Multimodal mechanophores that react under mechanical force to produce discrete product states with uniquely coupled absorption properties are interesting targets for the design of force-sensing polymers. Herein, we investigate the reactivity of a 2H-bis-naphthopyran mechanophore that generates thermally persistent mono-merocyanine and bis-merocyanine products upon mechanical activation in solution using ultrasonication, distinct from the thermally reversible products generated photochemically. We demonstrate that a force-mediated ester C(O)–O bond scission reaction following ring opening establishes an intramolecular hydrogen bond, locking one merocyanine subunit in the open form. Model compound studies suggest that this locked subunit confers remarkable thermal stability to bis-merocyanine isomers possessing a trans exocyclic alkene on the other subunit, implicating the formation of an unusual trans merocyanine isomer as the product of mechanochemical activation. Density functional theory calculations unexpectedly predict a thermally reversible retro-cyclization reaction of the bis-merocyanine species that could explain the mechanochemical generation of the unusual trans merocyanine isomer.
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Stereochemical effects on the mechanochemical scission of furan–maleimide Diels–Alder adducts
Clarifying the correlation between the chemical structure of mechanophores and their mechanical reactivity informs the design of mechanochemical systems. One specific correlation that has received much recent attention is that between stereoisomerism and mechanical reactivity. Here, we report previously unobserved differences in the mechanical reactivity of furan–maleimide Diels–Alder (DA) stereoisomers. We evaluated the internal competition between the mechanically triggered retro-DA reaction and the mechanochemical ring opening of gem -dichlorocyclopropane mechanophores in the pulsed sonication of polymer solutions. The relative extent of the two sonomechanochemical reactions in the same polymer shows that the endo DA isomer exhibits greater mechanical lability than its exo isomer. This result contrasts with recent measurements of the relative rates of scission in a similar system and points to potential enhanced sensitivity obtained through the use of internal competition as opposed to absolute rates in assessing mechanical reactivity in sonication studies.
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- Award ID(s):
- 1808518
- PAR ID:
- 10175557
- Date Published:
- Journal Name:
- Chemical Communications
- Volume:
- 55
- Issue:
- 81
- ISSN:
- 1359-7345
- Page Range / eLocation ID:
- 12263 to 12266
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Herein we report the discovery of the intrinsic mechanochemical reactivity of vinyl‐addition polynorbornene (VA‐PNB), which has strained bicyclic ring repeat units along the polymer backbone. VA‐PNBs with three different side chains were found to undergo ring‐opening olefination upon sonication in dilute solutions. The sonicated polymers exhibited spectroscopic signatures consistent with conversion of the bicyclic norbornane repeat units into the ring‐open isomer typical of polynorbornene made by ring‐opening metathesis polymerization (ROMP‐PNB). Thermal analysis and evaluation of chain‐scission kinetics suggest that sonication of VA‐PNB results in chain segments containing a statistical mixture of vinyl‐added and ROMP‐type repeat units.
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Abstract Herein we report the discovery of the intrinsic mechanochemical reactivity of vinyl‐addition polynorbornene (VA‐PNB), which has strained bicyclic ring repeat units along the polymer backbone. VA‐PNBs with three different side chains were found to undergo ring‐opening olefination upon sonication in dilute solutions. The sonicated polymers exhibited spectroscopic signatures consistent with conversion of the bicyclic norbornane repeat units into the ring‐open isomer typical of polynorbornene made by ring‐opening metathesis polymerization (ROMP‐PNB). Thermal analysis and evaluation of chain‐scission kinetics suggest that sonication of VA‐PNB results in chain segments containing a statistical mixture of vinyl‐added and ROMP‐type repeat units.
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Recent reports have shown that ferrocene displays an unexpected combination of force-free stability and mechanochemical activity, as it acts as the preferred site of chain scission along the backbone of highly extended polymer chains. This observation raises the tantalizing question as to whether similar mechanochemical activity might be present in other metallocenes, and, if so, what features of metallocenes dictate their relative ability to act as mechanophores. In this work, we elucidate polymerization methodologies towards main-chain ruthenocene-based polymers and explore the mechanochemistry of ruthenocene. We find that ruthenocene, in analogy to ferrocene, acts as a highly selective site of main chain scission despite the fact that it is even more inert. A comparison of ruthenocene and ferrocene reactivity provides insights as to the possible origins of metallocene mechanochemistry, including the relative importance of structural and thermodynamic parameters such as bond length and bond dissociation energy. These results suggest that metallocenes might be privileged mechanophores through which highly inert coordination complexes can be made dynamic in a stimuli-responsive fashion, offering potential opportunities in dynamic metallo-supramolecular materials and in mechanochemical routes to reactive intermediates that are otherwise difficult to obtain.more » « less
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The encapsulation of guests in supramolecular capsules has long been used to trap reactive intermediates and enhance or reduce the kinetic stability of reactants, and alter the products of chemical reactions that occur within the capsule interior. In recent years, multiple studies have shown that variations of normal reactivity patterns can be induced by trapping reactants under tension, for example along a backbone of an overextended polymer chain, in a manner that is fundamentally very different from, but reminiscent of, encapsulation. Here, we describe the formation of a mechanochemically generated isocyante via a mechanical retro [2 + 2] cycloaddition of a 1,2-diazetidinone (DAO) mechanophore. A single DAO mechanophore is incorporated into the chain center of a poly(methyl acrylate) (PMA) backbone via single electron transfer-living radical polymerization (SET-LRP). Mechanical activation of the DAO via ultrasonic sonication leads to the formation of isocyanate and imine products, as supported by trapping experiments using 9-(methylaminomethyl)anthracene labelling and 1 H NMR spectroscopy. Further, we examine the relative mechanical susceptibility of chain-centered DAO mechanophores through a variety of methods, and evaluate the advantage and disadvantage of each.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9CC06361G
