We have discovered a new flex-activated mechanophore that releases an N-heterocyclic carbene (NHC) under mechanical load. The mechanophore design is based upon NHC-carbodiimide (NHC-CDI) adducts and demonstrates an important first step toward flex-activated designs capable of further downstream reactivities. Since the flex-activation is non-destructive to the main polymer chains, the material can be subjected to multiple compression cycles to achieve iterative increases in the activation percentage of mechanophores. Two different NHC structures were demonstrated, signifying the potential modularity of the mechanophore design.

Mechanical forces can lead to qualitative changes in reaction mechanism, which depend on the specific mode of force induction and inherent chemistries of the mechanophore. To demonstrate these effects at an atomistic level, three challenging mechanochemical transformations of recent interest are herein studied: spiropyran ring opening, flex-activated small molecule release, and cyclopropane ring opening. These examples show that mechanical load can eliminate intermediates and transition states along a reaction path, preactivate reactions where the force is not parallel to the reaction path, and modulate force conveyance to the mechanophore in a polymerdependent fashion. Two different merocyanin products were identi-fiedcis and trans isomersfrom the spiropyran ring-opening reaction, and tuning the magnitude of the force allows for selection of one over the other. The reaction involving oxanorbornadiene where bonds perpendicular to the force are flexed through angular distortions has the interesting property that while the central bond contracts, the overall molecule is lengthened to effect activation. Cyclopropane ring openings demonstrate that the larger the extension of the mechanophore, determined by the rigidity of the polymer backbone under applied force, the larger the change in activation barrier. In this case, the application of a force represents a preactivation mechanism where the initial structure rises in energy, while the transition state stays relatively constant. For these three motifs, the reaction paths are readily uncovered using the growing string method, showing it to be a widely useful tool for studying mechanochemistry.

Stereochemistry can have ap rofound impact on polymer and materials properties.U nfortunately,s traightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve.Inadeparture from traditional metal-mediated ring-opening metathesis polymerization (ROMP), we discovered ar emarkably simple method for controlling alkene stereochemistry in photoredox mediated metal-free ROMP.I on-pairing, initiator sterics,a nd solvation effects eachhad profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB with trans alkene content of 25 to > 98 %. High cis content was obtained from relatively larger counterions,t oluene as solvent, low temperatures (À78 8 8C), and initiators with low Charton values.C onversely, smaller counterions,d ichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with high trans content. Data from ac ombined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control.