Polymer Mechanochemistry: From Destructive to Productive

Abstract

When one brings "polymeric materials" and "mechanical action" into the same conversation, the topic of this discussion might naturally focus on everyday circumstances such as failure of fibers, fatigue of composites, abrasion of coatings, etc. This intuitive viewpoint reflects the historic consensus in both academia and industry that mechanically induced chemical changes are destructive, leading to polymer degradation that limits materials lifetime on both macroscopic and molecular levels. In the 1930s, Staudinger observed mechanical degradation of polymers, and Melville later discovered that polymer chain scission caused the degradation. Inspired by these historical observations, we sought to redirect the destructive mechanical energy to a productive form that enables mechanoresponsive functions. In this Account, we provide a personal perspective on the origin, barriers, developments, and key advancements of polymer mechanochemistry. We revisit the seminal events that offered molecular-level insights into the mechanochemical behavior of polymers and influenced our thinking. We also highlight the milestones achieved by our group along with the contributions from key comrades at the frontier of this field. We present a workflow for the design, evaluation, and development of new "mechanophores", a term that has come to mean a molecular unit that chemically responds in a selective manner to a mechanical perturbation. We discuss the significance of computation in identifying pairs of points on the mechanophore that promote stretch-induced activation. Attaching polymer chains to the mechanophore at the most sensitive pair and locating the mechanophore near the center of a linear polymer are thought to maximize the efficiency of mechanical-to-chemical energy transduction. We also emphasize the importance of control experiments to validate mechanochemical transformations, both in solution and in the solid state, to differentiate "mechanical" from "thermal" activation. This Account offers our first-hand perspective of the change-in-thinking in polymer mechanochemistry from "destructive" to "productive" and looks at future advances that will stimulate this growing field.

π-extended anthracenes as sensitive probes for mechanical stress

“…A common and straight-forward approach for the implementation of this function into polymeric materials is to incorporate molecular moieties (mechanophores) in the polymer backbone that alter their physicochemical properties upon exposure to mechanical stress. 1–5 One of the most simple modes of detection is the change in optical properties, as this method is not only non-invasive and spatiotemporally well resolved but in principle also detectable with the naked eye (Fig. 1).…”

“…Moore, White, Sottos and co-workers tackled this challenge and established spiropyran as now widely employed mechanochromic probe. 1,6,7 Other mechanochromic moieties have been successfully activated in polymers, including azobenzene, 8 diaryl-bibenzofuranone 9 and the coumarin dimer. 10 Also, the reorientation of chromophores in polymer blends has been exploited to modulate materials' absorption spectra.…”

Mechanical gating of a mechanochemical reaction cascade

“…Covalent polymer mechanochemistry12345 has been extensively explored in recent years for a variety of purposes, including biasing reaction pathways6789, trapping transition states and intermediates1011, catalysis12131415, release of small molecules and protons161718, stress reporting19202122, stress strengthening222324 and soft devices2526. These efforts are largely facilitated by the fact that, unlike other energy sources, mechanical force is directional and regulated by local molecular structure.…”

Stress-induced colouration and crosslinking of polymeric materials by mechanochemical formation of triphenylimidazolyl radicals

“…[1][2][3][4][5] Commonly, this is achieved by incorporating mechanically activatable molecular moieties (mechanophores) into polymer architectures. Stress-induced bond scission, a microscopic process, eventually causes macroscopic failure of the material.…”