Bicyclo[3.2.0]heptane mechanophores for the non-scissile and photochemically reversible generation of reactive bis-enones.

Force-induced transformations of polymer-bound functionalities have the potential to produce a rich array of stress-responsive behavior. One area of particular interest is the activation of non-scissile mechanophores in which latent reactivity can be unveiled that, under the appropriate conditions, could lead to constructive bond formation in materials exposed to typically destructive stress. Here, the mechanical activation of a bicyclo[3.2.0]heptane (BCH) mechanophore is demonstrated via selective labeling of bis-enone products. BCH ring-opening produces large local elongation (>4 Å) and products that are reactive to conjugate 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.

Tension trapping of carbonyl ylides facilitated by a change in polymer backbone.

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 (cis to trans 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.

Microstructure of Copolymers Formed by the Reagentless, Mechanochemical Remodeling of Homopolymers via Pulsed Ultrasound.

The high shear forces generated during the pulsed ultrasound of dilute polymer solutions lead to large tensile forces that are focused near the center of the polymer chain, but quantitative experimental evidence regarding the force distribution is rare. Here, pulsed ultrasound of quantitatively geminal-dihalocyclopropanated (gDHC) polybutadiene provides insights into the distribution. Pulsed ultrasound leads to the mechanochemical ring-opening of the gDHC mechanophore to a 2,3-dihaloalkene. The alkene product is then degraded through ozonolysis to leave behind only those stretches of the polymer that have not experienced large enough forces to be activated. Microstructural and molecular weight analysis reveals that the activated and unactivated regions of the polymer are continuous, indicating a smooth and monotonic force distribution from the midchain peak toward the polymer ends. When coupled to chain scission, the net process constitutes the rapid, specific, and reagentless conversion of a single homopolymer into block copolymers. Despite their compositional polydispersity, the sonicated polymers assemble into ordered lamellar phases that are characterized by small-angle X-ray scattering.