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<p>The acceleration of chemical reactions under mechanical stress has been known since
the earliest days of polymer science. Once limited to the simple scission of polymer
chains, mechanical force can now be used to produce a wide array of productive chemistry.
The development of so-called "covalent mechanochemistry," has allowed chemists to
challenge and support classically held models of chemical reactivity, impacting both
synthetic chemistry and material science. This work aims to develop molecular tools
that respond to stress and explore the mechanisms behind that response. While a wide-range
of fields may be impacted, the overall inspiration for the work herein is the development
of materials with rich and robust molecular responses to otherwise destructive forces.
To this end we focus on: (a) developing new mechanochemically reactive organic molecules
(mechanophores) that undergo constructive covalent transformations in linear polymers
under stress; (b) probing the nature of force transduction across length scales, from
bulk (macroscopic) to microscopic stress, in networks thus informing material design;
(c) constructing systems that reversibly amplify mechanochemical signals via catalysis.
</p><p> Force induced transformations of polymer bound mechanophores have the potential
to produce a rich array of stress responsive behavior. One area of interest is the
activation of non-scissile mechanophores in which latent reactivity can be unveiled.
Under the appropriate conditions, this new reactivity could lead to constructive bond
formation, and potentially a pathway to mechanochemical stress strengthening. In chapter
2, the mechanical activation of a bicyclo[3.2.0]heptane (BCH) mechanophore is demonstrated
via selective labeling of bis-enone products. BCH ring-opening, via a formal [2+2]
cycloreversion, produces large local elongation (> 4 Å) and products that are reactive
to Michael-type additions under mild conditions. Subsequent photocyclization regenerates
the initial BCH functionality, providing switchable structure and reactivity along
the polymer backbone in response to stress and visible light.</p><p> In chapter 3,
the [2+2] cycloreversion of cyclobutane mechanophores is further explored through
the development of bicyclo[4.2.0]octane (BCO) mechanophores. Using carbodiimide polyesterification,
BCO units were incorporated into high molecular weight polymers containing up to 700
mechanophores per polymer chain. Under exposure to the otherwise destructive elongational
forces of pulsed ultrasound, these mechanophores unravel by ~7 Å per monomer unit
to form unsaturated esters that react constructively via nucleophilic thiol-ene conjugate
addition to form sulfide functionalized copolymers and cross-linked polymer networks.
The wide variety of possible product stereochemistry provided an opportunity to probe
the dynamics of the mechanochemical ring opening. A series of bicyclo[4.2.0]octane
derivatives that varied in stereochemistry, substitution, and symmetry were synthesized
and activated. Product stereochemistry was analyzed by conventional NMR and chromatographic
means, which enabled inquiry into the mechanism of the mechanochemical [2+2] cycloreversion.
These results support that the ring opening is not concerted, but proceeds via a 1,4
diradical intermediate. Additionally, insight is provided into the 1,4-diradical dynamics
prior to product formation. </p><p> We next turn our attention to the molecular level
responses of polymer materials under macroscopic stress. Hydrogels and organogels
made from polymer networks are widely used in biomedical applications and soft, active
devices for which the ability to sustain large deformations is required. The strain
at which polymer networks fracture is typically improved through the addition of elements
that dissipate energy, often strong, yet reversible interactions. The result is often
tougher materials, resulting from both greater nominal strains and elastic moduli.
These materials require extra work to achieve a desired level of deformation, however,
there is little evidence that large amounts of energy dissipation is required to achieve
greater nominal strains. In chapter 4, we show that the addition of mechanically "invisible"
supramolecular crosslinks causes substantial increases in the ultimate gel properties
without incurring the added energetic costs of dissipation. We then incorporated a
chemiluminescent stress-sensor, the bis(adamantyl)dioxetane covalent cross-linker,
first developed in the Sijbesma group, which emits light in the event of covalent
bond scission. In these experiments we demonstrate that the occurrence of macroscopic
failure (from stress-strain curves) coincides with the molecular level failure of
the underlying covalent network.</p><p> Finally, in chapter 5, we turn our attention
to the development of mechanocatalytic systems. By activating or otherwise altering
the activity of a catalyst using force, a single mechanochemical event may be amplified
(i.e. by catalyst turnover). Such systems have been previously reported in the form
of force-activated polymer bound transition metal complexes. Beyond these on/off systems,
we imagine that force may be used to tune catalyst selectivity, via the perturbation
of ligand geometry. Here, we report a catalyst that couples a photoswitch to the biaryl
backbone of a chiral bis(phosphine) ligand, thus allowing photochemical manipulation
of ligand geometry without significantly altering the electronic structure. The changes
in catalyst activity and selectivity upon switching can be attributed to intramolecular
mechanical forces, laying the foundation for a new class of catalysts whose selectivity
can be varied smoothly and in situ over a useful range by controlling molecular stress
experienced by the catalyst during turnover. Forces on the order of 100 pN are generated,
leading to measureable changes in the enantioselectivities of asymmetric Heck arylations
and Trost allylic alkylations.</p>
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