Molecular Ball Joints: Mechanochemical Perturbation of Bullvalene Hardy-Cope Rearrangements in Polymer Networks

The solution-state fluxional behavior of bullvalene, first investigated over 60 years ago, has fascinated physical organic and supramolecular chemists alike. Little effort, however, has been put into investigating bullvalene applications in the bulk, partially due to difficulties in characterizing such dynamic systems. To address this fundamental knowledge gap, herein we probe whether bullvalene Hardy-Cope rearrangements can be mechanically perturbed in bulk polymer networks. We first demonstrate the impact of bullvalene fluxionality in bulk thermoset elastomers using modulated differential scanning calorimetry; enhanced enthalpic relaxation events are observed in the non-reversing heat flow for bullvalene-containing materials relative to “static” control networks. Then, we use dynamic mechanical analysis to demonstrate that the activation barrier to glass transition is significantly elevated for bullvalene-containing materials (ca. 90 kcal/mol) relative to “static” control networks (ca. 50 kcal/mol). Furthermore, bullvalene rearrangements can be “mechanically activated” at low temperature in the glassy region; such behavior facilitates energy dissipation (at least ca. 3-fold increase in hysteresis energy) and polymer chain alignment to stiffen the material (at least ca. 2-fold increase in Young’s modulus) under load. Collectively, this work showcases bullvalene as a reversible chemical mechanophore in the modulation of viscoelastic behaviors.