The Use of Electroactive Ureidopyrimidones As Redox-Responsive Chain Terminators for Supramolecular Polymers

Abstract

Unlike conventional polymers, in which monomers are reacted to form strong covalent connections, supramolecular polymers are formed by linking together monomers via weaker, non-covalent interactions such as H-bonds, ion-dipole interactions and so on. One very 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, the use of our previously studied electroactive UPy’s, 1a and 1b, are being investigated as redox-active chain terminators. For these studies the known UPy monomer, 2, was synthesized. Viscosity measurements of solutions of 2 in 0.1 M NBu4PF6/CH2Cl2 give a critical polymerization concentration of 20 mM. Based on this, 60 mM 2 was chosen as the working concentration for initial experiments giving a solution with a specific viscosity of 3.0 mPa∙s. Upon addition of 2 mM of the ferrocene containing UPy 1a, the viscosity dropped to 1.1 mPa∙s, indicating that 1a is an effective chain terminator, decreasing the lengths of the chains which results in a decrease in the viscosity of the solution The ferrocenes were then completely oxidized to the ferrocenium form in a two compartment electrochemical cell. This caused the viscosity of the solution to increase from 1.0 to 1.7 mPa∙s, suggesting decreased effectiveness of the ferrocenium form as a chain terminator. This is consistent with poorer binding of the ferrocenium form to the bis-UPy. Importantly, reduction back to the ferrocene led to a decrease in the viscosity back to close to where it was initially. In this presentation, the results of further studies on the ability of both 1a and 1b to act as redox-responsive chain terminators for UPy-based supramolecular polymers will be reported. Figure 1