Unfolding and Mechanochemical Scission of Supramolecular Polymers Containing a Metal–Ligand Coordination Bond

Abstract

Mechanochemical scission of supramolecular polymer complexes by ultrasound is investigated using viscosity measurements and molecular dynamics (MD) simulations combined with constrained geometry optimization (COGEF) calculations. The supramolecular polymers used in this study consist of a poly(tetrahydrofuran) (PTHF) backbone that contains a silver(I)–NHC (NHC = N-heterocyclic carbene) coordination complex in the chain center. The limiting molecular weight (M lim) for mechanochemical chain scission is lower for supramolecular polymers than for their covalent analogues. The longest characteristic relaxation times of the supramolecular polymers were obtained from viscosity measurements, and they confirmed that the criterion for coil-to-stretch is fulfilled in typical sonication experiments. A model DFT study was performed to estimate the value of external force that is required to break a silver(I)–NHC coordination bond. A combination of ab initio MD simulations and COGEF provided an atomistic insight into the response of the supramolecular center of the polymer chain to external force. The calculations indicated that the force required to break the chain is between 400 and 500 pN. This is significantly lower than the force of several nN that is typically required to break e.g. covalent C–C bonds in polymer backbones. These results confirm that the reduction in M lim is due to the lower bond strength of the metal–ligand coordination bonds as compared to covalent bonds.

DFT Study on mechanochemical bond breaking in COGEF and Molecular Dynamics simulations

“…This value is even closer to the experiment, nevertheless our model is not fully consistent with real structure, i.e. simulations have been done in gas phase, not in the solvent; the aliphatic chains at N1 position of imidazole are shorter [12].…”

Toward a theory of mechanochemistry: Simple models from the very beginnings

“…It is well known that the incorporation of a single weaker bond within a polymer chain can alter the mechanochemical scission behavior (specifically the selectivity of the process). Examples are, for example, due to Sijbesma and coworkers which show that the limit below which a mechanochemical chain scission of a polymer is not possible, can be reduced when a covalent CC bond near the center of the polymer backbone is replaced by a weak metal‐ligand coordination bond. The effect of this metal–ligand can be well rationalized theoretically.…”

Theoretical Approaches for Understanding the Interplay Between Stress and Chemical Reactivity

“…Constrained geometries have been used extensively in the context of mechanochemistry, with applications including studies of bond rupture [82][83][84][85][86], reactivity of disulfide bonds [87], unfolding of supramolecular polymers [88], mechanochemical synthesis of phenyl cations [89], extraction of gold nanowires [90], evaluation of restoring forces in force probes [14,91], and calculation of free energy barriers [92][93][94][95][96][97][98]. Examples of selected applications are described below.…”