Mechanical Unfolding and Thermal Refolding of Single-Chain Nanoparticles Using Ligand-Metal Bonds

Covalent macromolecules tend to fragment under mechanical stress through the mechanochemical scission of covalent bonds in the backbone. However, linear polymers that have been intramolecularly collapsed by covalent bonds show greater mechanochemical stability compared to other thermoplastics. Here, rhodium-π bonds are used for intramolecular collapse in order to show that mechanical stress can be removed from the polymer backbone and focused on weaker intramolecular cross-links, leading to polymer unfolding instead of mechanochemical events at the backbone. Moreover, given rhodium-π bonds form spontaneously, by changing the time interval between ultrasound pulses, we demonstrate that entropic spring effects can lead to polymer refolding and reformation of the previously cleaved metal-ligand bonds, effectively repairing the intramolecular noncovalent cross-links. These findings provide the first example of an intramolecular repairing mechanism in synthetic molecules in solution, allowing for restoration of chemical bonds after mechanochemical events.