Diels-Alder polyphenylenes (DAPPs) are chemically and thermally stable polymers, used for dielectric resins, gas separation membranes, and fuel cell proton exchange membranes. Highly fluorinated DAPPs are expected to have better thermal stability and chemical resistance, higher glass transition temperatures, improved compatibility with other fluorinated polymers (like NafionTM), and better adhesion to certain surfaces such as some metals. This thesis proposes a synthesis of highly fluorinated DAPPs by reacting novel, fluorinated bis-cyclopentadienone (CPD) monomers with known aromatic dialkynes. This thesis starts with an introduction to DAPP synthesis, properties, and applications. The second chapter focuses on the synthesis of 4,4′-bis[1,2,4-tris(perfluoro-4-tolyl)cyclopentadien-5-one-3-yl]octafluorobiphenyl (CPD monomer). In the first step, disodiumbis(cyclopentadieny)octafluorobiphenyl was combined with 6 equivalents of octafluorotoluene in HMPA to give 4,4′-bis[1,2,4-tris(perfluoro-4-tolyl)cyclopentadiene-3-yl]octafluorobiphenyl. Oxidation to the corresponding diketone was effected using selenium dioxide. The synthesis of CPD monomer presented several unexpected challenges that were ultimately overcome. The third chapter describes a series of initial polymerization experiments as well as some model reactions that were carried out to understand monomer reactivity. Finally the future research plan of synthesizing different polymers by Diels-Alder reaction and nucleophilic substitution reaction has been discussed. / Master of Science

The first chapter of this dissertation reports the synthesis of highly fluorinated Diels-Alder polyphenylenes. The first section of this chapter describes the three-pot synthesis of a perfluoroarylated bis(cyclopentadienone) monomer. The synthesis begins with the previously reported substitution reaction of decafluorobiphenyl and sodium cyclopentadienide. To the resulting 4,4'-octafluorobiphenylene-linked bis(cyclopentadiene), six perfluoro-4-tolyl groups (three on each of the two cyclopentadienyl moieties) are attached by nucleophilic aromatic substitution (SNAr) reactions. The remaining ring methylenes are subjected to a selenium dioxide-catalyzed oxidation to obtain the desired bis(cyclopentadienone) monomer. The next part of this chapter describes the polymerization of the perfluoroarylated bis-(cyclopentadienone) monomer and bis(4-ethynylphenyl) ether. The reaction affords an oligomer (Mn ~ 14,000 g/mol according to size-exclusion chromatographic analysis) that is soluble in several solvents and that decomposes above about 300°C according to thermogravimetric analysis. The second chapter of this dissertation describes a novel method to oxidize per-fluoroarylated cyclopentadiene compounds to the corresponding ketones using catalytic selenium dioxide and stoichiometric hydrogen peroxide. The first part of this chapter shows the synthesis of some perfluoroarylated cyclopentadiene substrates, while the second part of the chapter explores the oxidation of these compounds along with other perfluoroarylated cyclopentadienes already available within our research group. This chapter also explains how the reactivity of the perfluoroarylated cyclopentadienes under the oxidation conditions depends on their structure. Generally more electron-deficient cyclopentadienes react more readily, while sterically crowded cyclopentadienes react more reluctantly. This third chapter of this dissertation describes the synthesis and characterization of a reversible Diels-Alder polymer from an octafluorobiphenylene-linked bis(cyclopentadiene). In the first section, the synthesis of a reversible homopolymer of the bis(cyclopentadiene) monomer is described. The polymer reaches an optimized molecular weight of 11,000 g/mol (degree of polymerization is 20) under the reaction conditions because there is an equilibrium between polymerization and depolymerization even at the mild polymerization temperature (65°C). The TGA trace of the polymer shows that chain degradation takes place beyond 300°C. The thermal reversibility of the polymer was examined by bulk thermolysis, and flash-vacuum thermolysis. The second section describes the synthesis of a methylated bis(cyclopentadiene) that does not undergo self-polymerization at comparatively lower temperature but instead reacts with a second bis(maleimide) monomer. The resulting polymer typically shows a number-average molecular weight of 15,400 g/mol. This polymerization also is limited by the attainment of steady-state end group concentrations. The reversibility of the polymerization is demonstrated by solution thermolysis experiments in which unmasked cyclopentadiene groups are trapped by a monofunctional maleimide. / Ph. D.

