Polymer mechanochemistry: the design and study of mechanophores

Abstract

The burgeoning field of polymer mechanochemistry has garnered significant interest in recent years. Mechanochemical transformations are those that are promoted by exogenous forces, and polymer mechanochemistry concentrates on the use of polymer chains to translate mechanical forces to chemical systems. Acoustic fields, particularly ultrasound, have proven to be highly efficient progenitors of tensile stresses within macromolecules and are frequently used to facilitate mechanochemical phenomena. Mechanochemical activation often arises when mechanophores, or functional groups that respond to mechanical perturbation in a controlled manner, are appropriately positioned within a polymer chain to experience tensile forces. A variety of interesting transformations have been realized when well‐designed mechanophores have been properly outfitted with polymer chains, including: thermally inaccessible isomerizations and cycloreversions, symmetry‐forbidden electrocyclic ring openings and activation of latent catalysts. Herein, the chemistry of known mechanophores is summarized and augmented with implications for new opportunities in synthesis and materials science. The focus of this mini‐review is limited to mechanophores that have been specifically adapted for polymer mechanochemistry under acoustic activation. Copyright © 2012 Society of Chemical Industry

Stress-augmented thermal activation: Tribology feels the force

“…Bell's 1978 paper is generally taken as the seminal starting point, although occasionally earlier work by Zhurkov and Eyring is cited. Mechanochemistry is now being applied to a wide range of areas of chemistry and biochemistry, including solvent free synthesis routes [108,111], molecular motors [112,113], single molecule studies, for example using molecular tweezers or AFMs [114,115], cell differentiation [116,117], and mechanophores [118,119]. The latter are molecular groups designed specifically to respond to mechanical force, to enable this force to be measured externally (sensors) or to produce materials that respond in desired ways to applied forces (smart materials).…”

Perspective: Mechanochemistry of biological and synthetic molecules

“…Using single-molecule force spectroscopy (SMFS), the Craig group characterized the same process at a molecular level by measuring the dependence of the isomerization rate k(F) on the applied force. 15 Spiropyran is just one example of many existing mechanophores [16][17][18][19][20][21] (i.e., mechanosensitive molecules that show interesting chemistry when subjected to stress or strain). The coupling between its mechanical and optical properties makes spiropyran an example of a molecular force probe; that is, it can be used to quantify how macroscopic stress (produced, e.g., by a hammer) translates into local forces within molecular chains.…”

Interplay Between Mechanochemistry and Sonochemistry

“…Mechanical energy can be harnessed in multiple chemical transformations, such as mechanochromism, analyte detection and self-healing. The basic strategy relies upon the concept of the mechanophore; small structural units embedded into long chain polymers which undergo site-selective scission [6][7][8]12]. Although numerous solid-state and solution methods can be used to activate polymers [11], sonication holds the unique ability of providing shear forces and strain in solution by virtue of the mechanical events associated with cavitational collapse.…”

“…This chapter aims to describe the mechanical bias of cavitational effects and how they are related to conventional mechanochemistry and force-induced physical fields in general. This subject has been well documented over the last decade [5][6][7][8][9], and particular attention has been paid to scenarios such as mechanically responsive polymers [10][11][12], micro-and nano-structured materials [13][14][15], and sonocrystallization [16], which are all expected to have a major impact on the fabrication of novel and smart materials as well as the pharmaceutical industry. Although we shall briefly mention these past achievements, our goal is to focus on the aspects of sono-mechanochemistry that have been overlooked in recent thematic issues.…”