Use of a Redox-Responsive 4 H-Bond Ureidopyrimidone (UPy) Array to Control Polymerization in a Upy-Based Supramolecular Polymer

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Ureidopyrimidinone (UPy) derivatives, introduced by Meijer and co-workers, are capable of forming dimers linked by four strong H-bonds. They have been widely used as reversible crosslinkers in self-healing supramolecular polymers and gels. The goal of the project to be presented is to create a redox-controllable UPy supramolecular polymer system by adding a redox-responsive UPy monomer as a capping agent to a solution of a non-redox-active UPy supramolecular polymer. Specifically, a UPy containing a phenylenediamine redox couple, UPyH, that we have previously studied,1 will be used as the capping agent. In previous work we have shown that UPyH dimerizes in methylene chloride with Kdim > 105 as expected. Oxidation of the phenylenediamine to the radical cation does not break apart the dimer at mM concentrations. However, further oxidation to the quinoidal dication weakens the binding enough to allow an external base to remove the most acidic protons and thereby create a non-self-complementary H-bonding motif that cannot dimerize. As long as the base is not too strong the process is reversible on the cyclic voltammetric time scale, leading to re-formation of the UPyH dimer upon reduction. Previous studies show that 1 equivalent of 4-trifluoromethylpyridine is a good choice for the base. To create the redox-responsive supramolecular polymer system, a known UPy oligomer will be used to prepare a supramolecular polymer solution in methylene chloride electrolyte solution. The degree of polymerization will be assessed by measuring the viscosity of the solution. UPyH plus an equal amount of 4-trifluoromethylpyridine will be then added until there is a significant decrease in viscosity indicating that the UPyH is H-bonding to the UPy’s in the non-redox active UPy oligomer and thereby decreasing the length of the supramolecular chains. The solution will then be oxidized either chemically or electrochemically to convert the UPyH to its fully oxidized, de-protonated form. Since this form is incapable of dimerizing with itself or another UPy, the UPyH will lose its viability as a chain terminator, thus allowing the non-redox active Upy oligomers to reform supramolecular chains. This will be signaled by an increase in the viscosity of the solution. 1L. A. Clare and D. K. Smith, Chemical Communications 2016, 52, 7253-7256.