Trehalose Polymers and Ruthenium-Catalyzed Polymerizations: Synthesis and Applications
Since the definition of polymers as covalently linked macromolecules by Hermann Staudinger in 1920, polymers have become indispensable components of our society as industrial materials, consumer products, and medical devices only to name a few applications. This explosive growth of polymer use in the past century, referred to as “The Plastics Revolution”, was driven by research into polymerization techniques for various monomer types appropriate for the desired application. The past two decades have been marked by the development of controlled polymerization methods in addition to advances in new chemical reactions that are highly efficient. The combination of controlled polymerization techniques with methodologies in other fields of organic chemistry is expected to lead to a second plastics revolution, enabling polymers to further enhance our society by materials with new applications.
The first five chapters of this dissertation are related to the use of controlled radical polymerization in combination with methods for protein-polymer conjugation, carbohydrate synthesis, and click chemistry to enhance the properties of protein drugs. Proteins have many desirable traits for therapeutic use, but their potential has yet to be fully realized due to their low physical stability and rapid clearance from the body by multiple elimination pathways. Synthetic polymers produced from the natural sugar trehalose, which stabilizes proteins in nature, would be effective in protecting various proteins from physical stressors and also in vivo clearance when covalently conjugated to proteins.
Even though polymers based on sugars are used for various applications, their syntheses are hindered by the difficulty in monomer synthesis. This is because the multiple hydroxyl groups in sugars have minimal reactivity difference under most reaction conditions, and the reported syntheses are multi-step and low yielding. In Chapter 1, an efficient one-step synthesis of a trehalose monomer was developed using the specific interaction of borinic acids with 1,3-diol of trehalose, thereby overcoming the challenge in carbohydrate monomer synthesis. In Chapter 2, trehalose polymers with varying attachment site of the polymer backbone to the trehalose side chain were synthesized, and the effect of the regioisomers on protein stabilization was evaluated.
In Chapter 3, polymeric hydrogels based on trehalose were used to stabilize proteins important for medical and industrial applications. In the first section, a glucose-responsive trehalose hydrogel was developed to stabilize insulin against heat stress and release insulin in response to high glucose level as in diabetic patients after a meal. The second section presents the industrially scalable synthesis of trehalose hydrogel that did not require any chromatographic purification. The hydrogel fully stabilized phytase, an important animal feed enzyme, under industrially relevant conditions when the hydrogel was used at 10 or higher weight equivalents.
In Chapter 4, trehalose polymers were conjugated to therapeutic proteins to enhance both their stability and in vivo pharmacokinetics. The first section demonstrates that this strategy was effective for insulin as a model protein drug, and the insulin-trehalose polymer conjugate exhibited significantly higher thermal stability and in vivo half-life compared to insulin itself. In the second section, the approach was extended to granulocyte colony-stimulating factor (G-CSF). Click chemistry was used for efficient synthesis of the G-CSF-trehalose polymer conjugate. In vivo evaluations including bioactivity, biodistribution, toxicity, and immunogenicity were conducted.
Although conjugation of a polymer to a therapeutic protein has benefits that outweigh the costs, the conjugates often have reduced activity due to steric hindrance by the attached polymer. In Chapter 5, a new traceless conjugation method was developed to address this drawback of polymer conjugation to proteins. Traceless conjugation such as the method described herein would help the protein regain full activity while retaining all the benefits of polymer conjugation.
In the last three chapters, ruthenium-catalyzed controlled polymerization techniques were combined with new organic chemistry methodologies to access new types of polymers. In Chapter 6, ruthenium-catalyzed living radical polymerization (Ru-catalyzed LRP) was used to prepare amphiphilic fluorinated random copolymers that encapsulated a fluorinated agrochemical and exhibited interesting self-assembly behavior. In Chapter 7, cyclic ketene acetal was used with Ru-catalyzed LRP to synthesize degradable fluorinated polymers, and their degradation rates were shown to be modulated by the shielding of degradable units by the fluorous side chains. In Chapter 8, aryne chemistry was used to prepare monomers for ring-opening metathesis polymerization (ROMP). Previous syntheses of benzonorbornadiene polymers showed that these polymers are highly unstable and spontaneously oxidize and degrade upon exposure to air. Aryne chemistry enabled the efficient syntheses of monomers with substitution at the benzylic/allylic position, which prevented the resulting polymer from oxidative deformation in air.