Molecular Mechanochemistry: Engineering and Implications of Inherently Strained Architectures

Abstract

Mechanical activation of chemical bonds is usually achieved by applying external forces. However, nearly all molecules exhibit inherent strain of their chemical bonds and angles as a result of constraints imposed by covalent bonding and interactions with the surrounding environment. Particularly strong deformation of bonds and angles is observed in hyperbranched macromolecules caused by steric repulsion of densely grafted polymer branches. In addition to the tension amplification, macromolecular architecture allows for accurate control of strain distribution, which enables focusing of the internal mechanical tension to specific chemical bonds and angles. As such, chemically identical bonds in self-strained macromolecules become physically distinct because the difference in bond tension leads to the corresponding difference in the electronic structure and chemical reactivity of individual bonds within the same macromolecule. In this review, we outline different approaches to the design of strained macromolecules along with physical principles of tension management, including generation, amplification, and focusing of mechanical tension at specific chemical bonds.

Instability of Surface-Grafted Weak Polyacid Brushes on Flat Substrates

“…10,11 Examples include polyelectrolytes in water 12−16 and branched macromolecules, 13,17 as well as polyacrylamide in methanol with a strong base. 18 A proposed mechanism 12 for the instability of polyelectrolyte brushes in water, illustrated in Figure 1a, suggests that strong swelling of the brush due to electrostatic charging in the brush generates increased tension along the grafted chain backbone.…”

Mechanical gating of a mechanochemical reaction cascade

“…Both experimental672427303132333435363738 and theoretical2939404142434445464748 studies have been conducted to understand force-coupled reactivity and mechanisms, and these investigations provide valuable insights into the design of mechanophore response. One concept for a useful reactivity response is mechanochemical gating, in which one mechanophore (a molecular gate) initially prevents another mechanophore (substrate) from experiencing force delivered along a polymer backbone.…”

Visualization of Mechanochemically-Assisted Degrafting of Surface-Tethered Poly(Acrylic Acid) Brushes

“…Recent reports pointed out that the stability of polymer brushes may get compromised under certain conditions (e.g., pH and ionic strength), resulting in degrafting and removal of polymer chains from the substrates. 4,5 Examples include degrafting of poly(poly-(ethylene glycol) methacrylate) (PPEGMA) brushes in cell culture media, 6,7 surface-tethered hyperbranched polyglycidol during polymerization of glycidol, 8 polyacrylamide brushes in methanol with a strong base, 9 and polyelectrolyte (i.e., charged polymer) brushes in buffer solutions. 10−19 The instability occurs more often in polyelectrolyte brushes than neutral polymer brushes.…”