Cascade Mechanochemical Transformation of a Benzobarrelane Polymer: A Neighboring Repeat Unit Effect Article Swipe
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· 2025
· Open Access
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· DOI: https://doi.org/10.21203/rs.3.rs-7133429/v1
· OA: W4412572656
<title>Abstract</title> Mechanical force can trigger non-destructive chemical transformations of force-responsive molecules, mechanophores, that are embedded in a long polymer chain to change or induce a variety of their physical and chemical properties. Studies on polymer mechanochemistry have thus far focused on the design and understanding of the reactivity of individual mechanophores. However, when mechanophores, or even molecular structures that are not typically considered force-responsive, are coupled with each other in the polymer backbone, new reactivity and force response that are absent from discrete molecules may arise to transform polymer backbone structures. Herein, we describe a system where mechanically triggered backbone bond scission generates biradical intermediates, which then undergo radical cascade reactions to transform polymer backbone structure. Our design is based on a polymer with benzobarrelane repeat units that are connected via backbone alkenes. The alkene linkage is crucial for the radical cascade ring-opening reactions to occur across multiple connected benzobarrelane units. In contrast, no mechanochemical reaction was observed in isolated benzobarrelane units that are separated by alkyl linkages, even when the alkenyl substituents of benzobarrelane were preserved. Despite the lack of weak covalent bonds or significant ring strain in the benzobarrelane polymer, it achieved similar degrees of mechanochemical transformation as our previously reported polyladderene systems consisting of repeat units of highly strained fused cyclobutanes. Ab initio steered molecular dynamics and force-modified potential energy surface calculations supported the force-induced radical cascade mechanism along the polymer backbone. Simulations indicated a strong preference for sequential mechanoactivation: upon ring-opening of one benzobarrelane unit, its immediately neighboring unit is most likely to be activated next. This work demonstrates a new strategy for designing mechanically responsive polymers by creating chemical pathways to promote cooperative mechanoactivation across neighboring units. It further underscores the potential to elicit novel reactivities through the collective interaction of otherwise inert individual molecules.
