Incorporation of small reactive moieties, the reactivity of which depends on externally imposed load (so-called mechanophores) into polymer chains offers access to a broad range of stress-responsive materials. Here, we report that polymers incorporating spirothiopyran (STP) manifest both green mechanochromism and load-induced addition reactions in solution and solid. Stretching a macromolecule containing colorless STP converts it into green thiomerocyanine (TMC), the mechanically activated thiolate moiety of which undergoes rapid thiol-ene click reactions with certain reactive C=C bonds to form a graft or a cross-link. The unique dual mechanochemical response of STP makes it of potentially great utility both for the design of new stress-responsive materials and for fundamental studies in polymer physics, for example, the dynamics of physical and mechanochemical remodeling of loaded materials.

Mechanochemistry offers exciting opportunities for molecular-level engineering of stress-responsive properties of polymers. Reactive sites, sometimes called mechanophores, have been reported to increase the material toughness, to make the material mechanochromic or optically healable. Here we show that macrocyclic cinnamate dimers combine these productive stress-responsive modes. The highly thermally stable dimers dissociate on the sub-second timescale when subject to a stretching force of 1–2 nN (depending on isomer). Stretching a polymer of the dimers above this force more than doubles its contour length and increases the strain energy that the chain absorbs before fragmenting by at least 600 kcal per mole of monomer. The dissociation produces a chromophore and dimers are reformed upon irradiation, thus allowing optical healing of mechanically degraded parts of the material. The mechanochemical kinetics, single-chain extensibility, toughness and potentially optical properties of the dissociation products are tunable by synthetic modifications.

We study the mechanical activation of spiropyran (SP) in a doubly cross-linked polyurethane elastomer. Besides chemical cross-linking, the elastomer comprises polytetrahydrofuran as soft segments and hydrogenbonding 2-ureido-4-pyrimidone (UPy) as hard segments. The material shows two color changes because of the ring-opening reaction of SP to merocyanine (MC) at strained state and the isomerization about the methane bridge of MC at relaxed state. Increasing tensile strain rate leads to stiffer and stronger elastomer as well as earlier activation of SP. The activation point of SP to MC always coincides well with strain hardening of the stress−elongation curves. We further use the two-color transitions of SP to study the fracture of the elastomer during crack propagation.

A mechanically active spiropyran (SP) mechanophore is incorporated into the backbone of prepolymer which is further end-capped with ureidopyrimidinone (UPy) or urethane. Strong mechanochromic reaction of SP arises in the bulk films of UPy containing materials whereas much weaker activation occurs in urethane containing counterparts, coincident with their stress−strain responses. The difference in the magnitudes of supramolecular interactions leads to different degrees of chain orientation and strain induced crystallization (SIC) in the bulk and consequently distinct capabilities to transfer the load to the mechanophores. This study may aid the design of novel mechanoresponsive materials whose mechanoresponsiveness can be tailored by tuning supramolecular interactions.

The development of polymers that possess superb mechanical properties and at the same time are capable of sensing damage and self-healing is presented. Coppercatalyzed azide−alkyne cycloaddition (CuAAC) based tridentate ligand 2,6-bis(1,2,3-triazol-4-yl)pyridine (BTP) and covalent mechanophore spiropyran (SP) units are incorporated into the polymer backbone to prepare ligand macromolecule. Upon coordinating with transition or lanthanide metal salts, metallosupramolecular films with phased-separated soft/hard morphology are spontaneously formed. The resulting materials show a rare combination of strong, tough, and elastic mechanical properties and are able to sense damage by changing optical properties. The Zn 2+ -containing material can self-heal in the presence of solvent and fully restore its mechanical properties. The underlying structure−property relationship is unveiled. In particular, the interplay between the covalent SP mechanophore and the noncovalent metal−ligand interactions and their hard phase is demonstrated.

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With the aim of designing novel polymeric materials that exhibit superb mechanical performance and at the same time can sense stress, the hydrogen bonding ureidopyrimidinone (UPy) and covalent mechanochromic spiropyran (SP) are incorporated into one polymer structure to program the mechanical and mechanochemical responses of the materials. Excellent correlation between the molecular and microscopic structures and the macroscopic properties has been successfully demonstrated. Tensile tests reveal a combination of excellent mechanical properties, that is, high strength, high elongation at break, and high toughness. The superior mechanical properties are shown to be the consequence of the successive fragmentation of the hard domains formed by stacks of UPy dimers and the dissociations of UPy dimers. Stress sensing before catastrophic failure is also readily achieved in the form of color changing, which coincides with the strain hardening event. Studies show that the fragmentation and dissociation events occur before the mechanical activation of SP.

A mechanically active spiropyran (SP) mechanophore is incorporated into the center of poly(n-butyl acrylate) (PnBA) block to construct a series of mechanoresponsive polystyrene (PS)-PnBA-SP-PnBA-PS triblock copolymers. Similar mechanical activations of SP occur in all of the copolymers in solution, whereas a unique PS fraction-dependent mechanochromism is observed in the bulk. Effective mechanical activation occurs in the copolymer with a medium PS block length, whereas a very weak color change is observed in the samples bearing low PS fractions and activation appears only in the vicinity of the fracture point in the copolymer bearing long PS blocks. The difference in chemical compositions of the triblock copolymers leads to different microphase separated structures in the bulk and consequently the unique stress–strain responses and mechanochemistry. This platform promises to open way to the design of a wide range of useful mechanoresponsive triblock copolymers having different hard/soft blocks and various types of mechanoresponsive motifs.