Does Selectivity of Molecular Catalysts Change with Time? Polymerization Imaged by Single‐Molecule Spectroscopy

The chemoselectivity of molecular catalysts underpins much of modern synthetic organic chemistry. However, little is known about the selectivity of individual catalysts because this single‐catalyst‐level behavior is hidden by the bulk catalytic behavior. Here, for the first time, the selectivity of individual molecular catalysts for two different reactions is imaged in real time at the single‐catalyst level. This imaging is achieved through fluorescence microscopy paired with spectral probes that produce a snapshot of the instantaneous chemoselectivity of a single catalyst for either a single‐chain‐elongation or a single‐chain‐termination event during ruthenium‐catalyzed polymerization. Superresolution imaging of multiple selectivity events, each at a different single‐molecular ruthenium catalyst, indicates that catalyst selectivity may be unexpectedly spatially and time‐variable. Little is known about the selectivity behavior of individual molecular catalysts, because this information is typically hidden by their behavior in bulk. Superresolution fluorescence microscopy with single‐turnover counting characterizes the instantaneous chemoselectivity of different molecular catalysts in a polymerization reaction and shows it may be variable in time and space.