Polymers with the ability to repair themselves after sustaining damage could extend the lifetimes of materials used in many applications 1 . Most approaches to healable materials require heating the damaged area [2][3][4] . Here we present metallosupramolecular polymers that can be mended through exposure to light. They consist of telechelic, rubbery, low-molecular-mass polymers with ligand end groups that are non-covalently linked through metal-ion binding. On exposure to ultraviolet light, the metal-ligand motifs are electronically excited and the absorbed energy is converted into heat. This causes temporary disengagement of the metal-ligand motifs and a concomitant reversible decrease in the polymers' molecular mass and viscosity 5 , thereby allowing quick and efficient defect healing. Light can be applied locally to a damage site, so objects can in principle be healed under load. We anticipate that this approach to healable materials, based on supramolecular polymers and a light-heat conversion step, can be applied to a wide range of supramolecular materials that use different chemistries.The healing of cracks in amorphous polymers by heating above the glass transition temperature (T g ) involves surface rearrangement and approach of polymer chains, followed by wetting, diffusion and reentanglement of the chains 6 . Because the rates of the final two steps are inversely proportional to the molecular mass, healing is generally slow and inefficient. This problem can be overcome by exploiting thermally reversible, covalent bonds 7,8 or non-covalent supramolecular motifs 5,9,10 that allow the reaction equilibrium to be temporarily shifted to lower-molecular-mass species 11 on exposure to heat. This reduces the viscosity of the material, such that defects can be mended, before the equilibrium is shifted back and the polymer is reformed. Supramolecular polymers that phase separate into physically crosslinked networks (Fig. 1a) should be especially well suited for this purpose, because such morphologies generally bestow the material with high toughness. The supramolecular motifs can disengage in the solid state on exposure to heat or a competitive binding agent 12,13 , causing disassembly into small molecules 14 and viscosity reductions. Reporting a series of supramolecular materials formed by metal-ligand interactions, we demonstrate here that this architecture is an excellent basis for elastomeric materials in which defects can be efficiently repaired. We show that the use of light 15 as a stimulus for the dissociation of supramolecular motifs has distinct advantages over thermally healable systems, including the possibility of exclusively exposing and healing the damaged region.The new polymers are based on a macromonomer comprising a rubbery, amorphous poly(ethylene-co-butylene) core with 2,6-bis(19-methylbenzimidazolyl)pyridine (Mebip) ligands at the termini (Fig. 1b, 3). This design was based on the assumption that the hydrophobic core and the polar metal-ligand motif would phase separate 16 . Metal-Mebip com...

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.

A series of stimuli-responsive films based on metallo-supramolecular polymers have been prepared and studied. The metallo-supramolecular polymers are comprised of a 4-oxy-2,6-bis-(1 0 -methylbenzimidazolyl)pyridine ditopic endcapped poly(tetrahydrofuran) with different ratios of Zn 2+ and Eu 3+ . A combination of the optical properties of the Eu 3+ complex along with its more labile nature results in the formation of highly stimuli-responsive materials. The films of the Eu 3+ -containing metallo-supramolecular polymers show a pronounced optical response to an increase in temperature or upon exposure to chemicals such as triethyl phosphate, which was used as a mimic for organophosphate pesticides and nerve gas agents.

Detailed rheological studies of metallosupramolecular polymer films in the melt were performed to elucidate the influence of the metal ion and polymer components on their mechanical and structural properties. 4-Oxy-2,6-bis(N-methylbenzimidazolyl)pyridine telechelic end-capped polymers with a low-T g core, either poly(tetrahydrofuran) or poly(ethylene-co-butylene), were prepared with differing ratios of Zn 2+ and Eu 3+ to determine the influence of polymer chain chemistry and metal ion on the properties. Increasing the amount of the weaker binding europium yielded more thermoresponsive films in both systems, and results show that the nature of the polymer core dramatically affected the films mechanical properties. All of the films studied exhibited large relaxation times, and we use this to explain the pure sinusoidal behavior found in the "nonlinear" viscoelastic region. Basically, the system cannot relax during a strain cycle, allowing us to assume the network destruction and creation rates to be only a function of the strain amplitude in a simplified network model used to rationalize the observed behavior.

Square-planar platinum(II) complexes of the 4-dodecyloxy-2,6-bis(N-methylbenzimidazol-2 0 -yl) pyridine ligand have been blended into a series of methacrylate polymers. Each of the polymer films displayed vapochromic (yellow to red) and vapoluminescent behaviour as a result of the vapour induced change in the Pt-Pt interactions. The solid-state absorption and emission properties of the films were characterized, and the influence of the polymer matrix on the vapochromic response was investigated. Notably, the rate of recovery of the yellow colour after vapour-exposure was found to be dependent on the T g of the matrices, and the response can be effectively erased and the materials restored to the original yellow colour within minutes by simply heating above their T g . The polymer-complex blends also displayed interesting mechanochromic and mechanoluminescent properties upon deformation either by compression, scratching, or stretching. The accumulated data are consistent with a solvato-or mechanoinduced structural rearrangement that results in a shortening of the Pt-Pt distances.