Metal-containing polymers are a diverse class of materials that can combine the functional elements imparted by the metals with the processability of polymers and as such, are ideal candidates to access new functional and stimuli-responsive materials. The applications span a diverse set of fields ranging from actuators to healable materials and molecular logic to active coatings, among many others. One subset of this field is metallo-supramolecular polymers (MSPs) in which the coordination between the organic and metal ion components are reversible. If the metal ion is redox active, then there is potential for such MSPs to be redox-responsive. The Rowan group has utilized MSPs to access stimuli-responsive gels and films that are sensitive to mechanical forces, heat, chemicals, and light. In all of these cases, ditopic 2,6-bis(N-methylbenzimidazolyl)pyridine (MeBip) encapped macromonomers are used as the organic component and either zinc (II), lanthanide (III), or Cu(I)/Cu(II) as the metallic component.

Redox-induced polymerisation/depolymerisation of metallo-supramolecular polymers

Abstract: 

Redox-responsive metallo-supramolecular polymers (MSPs), consisting of a ditopic ligand end-capped poly(tetrahydrofuran) (pTHF) macromonomer self-assembled with copper ions, have been developed where the degree of polymerization of the MSP, and therefore the viscosity of its solutions, depends on the oxidation state of the copper ion. We use a combination of UV-Vis, viscosity, light scattering, cyclic voltammetry and SAXS to show that the ligands, 2,6-bis(N-methylbenzimidazolyl)pyridine (MeBip), at the ends of the pTHF, bind copper(II) in a 2 : 1 ratio and copper(I) in a 2 : 2 ratio in solution. Thus at fixed 1 : 1 or 1 : 2 stoichiometry of macromonomer to copper ions the viscosity of their solutions shows dramatic changes in response to addition of a chemical oxidant (nitroso tetrafluoroborate) or reductant (aqueous ascorbic acid).