Chemists from the USA, South Korea, Hungary and Switzerland have developed a heterogeneous ruthenium-based catalyst for the synthesis of cyclic polymers from cyclopentene. They also managed to come up with a reaction installation that makes it easy to separate the catalyst from the reaction products. The study was published in Nature Chemistry.
Linear polymers containing double bonds in their structure are most often used for the manufacture of lubricants. But these materials wear out quickly because frequent use breaks the carbon-carbon bonds in the polymer chains, causing the material to become less viscous. In contrast, cyclic polymers become more viscous over time because the first bond cleavage in a cyclic polymer molecule results in ring opening. In this case, a linear polymer is formed, and the length of the carbon chain does not change.
One of the main methods for the synthesis of linear polyolefins is ring-opening metathesis (ROMP). For this reaction, chemists use Grubbs catalysts—ruthenium carbene complexes with various ligands. But recently, scientists were able to develop a similar catalyst for the synthesis of cyclic polymers using the reaction of ring expansion metathesis (REMP). It is a ruthenium carbene complex with a cyclic fragment combining a ruthenium atom and two carbene ligands.
Grubbs catalysts are in solution during the reaction, and the polymer product often contains impurities of ruthenium complexes. In addition, ruthenium itself is an expensive metal, costing about 20 thousand US dollars per kilogram, and after the polymerization reaction the catalyst cannot be regenerated. Therefore, chemists led by Robert Grubbs from the California Institute of Technology decided to develop a catalyst that, after the reaction, could be isolated in its pure form and used again.
To do this, the researchers prepared two ruthenium catalysts on a silica gel substrate—fine silicon oxide SiO2. Both substances turned out to be stable in air, and the ruthenium content in them was determined by chemists using mass spectrometry: it was 4 and 2.8 micromoles per gram of substance.
To test the new catalysts, the scientists polymerized cyclopentene. The reaction was successful, but in the NMR spectrum of the product, chemists observed peaks characteristic of linear polymers. They assumed that this was due to impurities of linear alkenes in the original cyclopentene, and decided to further purify it. To do this, chemists used a selective hydroboration reaction, which occurs only with linear alkenes. The resulting mixture of cyclopentene and organoboron compounds was separated by simple distillation, and the polymerization reaction with purified cyclopentene led to a pure polymer product.
One problem remained – to isolate the catalyst back from the reaction mixture, it was necessary to use dehydrated solvents and an inert atmosphere, because during the reaction it passed into a more active unstable form. To simplify this process, chemists assembled a reaction unit in which the catalyst was loaded into a separate vessel, and cyclopentene flowed through it. The product was collected in a separate flask, and once the reaction was complete, the vessel containing the catalyst could simply be separated and stored in an inert atmosphere.
As a result, chemists have developed a method for producing pure cyclic polymers with the ability to regenerate the expensive catalyst. The yield of polymerization reactions in most experiments was about 70 percent, and the molar mass of the product was from 10,000 to 100,000 daltons, depending on the reaction conditions.
We recently talked about how chemists managed to obtain a biocompatible and elastic polymer that can withstand 500 stretching cycles. Scientists also managed to use this material to monitor the muscle activity of animals.