Different forms of molecular recognition – such as hydrogen bonding or π-π stacking for example – have proven to be powerful tools in biology and play a major role in mediating communication across cellular membranes and controlling the conformation and function of nature’s biopolymers. Chemists have traditionally designed synthetic polymers that adopt similar forms of molecular recognition as a way to enhance the properties of a material. The Barnes group at Washington University in St. Louis is interested in programming precision polymers with non-natural forms of molecular recognition to obtain new materials with unique functions. Specifically, my group has developed a library of well-defined main-chain polyviologens that undergo chain folding (intramolecularly) in response to reduction of each viologen subunit (aka, N,N′-dialkyl-4,4′-bipyridinium) from its dicationic oxidation state to the corresponding radical cation (i.e., V2+ to V●+). The molecular recognition process is driven by thermodynamically favorable radical-radical pairing interactions, the extent of which we hypothesize can be tuned depending on the linker units that are installed in between each viologen subunit. Conversion of the polyviologens into macrocrosslinkers through end-capping reactions that place a styrene or an acrylate group at each terminus allows for polymer networks to be established using free radical polymerizations. These polymer networks (hydrogels) can be induced reversibly to undergo large amplitude changes in volume (contraction/expansion) and mechanical properties (higher Young’s modulus, tensile strength, and elasticity) when the constituent polyviologens are reduced in situ. We have demonstrated the versatility of this new mechanism for actuating soft materials by introducing photocatalysts into the hydrogels and irradiating with different wavelengths of visible light, depending on the photocatalyst employed. In my talk, I will describe my laboratory’s progress in these areas, and I will discuss new polyviologen designs and other applications currently being pursued in my group.

Synthesis and application of viologen-based polymers and photoredox-responsive soft materials

ACS Fall 2021

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