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      Polymer mechanochemistry-enabled pericyclic reactions

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          Abstract

          Polymer mechanochemical pericyclic reactions are reviewed with regard to their structural features and substitution prerequisites to the polymer framework.

          Abstract

          Over the past decades, it became clear that next to heat and light, pericyclic reactions can be induced mechanochemically when the reacting motifs are embedded as latent force-responsive groups (mechanophores) into polymer architectures. Not only does this enable a variety of functions and applications on a material level, but moreover grants access to symmetry-forbidden reaction products with respect to the Woodward–Hoffmann rules. The latter indicates that polymer mechanochemistry follows its own set of rules that, however, regarding underlying mechanisms and design rationales is far from being holistically understood. Here we review the existing body of literature and identify common structural features and substitution prerequisites to the polymer framework shining light on the differences between polymer mechanochemical pericyclic reactions and their traditional counterparts. By this, we believe to contribute to the major challenge of not only retrospectively describing force-induced reactivity but eventually finding a common molecular design guideline.

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          Most cited references 211

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          A thermally re-mendable cross-linked polymeric material.

          We have developed a transparent organic polymeric material that can repeatedly mend or "re-mend" itself under mild conditions. The material is a tough solid at room temperature and below with mechanical properties equaling those of commercial epoxy resins. At temperatures above 120 degrees C, approximately 30% (as determined by solid-state nuclear magnetic resonance spectroscopy) of "intermonomer" linkages disconnect but then reconnect upon cooling, This process is fully reversible and can be used to restore a fractured part of the polymer multiple times, and it does not require additional ingredients such as a catalyst, additional monomer, or special surface treatment of the fractured interface.
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            Force-induced activation of covalent bonds in mechanoresponsive polymeric materials.

            Mechanochemical transduction enables an extraordinary range of physiological processes such as the sense of touch, hearing, balance, muscle contraction, and the growth and remodelling of tissue and bone. Although biology is replete with materials systems that actively and functionally respond to mechanical stimuli, the default mechanochemical reaction of bulk polymers to large external stress is the unselective scission of covalent bonds, resulting in damage or failure. An alternative to this degradation process is the rational molecular design of synthetic materials such that mechanical stress favourably alters material properties. A few mechanosensitive polymers with this property have been developed; but their active response is mediated through non-covalent processes, which may limit the extent to which properties can be modified and the long-term stability in structural materials. Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups-so-called mechanophores-that the directional nature of mechanical forces can selectively break and re-form covalent bonds. We now demonstrate that such force-induced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process. We anticipate that force activation of covalent bonds can serve as a general strategy for the development of new mechanophore building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing.
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              A Correlation of Reaction Rates

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                Author and article information

                Contributors
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                Journal
                PCOHC2
                Polymer Chemistry
                Polym. Chem.
                Royal Society of Chemistry (RSC)
                1759-9954
                1759-9962
                March 31 2020
                2020
                : 11
                : 13
                : 2274-2299
                Affiliations
                [1 ]DWI – Leibniz Institute for Interactive Materials
                [2 ]52056 Aachen
                [3 ]Germany
                [4 ]Institute for Technical and Macromolecular Chemistry
                [5 ]RWTH Aachen University
                Article
                10.1039/C9PY01937E
                © 2020
                Product
                Self URI (article page): http://xlink.rsc.org/?DOI=C9PY01937E

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