Mechanochemistry

At the intersection of mechanics and chemistry, mechanochemistry is a subject that embraces many everyday phenomena including wear and abrasion, friction and lubrication, and stress-accelerated degradation of materials.

Through collaborative research at the University of Illinois Beckman Institute, we are testing the "mechanophore hypothesis" which states that force drives chemical change in selective and productive ways. The goal is to invent materials that have new functionality, such as the ability to repair themselves when damaged. Such materials promise to be safer and last longer than conventional materials.

Our concept of a mechanophore is a stress or strain activated molecular unit that is inserted into a polymeric material to provide a molecular-scale reading of the local mechanical state or to transform materials properties in response to the local mechanical environment. Molecular designs and research activities are founded on fundamental principles borrowed from polymer science and physical organic chemistry. Building on the mechanophore concept, research encompasses a variety of goals and challenges. Most of our research in mechanochemistry falls into one of four categories: Sustainable materials, Mechanically triggered chemistry, Understanding mechanophore activation and Shock wave energy dissipation (Fig 1.)

• Alkyne Metathesis Dynamic Covalent Chemistry (DCC)

In recent years the utility of dynamic covalent chemistry (DCC) has been well demonstrated for the preparation of shape persistent nanostructures including arylene ethynelene macrocycles and cages. We have recently prepared kinetically trapped tetrahedral cages via alkyne metathesis DCC. From these results, we have been inspired to further study reaction pathways in dynamic alkyne metathesis. While the field of DCC largely has focused on diverting reactivity to thermodynamically favored products, our results demonstrate that kinetic parameters can play a crucial role in reaction selectivity. We seek to study alkyne metathesis DCC pathways in order to understand the key intermediates involved in these processes and leverage this mechanistic understanding to prepare more complex macromolecules in an iterative fashion. Current studies are also being directed to the incorporation of orthogonal DCC chemistries with alkyne metathesis in order to prepare desymmetrized cage structures and to further expand the scope of alkyne metathesis DCC for the synthesis of complex macromolecular architectures.

• Mechanically-triggered productive chemistry

Mechanical stress can be used as an external stimulus to trigger downstream, productive chemistry. Mechanical stress has been recognized as one of the most basic and universal signal inputs for cell differentiation and growth. Cellular matrix contains complex chemical network to sense and response to external stress. The group is interested in design and synthesis of new mechanophores that transduce mechanical input into chemical stimuli: e.g., mechanically-triggered depolymerization of metastable polymers to enhance recyclability of materials, generation of protons under force, and recently generation of light. These adaptive mechanophore containing systems have promising potential applications in progenerative medicine.

• Fundamental understanding of mechanophore activation

We are developing a systematic understanding of how mechanophores activate. We strive to probe how mechanical force acts differently from other energy input in activating chemical reactions. Parameters of interest include chemospecificity, regiospecificity, stereospecificity, and force application methods. By varying the backbone chemical designs, tethering positions of polymers, relative stereochemical arrangement of polymer linkers, and ways in which force is applied, we anticipate a sophisticated design roadmap as guideline for novel mechanophores and mechanochemistry.

• Autonomous Control of Materials Life-cycle

Waste reduction is key to a sustainable materials landscape, and it is achievable through life extension and recycling of polymer materials. We seek to extend the lifetimes of widely used polymer materials by incorporating mechanically triggered damage sensing and healing functionalities. When damage occurs, the mechanophores locally activate to report damage or initiate healing. Major areas of focus to achieve these goals are learning structure-property relationships for efficient force transduction in polymers, chemical synthesis of functional mechanophores, and streamlined analytical tools to study these mechanochemical reactions.

Ultimately we believe our group is well positioned to realize the next major advancement in the field of mechanochemistry as well as further our understanding of matter as it experiences mechanical energy input.