This dissertation describes a general synthetic platform for segmented polymers that have main-chain reversible linkages based on cyclopentadiene-maleimide Diels-Alder chemistry. Research in the area of thermally reversible (self-healing) polymers has been an ever-expanding area of interest in the current scientific literature. However most of the emphasis has been on systems containing furan-maleimide linkages. While inexpensive and synthetically accessible, furan chemistry is mostly limited to crosslinked and hyperbranched architectures due to its relatively weak binding with maleimides at suitable propagation temperatures. Following a general review of the literature in this area (Chapter 1) the first stage of our research (Chapter 2) entails the synthesis of 2-substituted hydroquinones, which are needed as monomers in the later stages. The novelty of our hydroquinone synthesis stems from the use of allylic and other alkenyl ethers as the source of the ring substituent, and from the utilization of catalytic hydroboration to improve atom-efficiency. We showed that hydroquinones with widely varying functionality can be prepared efficiently by our method; these findings were published in the journal Tetrahedron in 2018. The second stage (Chapter 3) involves the use of the new hydroquinones in step-growth syntheses of hydroquinone-terminated telechelic and chain-extension of these telomers via Diels-Alder chemistry to form segmented polymers having thermally reversible linkages. The novelty of our approach rests with the use of cyclopentadiene-maleimide chemistry for the linkages, while the overall physical properties such as the glass transition temperature were established by using well-defined aromatic polymers — poly(ether ether ketones) or PEEK and poly(aryl ether sulfones) or PAES — as segments. This approach represents an important departure from earlier work in our group in which reversible linkages were present in every repeat unit of a step-growth Diels-Alder polymer that showed thermal reversibility in solution but not in the bulk, owing to glass transition temperatures that were too high. Using scratch-healing and mechanical (tensile) tests, we show that our new segmented polymers exhibit self-healing characteristics that are competitive with or superior to previously reported systems based on different Diels-Alder chemistry. The third stage (Chapter 4) aims to explore new application areas for some of the more novel functionalized hydroquinones reported in Chapter 2. First we developed an efficient synthesis of a PAES derivative bearing 5-phenoxypentyl groups on the hydroquinone moiety. Then we showed that the 5-phenoxy group can be cleanly cleaved, post-polymerization, to afford a PAES having 5-bromopentyl substituents. The promise of our method rests with the potential of the pendant electrophiles to undergo reactions with nucleophilic reagents to post-modify these polymers further. As proof of concept, we showed that substitution of the pendant bromides with furfuryloxy groups enabled thermally reversible crosslinking with a bis-maleimide reagent to form a polymeric material that demonstrates partial scratch healing. Finally we are exploring the synthesis of new ion-containing polymers by substituting the pendant bromides with tertiary amines. / PHD / This dissertation describes a new synthetic approach to polymeric materials that can heal themselves (for example, repair small cracks that may have formed due to stress or aging) simply by heating the damaged area. Our approach uses a thermally reversible chemical reaction (called the Diels-Alder reaction) to connect several shorter polymer segments into longer chains. Upon heating, the segments can come apart, diffuse into and through the damaged area, and then rejoin. The first chapter is a review of background in the published literature as well as previous not-yet-published work in our laboratory. The second chapter describes the creation of new building-block molecules (monomers) that will help control the temperature range necessary to induce self-healing after incorporation into the polymer segments. The third chapter details the process of forming the segments, the incorporation of self-healing functionalities on the ends of the segments, the joining of the segments into longer polymeric chains, and the testing of all of the physical properties of these new materials, including their self-healing capabilities. The fourth chapter represents a preliminary study of a new method of preparing ion-containing polymers. The latter materials have potential use in various membrane technologies including fuel cell devices for the harnessing of renewable energy.

Curved graphene nanoribbons (GNRs) with hybrid edge structures have recently attracted increasing attention due to their unique band structures and electronic properties as a result of their nonplanar conformation. This work reports the solution synthesis of a long and curved multi-edged GNR (cMGNR) with unprecedented cove–armchair–gulf edge structures. The synthesis involves an efficient A2B2-type Diels–Alder polymerization between a diethynyl-substituted prefused bichrysene monomer (3b) and a dicyclopenta[e,l]pyrene-5,11-dione derivative (6) followed by FeCl3-mediated Scholl oxidative cyclodehydrogenation of the obtained polyarylenes (P1). Model compounds 1a and 1b are first synthesized to examine the suitability and efficiency of the corresponding polymers for the Scholl reaction. The successful formation of cMGNR from polymer P1 bearing prefused bichrysene units is confirmed by FTIR, Raman, and solid-state NMR analyses. The cove-edge structure of the cMGNR imparts the ribbon with a unique nonplanar conformation as revealed by density functional theory (DFT) simulation, which effectively enhances its dispersibility in solution. The cMGNR has a narrow optical bandgap of 1.61 eV, as estimated from the UV–vis absorption spectrum, which is among the family of low-bandgap solution-synthesized GNRs. Moreover, the cMGNR exhibits a carrier mobility of ≈2 cm2 V−1 s−1 inferred from contact-free terahertz spectroscopy.