Cutting Down On The Ruthenium In ROMP

In the polymer-making reaction known as living ring-opening metathesis polymerization, or ROMP, a transition-metal complex knits ring-strained olefins together into a chain. The reaction offers chemists control over the molecular weight of the resulting polymers, but it’s expensive because one molecule of the metal complex is needed for each polymer chain. What’s more, ruthenium is the most commonly used metal for ROMP, and all traces must be laboriously removed if the polymer is going to be used in biomedical applications. Seeking a way to minimize ruthenium, Andreas F. M. Kilbinger and Amit A. Nagarkar of the University of Fribourg, in Switzerland, report that by adding a chain-transfer agent to the polymerization process they are able to use just 2% of the amount of a ruthenium complex typically used for ROMP (Nat. Chem. 2015, DOI: 10.1038/nchem.2320). In the new polymerization, a polymer chain is temporarily activated by the complex to react with monomer and then deactivated by the chain-transfer agent. The chemists found that a substituted cyclohexene (one shown) serves well for that purpose. The ruthenium then gets passed along to another chain. The process repeats over and over, giving the chemists precise control over the polymer’s molecular weight—they can decide when to permanently stop the reaction. “This new technique provides access to well-defined polymers for industrial, biomedical, and academic use at a fraction of the current costs and significantly reduced levels of residual ruthenium catalyst,” the researchers note.