Mechanochromism and Mechanical‐Force‐Triggered Cross‐Linking from a Single Reactive Moiety Incorporated into Polymer Chains

Abstract

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.

Stress-induced colouration and crosslinking of polymeric materials by mechanochemical formation of triphenylimidazolyl radicals

“…Stress-induced bond scission, a microscopic process, eventually causes macroscopic failure of the material. [11][12][13][14] Consequently, the implementation of both the ability to visualize as well as repair broken bonds in a polymeric material can be seen as a significant advance in the field of materials science, as recently demonstrated by Weng, Boulatov and co-workers. [1][2][3][4][5] Commonly, this is achieved by incorporating mechanically activatable molecular moieties (mechanophores) into polymer architectures.…”

“…Recent research demonstrates impressively how generally unspecific and undesired bond scission can be altered into constructive processes. 11 Motifs relying on the generation of stabilized radicals are particularly attractive for this endeavour as they combine features favourable for optical detection and for the formation of new chemical bonds. Those mechanophores then either act as probes for the visualization of stress or initiate the repair of the damaged material.…”

Mechanothermally induced conformational switch of a porphyrin dimer in a polymer film

“…4 A complementary approach stems from structural changes in the physical supramolecular arrangement of molecules. 5 To date, various compounds have been reported to show mechanical force-induced emission changes in the solid state using the abovementioned approaches.…”

Recent advances in mechanoluminescent polymers

“…For instance, mechanochromic 3D-printed structures [90] and soft robots [91] have been successfully developed using spiropyran as the mechanophore. As for the self-healing materials, Ramirez [28] and Zhang [27] both demonstrated the mechanochemical strengthening of their synthetic polymers in response to destructive forces, with gem-dibromocyclopropanes and spirothiopyran as the mechanophores, respectively. These pieces of work are inspiring, since we are anticipating huge benefits will be reaped if mechanoluminescence is coupled instead.…”

“…The last decade has witnessed tremendous progress on the creation of mechano-responsive polymers [12][13][14][15][16][17][18][19][20]. Mechanical force has been used, for instance, to activate ring-opening [13,17,21] and cycloreversion reactions [14,15,22], to trigger latent polymeric catalysts [12,[23][24][25], to guide "flex activation" [16,26], to induce cross-linking reactions [24,27,28], to assemble or disassemble supramolecular polymers in a controlled manner [29][30][31][32][33][34], and so on.…”