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 (Mlim) 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 Mlim is due to the lower bond strength of the metal–ligand coordination bonds as compared to covalent bonds.