Thermo-, photo-, and chemo-responsive shape-memory properties from photo-cross-linked metallo-supramolecular polymers

Films exhibiting multiresponsive shape-memory properties have been accessed using covalently cross-linked metallo-supramolecular polymers. Low molecular weight poly(butadiene) was end-capped with 4-oxy-2,6-bis(N-methylbenzimidazolyl)pyridine (-OMebip) ligands that upon addition of metal salts spontaneously formed high molecular weight metallo-supramolecular polymers. The addition of a tetra-functional thiol along with a photoinitiator results in mechanically stable films via solution-casting. These films consist of a soft poly(butadiene) phase and a hard metal-ligand phase. Photo-cross-linking of the poly(butadiene) soft phase, via the thiol-ene reaction, upon exposure to relatively low intensity light, allows access to a diverse range of permanent shapes. Investigations into the temporary shape fixing and recovery of these materials were undertaken to determine the effects of cross-link density and the nature of the metal salts. The key component in fixing and releasing the temporary shape is the metal-ligand hard phase, and as such any stimulus that can disrupt this phase (light, heat, or chemicals) can be used to create the temporary shape and induce its recovery back to the permanent shape.