By tethering two reactive diazirine groups together, chemists have created a compound that cross-links essentially any organic polymer, potentially improving properties like mechanical strength and corrosion resistance. The new cross-linking molecule also functions as a universal adhesive for polymers, making it possible to stick virtually any polymer to itself or a different material.
Cross-linking polymers simply means adding bonds across polymer strands, explains Jeremy E. Wulff, a chemistry professor at the University of Victoria who led the work. Silicone bakeware, certain types of joint replacements, and many modern plumbing lines are made from cross-linked polymers.
“There are plenty of methods available for cross-linking polymers, but each of them is custom designed for the particular polymer,” Wulff says. “What we tried to do was to come up with a way that we could cross-link pretty much any polymer using the same set of conditions.”
To do so, Wulff’s team took a cue from chemical biologists, who have been using diazirines—three-membered rings with one carbon and two double-bonded nitrogens—for decades to link small molecules to proteins. When exposed to light or heat, diazirines break down into carbenes, which promiscuously insert into C–H, N–H, and O–H bonds. By creating a molecule with a diazirine at each end, Wulff and coworkers reasoned, they could create a general cross-linker (Science 2019, DOI: 10.1126/science.aay6230).
Using just a small amount of the bisdiazirine, the chemists were able to cross-link several types of polymers, including polypropylene, which resists cross-linking. “We see this cross-linker as a magic pixie dust, where we can take an existing polymer material and just sprinkle in a little bit of the cross-linker and change the material’s properties,” Wulff says.
The cross-linker also works as a universal adhesive for polymers, allowing the chemists to glue pieces of notoriously slippery materials like polyethylene together or to other plastics or metals.
Karen L. Wooley, a polymer expert at Texas A&M University, says the cross-linker could add value to polyolefins, which can be tough to recycle because of their chemical inertness. “The reported bisdiazirine-generated carbene chemistry could be useful for cross-linking-based upcycling of polyolefin properties while also serving as a strategy for further chemical modification for polyolefins and other polymer materials as well,” she says.