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

Abstract

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.

Effect of intramolecular crosslinker properties on the mechanochemical fragmentation of covalently folded polymers

“…The results showed that addition of intramolecular CLs to linear polymer chains increases the polymer's resistance to fragmentation. Similarly to our previous studies, all the linear precursors ( L1 – L4 , which are all similar to each other), presented significant faster fragmentation rates compared to all SCPNs derived from them (see Tables ). That is true even when comparing the linear polymers to the least mechanochemically stable SCPNs, such as those folded by weak bonds like sulfide‐containing CLs.…”

“…Importantly, the measured rates correlate well to the DFT prediction. CO bond is the strongest chemical bond and SCPN CL2‐2 is indeed undergoing slower mechanochemical fragmentation, given now the force induces a CC bond scission which may occur in the main chain instead of the crosslinker . Polymers folded by CLs with nothing but CC bonds undergo statistically faster fragmentation compared to those with CO CL ( CL2‐1 and CL2‐2 , respectively).…”

Macromolecular Superstructures: A Future Beyond Single Chain Nanoparticles

“…The largest hurdle in the scale‐up of SCNP synthesis is currently the highly dilute concentrations of the intramolecular crosslinking, which prevent the development of strategies to produce SCNPs beyond the gram scale. As the desired intrachain reaction competes with the parasitic interchain reaction, concentrations <1 mg mL −1 are the current standard to achieve a selectivity for the intramolecular folding . Some examples require even lower concentrations of 0.02 mg mL −1 , 0.03 mg mL −1[17] or 0.04 mg mL −1 .…”

“…In addition to light, mechanical fields are a trigger that enables high temporal control through immediate responses of the material. The group of Diesendruck recently reported a fascinating example of mechanically triggered SCNP unfolding (Figure ) . Based on the transition metal‐π interaction induced folding developed in the laboratory of Lemcoff, poly(butadienes) containing additional mechanophores where folded upon addition of rhodium(I)‐ethylene complexes.…”

“…SCNPs were obtained by treating linear polybutadiene (PB) with a rhodium(I)‐ethylene complex in high dilution, where the ethylene ligands are substituted by the double bonds in the polymer backbone. An advantage to using PB is that it allows the incorporation of gem‐dichlorocyclopropane (gem‐DCC) mechanophores to the backbone B) Nonchain scissile mechanochemical events of folded polymer during sonication . Reprinted with permission from ( J.…”