Polyacetylene comes full circle

Many of today’s flexible electronic devices can trace their ancestry back to the first synthesis of linear polyacetylene in 1958. Now, researchers have used a highly active catalyst to create cyclic polyacetylene, providing a rapid way to make films of the semiconducting polymer (Nat. Chem. 2021, DOI: 10.1038/s41557-021-00713-2).

The material could be a useful addition to the polymer electronics toolbox, and researchers are keen to understand how its physical and electronic properties compare to those of its linear sibling. “I think people will be studying this material, just like they have linear polyacetylene, for a long time,” says Adam S. Veige of the University of Florida, who led the work with his colleague Brent S. Sumerlin.

Cyclic polyacetylene is the largest annulene isolated to date, forming a circle of over 200 C–H units connected by alternating double and single bonds.

The researchers made the polymer in several different ways, all of them catalyzed by an unusual tungsten complex. For example, simply wafting acetylene over a vial coated with a solution of the catalyst created a flexible, shiny film of the cyclic polymer in a few seconds. “You turn the nozzle on the acetylene and boom, it’s made,” Veige says. This could be a fast, simple way to lay down high-purity polyacetylene coatings on flexible substrates. Another approach produced cyclic polyacetylene in a soluble form that could be chemically modified.

“Now that people know they can make it, it should open the door to look at its properties in more detail,” says Christine K. Luscombe of the University of Washington, who works with semiconducting polymers.

The Florida team first made the circular compound in 2016, and spent the intervening time confirming that it was indeed cyclic, rather than linear, polyacetylene. One telling feature is that the double bonds in cyclic polyacetylene are almost entirely trans, the thermodynamically preferred conformation in larger annulenes. In contrast, linear polyacetylene generally contains cis double bonds.

Seeing is believing, though. So the researchers modified cyclic polyacetylene with polystyrene molecules to form circular ‘bottlebrush’ structures that were large enough to visualize using atomic force microscopy. “When my student Zhihui Miao was able to get that image, it was really the final piece of the puzzle that gave absolute proof,” Veige says.

The researchers found that doping the polymer with iodine makes it slightly more conductive than germanium, raising hopes that it could be used in electronic devices. “There’s a lot of work to be done in terms of understanding the morphology, and how it can be processed and adapted, but I think there’s a lot of potential there,” says Helen Tran of the University of Toronto, who develops polymeric materials for biocompatible electronics.