Suzanne Blum’s group at UC Irvine has taken a closer look at ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD) by the second generation Grubbs catalyst using a high-sensitivity and high-resolution single-particle fluorescence microscopy technique. This method was based on direct imaging of the location of the early stages of catalysis. The researchers were trying to answer a fundamental question: Does the reaction occur by heterogeneous catalysis, homogeneous catalysis or both? On macro scale it seemed that the polymerization occurred on the solid catalyst surface and the poly-DCPD encapsulated the solid catalyst. But after taking a closer look at the reaction at a single particle level, it turned out the polymerization was solely homogeneous.
The analytical technique described in the paper is capable of directly imaging the location of polymer growth to differentiate between homogeneous and heterogeneous catalysis. The system studied was ROMP of DCPD in the presence of fluorescent boron dipyrromethene (BODIPY)-tagged olefin. The olefin was expected to be incorporated during the polymerization yielding fluorescent polymers. A sample of solid Grubbs second gen was placed in the well of the microscope slide. The catalyst crystals were identified by ambient light imaging. A heptane solution of a mixture of DCPD and the BODIPY-tagged olefin was injected into the well and the polymers were detected by total internal reflection fluorescence mode. The metathesis catalyst was sparingly soluble in the system, but no fluorescent polymers were observed growing from the surface of the catalysts crystals. Instead, polymers started precipitating from the solution when they reached high enough molecular weights. The location of each polymer could be determined and it wasn’t associated with the location of the catalyst crystals, which suggests that polymer growth occurred exclusively in the solution by soluble metathesis catalyst.
In addition, it was also shown that the solid catalyst doesn’t undergo a metathesis reaction with the BODIPY-tagged olefin. When the olefin was added to crystals of the catalyst, no incorporation of the fluorescent olefin on the surface was observed. This experiment ruled out the possibility that some polymer growth occurred on the surface of the catalyst, but then the polymer dissolved prior to being tagged with the fluorescent olefin.
The method is a part of a research project in the Blum group to develop single molecule techniques to image catalytic reactions at individual transition metal centers. These single molecule techniques allow us to watch reactions live, one molecule at a time. Even though you need a fluorophore probe molecule, it is present in very low concentrations due to the sensitivity of the method and wouldn’t effect the catalytic reaction. This approach can be widely applicable and used to help understand the reactivity of transition metal catalysts better.