In the pharmaceutical and chemical industries, toxic and carcinogenic organic solvents used in molecular synthesis account for over 80% of waste produced. Mechanochemistry, where reactions are driven via mechanical energy primarily in powder form, has the potential to significantly reduce the environmental impact of industrial scale synthesis by reducing or eliminating toxic solvent use. Although several organic reactions have been conducted mechanochemically via ball milling, the driving force for these reactions is poorly understood due to the large variance in stresses, temperature, and particle size in the reactor. The stress-assisted thermal activation model proposes that applied mechanical stress can reduce activation energies, causing reactions to become thermodynamically favorable at lower temperatures. Here, we investigate shear-driven Diels-Alder cycloaddition using a surface-bound reaction in a ball-on-disc tribometer to elucidate the role of stress-assisted thermal activation in mechanochemical reactions. We modify the surface of a glass disc with anthracene and dissolve maleimide in a mixture of polyisobutylene and alkylated naphthalene, which is then used in the tribometer where shear and normal stresses are precisely measured. We conduct a preliminary reaction in full elastohydrodynamic lubrication at 22.7 MPa of shear stress based on Hertzian contact theory. The substrate is characterized with atomic force microscopy, surface-enhanced Raman spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS shows that elastohydrodynamic lubrication prevents anthracene removal from the substrate–which is an issue for product characterization–but does not provide evidence of product formation at the selected shear stress, temperature, and reaction time. This work develops reaction conditions and product characterization methods to be used in future study of shear-driven Diels-Alder cycloaddition using a ball-on-disc tribometer, which will ultimately isolate the effects of normal and shear stress on organic reaction kinetics.

Using shear stress to drive Diels-Alder cycloaddition

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