Written by Renée Haran
Topochemical polymerization (TCP) represents a powerful method for creating crystalline polymers directly from solid-state monomers without expensive solvents or catalysts. This unique approach opens new possibilities for polymer synthesis that would be difficult or impossible to achieve through conventional liquid-phase methods with the added benefit of minimizing waste, reducing energy consumption, and providing pathways for polymers that leverage bio-based resources.
However, TCP’s solid-state nature presents significant challenges. Achieving precise polymerization states—where crystals transform while maintaining their structure and chemical composition—can be tricky. The delicate crystals can easily disintegrate when treated with heat, light, or pressure, which makes them difficult to study. As a result, the mechanisms by which monomers are polymerized during TCP remain largely hidden.
In a series of breakthroughs published in Nature Communications and the Journal of the American Chemical Society, a team of researchers led by Yi Liu from the Foundry’s organic and macromolecular synthesis facility has overcome these obstacles. They successfully used X-rays to simultaneously transform and characterize single-crystal monomers in real-time, without degradation.
This breakthrough could significantly reduce manufacturing costs for many products. By eliminating the need for expensive solvents and catalysts, and by allowing precise control over the building process, companies could produce better materials more cheaply. Potential applications include advanced electronics, medical devices, and novel materials with unique mechanical and optical properties.
“Using X-rays, we were able to reveal how small (AQM) molecules of the same kind can transform neatly into a polymer. We got precise information of every atom in the polymer crystal, which is usually very hard to do,” said Liu.
In both studies, the researchers focused on a special type of monomers called azaquinodimethane (AQM).
The researchers observed that X-rays penetrate AQM monomers more deeply compared to UV-visible light (commonly used in TCP) resulting in a more uniform reaction throughout the crystal. They posited that high-energy photons initiate polymerization through processes other than the direct excitation of valence electrons seen with white light.
“This process is like illuminating a human with X-rays versus with light. When X-rays pass through a person’s body, you see their bones, but if white light shines on them, you will see something else, but it’s very complementary,” said Carolin Sutter-Fella, whose lab partnered with Liu on studying how fast the monomers polymerize.
An unexpected result of the researcher’s nuanced process was the discovery of a rare, temporary crystalline state that can usually not be seen.
“Often, people are interested in making a material for a device application, for example, to get a super high-performing battery,” said Carolin Sutter-Fella. “They basically make a material that is sandwiched between other functional layers, and then they test the device to see if the material works or not, then they go back to change something in the way the material is made. This is a somewhat indirect way to characterize the material.”
Instead of focusing on the desired outcome and working backwards, Sutter-Fella wants to know what’s happening under the hood to make characterization and development of materials more direct. Her lab builds tools that do just that.
“I want to lift this black box,” she said.
Together, Liu and Sutter-Fella’s teams, including their respective postdocs, Chongqing Yang and Maged Abdelsamie, explored how TCP transformations occur in powders, thin films – and, in a radical move – in a liquid medium, creating polymer nanofibers with high crystallinity, similar to those formed in the solid state.
Sutter-Fella’s lab provided a custom, Foundry-developed spin coater that attached directly to Berkeley Lab’s Advanced Light Source (ALS) to create thin AQM films that could be characterized while being blasted with X-rays.
In addition to these findings, and for the first time, the Foundry researchers also developed a strategy for controlling TCP pathways by modifying electron density in the AQM system’s aromatic end groups.
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By substituting phenyl groups (a benzene ring with one hydrogen atom removed) with five-membered aromatic rings containing one oxygen atom, the researchers were able to delocalize spin density across AQM molecules when activated with heat.
Their experiments showed that heating the molecules in their solid form created polymers with new types of linkages. The researchers confirmed their findings using X-ray crystallography and theoretical modeling.
“In the second paper, we moved the electron “hotspot” to the sticky (aromatic) end (of the molecule) instead of its usual place,” said Liu. “The reaction now follows two pathways: the original pathway and one where the hotspot moves all the way to the end of the new aromatic group.”
This body of innovative work uncovers the complex mechanisms of TCP and paves the way for cost-effective polymer manufacturing. By utilizing the power of X-rays, researchers can achieve more precise polymer transformations, including versatile aromatic systems, and open new avenues for developing advanced polymers without using expensive solvents and catalysts.
