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Nature's ability to precisely assemble macromolecules into highly cooperative and functional assemblies, such as DNA, provides an inspiration for our proposed efforts. Our research program involves novel synthetic strategy to tailor the relationship between macromolecular structure and non-covalent interactions for the discovery of novel functional, hydrogen bonding macromolecules. Our current research efforts involve the discovery of nucleobase-containing hydrogen bonding polymers with tunable mechanical properties for melt processing and self-healing applications. The primary principle is the ability to control the mobility and accessibility of the associative site, and disassociation in the melt step will enable more facile processing of thermoplastic elastomers containing ionic sites. Complementary multiple hydrogen bonding (CMHB) has received relatively sparse attention as a site for molecular recognition and intermolecular interactions. The foundation for this program is based on free radical copolymerization for the preparation of nucleobase-containing polyelectrolytes and polyacrylates.

Complementary multiple hydrogen bonding polymers offer potentially superior mechanical integrity compared to conventional hydrogen bonding polymer systems.  Moreover, CMHB units provide an efficient molecular recognition and self-assembly process due to the high hydrogen bonding association constant. Nucleobase-containing linear and branched copolymers exhibited DNA-like melting behavior and a stronger temperature dependence of melt viscosity compared to non-hydrogen-bond containing polymers, suggesting possible advantages as thermo-responsive polymers to self-heal at relatively low temperatures. Utilizing CMHB units in the design of a self-healing polymer that is both reusable and durable may facilitate recycling of hard-to-dispose plastics for environmental security and energy saving.

  Our studies start with determining the reactivity ratios of styrenic adenine monomer (VBA) and tert-butyl acrylate (tBA) utilizing our newest instrument, a Mettler-Toledo ReactIR 45M, which allows us to monitor in situ the infrared spectrum of the reaction contents and ultimately the reaction kinetics. (Figure 1) According to the Mayo-Lewis equation, the reactivity ratios for styrenic adenine and tBA were determined to be 0.41±0.02 and 0.42±0.02, respectively. The literature reactivity ratios for styrene and tBA were 0.89 and 0.29, respectively, in toluene. The difference in the reactivity ratios resulted from the change of solvent, which led to a higher incorporation of tBA than expected. Subsequent deprotection of tBA yielded water dispersible nucleobase-containg polyanions, which mimic DNA structures. Nucleobase-containing polycation analogs were salt-responsive in NaCl aqueous solution. Incorporating CMHB sites in polyelectrolytes provide the potential for rheology modifiers to assist nanofiber formation in the electrospinning process. (Figure 2) Additionally, the intermolecular interaction of the complementary hydrogen bond and pi-pi stacking interaction possibly plays a similar kinetic role as in nylon-6 in regulating the chain orientation process and allowing tunable fiber rigidity. Future studies will also address the salt responsiveness of nanofibers for water purification application.

  Collaborations with Eastman Chemical and Kraton^TM Polymers revealed the suitability of this novel functional hydrocarbon platform for both adhesive and elastomer technologies, respectively. Studies showed that CMHB greatly influenced corresponding polymer physical properties, such as mechanical and morphological behavior. Our attention on tailored, self-assembled nucleobase-containing polymers as membranes provides structure and ionic transport relationships that suggest block copolymer structures for desalination and ion-transport.

  Our previous studies on styrenic adenine- and thymine-containing polyacrylates showed that the bulky styrene units connected to nucleobase functionality reduced polymer backbone mobility and nucleobase binding efficiency in polymer blends. This observation catalyzed our current research program on the original preparation of a novel acrylate nucleobase monomer containing a dangling nucleobase (adenine and thymine) to improve binding and demonstrate the CMHB influence on polymer adhesive properties. We have synthesized a series of acrylate adenine-containing random copolymers. Figure 3 shows that adenine units microphase separated from the amorphous low Tg polymer matrices and formed percolated nano structures with a diameter of around 20 nm. This microphase separation might be driven by pi-pi stacking interactions between adenine units, which then contribute to self-healing as well as mechanical integrity. The strength and reversibility of non-covalent interactions are highly dependent on environmental conditions, and dynamic molecular recognition processes for self-healing have not been significantly addressed in the literature. Figure 4 depicts the building blocks for the introduction of pendant nucleobases and the implications of tailored hydrogen bonding on self-healing.

Current efforts are focused on the preparation of adenine and thymine based on acrylate functionality. The monomers will be used to demonstrate the role of sequenced hydrogen bonding on morphological and rheological performance.  Moreover, our efforts will continue to address the new area of "supramolecular click" chemistry wherein the functional group is located on the polymer template using molecular recognition. An important question arises concerning the selective nature of the functionalization, and the ability for a molecular guest to selectively locate the complementary pair in the presence of other nucleobases is an important aspect of continued research efforts.

 ADDIN EN.REFLIST