Tetrazine Usage in the Synthesis and Post-functionalization of Polymers Towards Generating Foams, Antioxidant-rich Materials, and Optical Waveguides

Abstract

This dissertation is composed of 5 chapters detailing advances in the synthesis and post-modification of polymers using tetrazines. The research described herein conveys three new discoveries each of which should forge new fields of research concerning tetrazines in polymer chemistry, that is the use of tetrazines as chemical blowing agents in generating polymer foams, as agents for incorporating large concentrations of antioxidants into polymers, and their attachment to polymers, adding a photodegradable functional unit which alters the refractive index of the materials after degradation. The first chapter is a review which highlights the use of tetrazines in polymer chemistry as repeat units incorporated into polymers, tools for post-modification, and cross-linkers for the formation of gels. Tetrazines are unique molecules which been utilized in many different types of materials. Their tetra-azo core has provided the basis for preparing high energetic explosive materials, and the high nitrogen content has also been exploited in metal chelating polymers. Being highly electron deficient, the tetrazine ring has found use in donor – acceptor (D – A) copolymers for use in polymer solar cells (PSCs). Its ability to undergo an irreversible cycloaddition reaction without the requirement of metal catalysis, has created an entire field of research in bioorthogonal ligation and has also found use in polymer chain extension, post-modification, and gel formation. The versatility of the tetrazine ring is demonstrated in this review with the shear variety of its applications in polymer chemistry. The second chapter reports on the discovery of 3,6-dichloro-1,2,4,5-tetrazine's (DCT) use as a chemical blowing agent in preparing polymer foams. This discovery demonstrates the first reported use of tetrazines as chemical blowing agents, and the production of a new class of polymers foams through exploitation of this chemistry. Nitrogen is released from a cycloaddition reaction between DCT and polybutadiene (PBD). As the reaction proceeds, the foam grows and increases in viscosity eventually setting up into a solid material. The product of the cycloaddition reaction, a dihydropyridazine ring, is demonstrated to provide the foams with a built-in antioxidant, and a change in fluorescence of the foams provides indication for extent of oxidation. Also in this chapter, a new method is proposed for comparing antioxidant properties of small molecules, and two dihydropyridazines are shown to outperform commercial antioxidant BHT with this method. Chapter 3 follows up on the results from the previous chapter, with the synthesis of four new polymers through solution-based cycloaddition reactions between dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (DMDT) and PBD. Near quantitative conversions results in the formation of two new classes of dihydropyridazine polymers, providing an entirely new class of polymer which contains its own built-in antioxidant on nearly every repeat unit along the polymer backbone. Oxidation of these materials results in an additional 2 new classes of heteroaromatic pyridazine polymers reported for the first time in this chapter. The properties of these new materials are reported and the antioxidant properties of the dihydropyridazine ring are further explored through cyclic voltammetry (CV). Quenching of fluorescence is observed upon oxidation of the materials, providing a visual indicator for extent of oxidation, and an explanation for the quenching is also provide through CV analysis. Chapter 4 diverges from tetrazine IEDDA modification of polybutadienes and instead focuses on the post-modification of poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) with DCT to generate photobleachable materials. This chapter represents the first reported use of tetrazine photobleaching in modifying the optical properties of polymers, and the generation of a new copolymer containing pendant tetrazine repeat units. The modified copolymer is demonstrated to be solvent processable, but crosslinks upon heat treatment. Extended UV or ambient light excitation of the copolymer results in degradation of the tetrazine ring and bleaching of the polymer. A refractive index change for the polymer is observed after degradation, leading to the hypothesis that these polymers may find use in optical waveguide materials. Finally, chapter 5 provides a summation of the dissertation and discusses unfinished projects that can be pursued further by future Loy group graduate students.