Expanding the Scope of Mechanically Active Polymers
The addition of mechanically active functional groups (mechanophores) to polymer scaffolds has resulted in new chemical transformations and materials properties. The novel functions in these polymers are achieved in response to a universal input: mechanical force. This dissertation describes studies that expand our ability to elicit and modify chemical reactivity through the application of force (mechanochemistry), both through fundamental studies of mechanochemical coupling and through the synthesis and characterization of new mechanophores.
In order to probe mechanochemical coupling, single molecule force spectroscopy was used to directly quantify and compare the forces associated with the ring opening of <italic>gem<italic>-dibromo and <italic>gem<italic>-dichlorocyclopropanes (<italic>g<italic>DBCs and <italic>g<italic>DCCs) affixed along the backbone of <italic>cis<italic>-polynorbornene (PNB) and <italic>cis<italic>-polybutadiene (PB). At a tip velocity of 0.3 μ sec<super>-1<super>, the isomerization of <italic>g<italic>DBC-PNB, <italic>g<italic>DCC-PNB, <italic>g<italic>DBC-PB, and <italic>g<italic>DCC-PB to their respective 2,3-dihaloalkenes occurs at 740, 900, 1210 and 1330 pN, respectively. In contrast to their relative importance in determining the rates of the thermal <italic>g<italic>DHC ring openings, the polymer backbone has much greater impact on <italic>g<italic>DHC mechanochemistry than does the halogen. The root of the effect lies in more efficient chemomechanical coupling through the PNB backbone, which acts as a phenomenological lever with greater mechanical advantage than the PB backbone. The ability to affect the reactivity of a mechanophore by polymer backbone manipulation provides a previously underappreciated means to tailor mechanochemical response. The experimental results are supported computationally and provide the foundation for a new strategy by which to engineer mechanical reactivity.
The ability to increase the reactivity of mechanophores by changing their polymer scaffold can lead to the realization of mechanically-induced transformations that were otherwise inaccessible. To probe this increased mechanophore reactivity, epoxidized polybutadiene and epoxidized polynorbornene were subjected to pulsed ultrasound in the presence of small molecules capable of being trapped by carbonyl ylides. When epoxidized polybutadiene was sonicated, there was no observable small molecule addition to the polymer. Concurrently, no appreciable isomerization (<italic>cis<italic> to <italic>trans<italic> epoxide) was observed, indicating that the epoxide rings along the backbone are not mechanically active under the experimental conditions employed. In contrast, when epoxidized polynorbornene was subjected to the same conditions, both addition of ylide trapping reagents and net isomerization of cis to trans epoxide were observed. The results demonstrate the mechanical activity of epoxides, show that mechanophore activity is determined not only by the functional group but also the polymer backbone in which it is embedded, and facilitate a characterization of the reactivity of the ring opened dialkyl epoxide.
Commercially available fluorinated polymers were also investigated as previously unrealized mechanophore-bearing polymers and as candidates for thermally re-mendable materials by examining their response to applied stress. Perfluorocyclobutane (PFCB) polymer solutions were subjected to pulsed ultrasound, leading to mechanically induced chain scission and molecular weight degradation. <super>19<super>F NMR revealed that the new, mechanically generated end groups are trifluorovinyl ethers formed by cycloreversion of the PFCB groups- a process that differs from thermal degradation pathways. One consequence of the mechanochemical process is that the trifluorovinyl ether end groups can be re-mended simply by subjecting the polymer solution to the original polymerization conditions, i.e., heating to >150 °C. Stereochemical changes in the PFCBs, in combination with radical trapping experiments, indicate that PFCB scission proceeds via a stepwise mechanism with a 1,4-diradical intermediate, offering a potential mechanism for localized functionalization and cross-linking in regions of high stress.
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