of starch that serves as an energy storage compartment), are appealing candidates for a wide variety of applications including drug delivery systems and sustainable plastics as they are biodegradable, readily available, and generally exhibit low toxicity. Polysaccharides are decorated with reactive sites (called functional groups) on each repeating sugar unit that can be modified to tailor physical and chemical properties. The properties of polysaccharide derivatives depend on which functional groups are substituted, how many are substituted along the polymer chain (i.e., degree of substitution), and the type of chemical substituent present. As a result, polysaccharide derivatives can be extremely complex because of the variety of chemical structures that can be present due to those three variables. An emerging class of polysaccharide derivatives gaining increased attention are polysaccharide graft (co)polymers, where a second chain (that we term a polymer) is chemically attached to a polysaccharide backbone to generate a comb-like structure. In addition to the structural complexity of polysaccharide derivatives mentioned previously, polysaccharide graft (co)polymers add additional complexity due to factors including the chemical composition of the grafted side chain, the length of the grafted side chain (i.e., degree of polymerization), the differences in lengths of grafted side chains (i.e., dispersity), the space between each grafted side chain (i.e., graft density), and the position of the grafted side chain on each individual repeating sugar unit. The structures of polysaccharide derivatives, including polysaccharide graft (co)polymers, dictate the physical and chemical properties of the final material. These properties can include the temperature at which the material softens (i.e., glass-rubber transition temperature), the stiffness of the material (i.e., elastic modulus), and the three-dimensional shape the chains take in the solid state (i.e., morphology). Consequently, in order to effectively design polysaccharide derivatives for specific applications, it is imperative to have a deep understanding of the structure of polysaccharide derivatives, and particularly polysaccharide graft (co)polymers, to accurately relate chemical structure to material properties. A valuable tool in polysaccharide chemistry is regioselective modification, where specific functional groups on each repeating sugar unit are selectively modified while leaving other similar functional groups untouched. Regioselective modification can be difficult due to the comparable reactivities of functional groups along the backbone, but can be achieved through careful reagent selection and chemical reaction conditions. Efficient synthetic methods were developed utilizing established chemistries to widen the toolbox available to polysaccharide chemists, permitting facile access to polysaccharides with higher degrees of structural control than those attainable by conventional methods. This dissertation focuses on designing polysaccharide graft polymers where cellulose or amylose comprise the backbone that can be valuable as biobased renewable materials that could potentially mitigate the negative environmental effects from the production and disposal of petroleum-based commodity plastics. We developed a polysaccharide graft polymer with an amylose backbone and poly(lactic acid) (PLA) side chains by attaching end-reactive PLA with varying lengths to an amylose backbone with varying functionalization density capable of selectively coupling with PLA through an efficient "click" reaction (i.e., grafting-to synthesis) to create a graft copolymer with tunable graft lengths and densities. This graft polymer was investigated as an additive to improve blends of starch and PLA, two common biodegradable and biobased polymers (appealing in single-use plastic applications such as fast food cutlery) to see how the structure (i.e., topology) of polysaccharide graft polymers affect these blends. We also developed an efficient pathway to cellulose derivatives with tailorable properties, including those containing specific functional groups that can act as sites from which polymer grafts can be grown (i.e., grafting-from synthesis). In short, the work described herein aims to develop new methods to design well-defined polysaccharide graft polymers that can be used as valuable renewable materials, driving innovation for a more sustainable future. | en |
dc.description.degree | Doctor of Philosophy | en |
dc.format.medium | ETD | en |
dc.identifier.other | vt_gsexam:44288 | en |
dc.identifier.uri | https://hdl.handle.net/10919/135754 | en |
dc.language.iso | en | en |
dc.publisher | Virginia Tech | en |
dc.rights | In Copyright | en |
dc.rights.uri | http://rightsstatements.org/vocab/InC/1.0/ | en |
dc.subject | polysaccharide esters | en |
dc.subject | regioselectivity | en |
dc.subject | graft (co)polymers | en |
dc.subject | structure-property relationships | en |
dc.subject | biobased materials | en |
dc.title | Regioselective Design of Polysaccharide Ester Graft Copolymers | en |
dc.type | Dissertation | en |
thesis.degree.discipline | Macromolecular Science and Engineering | en |
thesis.degree.grantor | Virginia Polytechnic Institute and State University | en |
thesis.degree.level | doctoral | en |
thesis.degree.name | Doctor of Philosophy | en |
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