Redox Control of Supramolecular Polymerization Using Electroactive Ureidopyrimidones

Abstract

Unlike conventional polymers, in which monomers are reacted to form strong covalent connections, supramolecular polymers are formed by linking monomers via weaker, non-covalent interactions such as H-bonds, ion-dipole interactions and so on. These imbue supramolecular polymers with unique properties since the linkages are readily broken, but also easily reformed as environmental conditions change. One particularly well-studied class of supramolecular polymers are based on the ureidopyrimidones, UPy’s, which dimerize via 4 strong, linear H-bonds in non-competitive solvents. Monomers for supramolecular polymerization can be made by linking together 2 or more UPy units with short covalent chains. Under dimerization conditions, the UPy’s link up to form longer polymeric chains. One of the useful attributes of such polymers is their self-healing ability. Application of heat or mechanical stress will readily break the monomers apart at the H-bonds, but, upon cooling or relief of stress, the bonds reform. The goal of this research is to see if electron transfer can be used as an alternative, more selective stimulus to break apart the UPy chains. As a first step in this direction, we have synthesized a known UPy monomer, 1, which is expected to polymerize at concentrations above ~20 mM in CH2Cl2. This can be verified by measuring the change in specific viscosity as a function of the concentration of 1. By adding simple UPy’s as chain terminators the degree of polymerization should decrease, with a corresponding decrease in the viscosity of solution. For this work, we plan to add our previously studied electroactive UPy’s, 2a and 2b. We have previously shown oxidation breaks apart the 2a and 2b dimers at mM concentrations in CH2Cl2. Thus, upon electrochemical oxidation, 2a or 2b will no longer be able to act as chain terminators and the linked UPy 1 should reform polymer chains. This can be detected by an increase in the viscosity of solution upon oxidation. Figure 1