Elemental sulfur enables a thione-mediated, non-radical polycondensation toward poly(benzodifurandione)

In the world of flexible electronics, scientists have long sought a material that can carry electricity as efficiently as copper but with the flexibility of plastic. While materials that carry positive charges have been perfected for decades, their counterparts that carry negative charges have remained stubbornly difficult to manufacture. These negative-charge carriers are essential for building balanced, high-performance circuits, yet creating them has often required toxic chemicals, extreme conditions, or purification steps that take weeks and generate massive amounts of waste. This bottleneck has kept the most promising negative-charge materials locked away in small laboratory samples, unable to reach the scale needed for real-world devices like wearable sensors or printed circuits.

A team of researchers has now found a way to break this deadlock using a substance as common and harmless as the sulfur found in a match head. By harnessing elemental sulfur, they have developed a new method to create a specific type of high-performance plastic called poly(benzodifurandione), or PBFDO, which is known for its ability to conduct electricity exceptionally well. The breakthrough lies in a chemical process that avoids the need for weeks of tedious cleaning or dangerous additives. Instead, the researchers mixed the plastic's building blocks with sulfur in a single pot, heated the mixture, and filtered the result to obtain a ready-to-use conductive ink. This approach not only produces a material with record-breaking conductivity but also does so in a way that is safe, scalable, and free from the toxic byproducts that have plagued previous methods.

The journey to this solution began with a recognition that existing ways to make this plastic were stuck in a difficult trade-off. The methods that produced the highest quality, most conductive material required the product to be washed in a solvent called dimethyl sulfoxide for weeks to remove impurities, a process that is slow and expensive. Other methods were faster but relied on toxic metals or corrosive chemicals that made the process unsafe for large-scale production. The researchers realized that to move forward, they needed a catalyst that was abundant, non-toxic, and capable of driving the reaction without leaving behind harmful residues. They turned their attention to elemental sulfur, a material that is plentiful and generally considered benign.

In their experiments, the team mixed the plastic's monomer units with sulfur in a solvent and heated the mixture. They discovered that the sulfur did not just sit there; it actively participated in a chemical dance that linked the building blocks together into long chains. Crucially, this process did not involve the chaotic, high-energy collisions typical of radical reactions, which are often hard to control. Instead, the sulfur opened up its ring-shaped structure to form a specific intermediate that allowed the building blocks to join together in an orderly fashion. This non-radical pathway was key to creating a clean, high-quality polymer without the defects that usually lower electrical performance.

As the chains grew, the sulfur performed a second, equally important job. The reaction naturally produced small amounts of hydrogen sulfide and dimethyl sulfide, gases that acted as internal boosters for the material's electrical properties. These gases spontaneously doped the plastic, a process that fills the material with extra electrons and allows it to conduct electricity much more efficiently. Because these dopants were generated right inside the reaction mixture, the final product emerged already primed for high performance, eliminating the need for separate, complex doping steps.

The researchers tested this new method under various conditions to find the perfect balance. They found that using just a small amount of sulfur was enough to drive the entire reaction, as the sulfur could be recycled within the chemical cycle. When they optimized the temperature and concentration, they achieved a material with a conductivity of 2,059 Siemens per centimeter, a figure that matches the best results ever recorded for this type of plastic. Perhaps most impressively, the crude mixture required only a simple filtration to remove large clumps, bypassing the weeks of dialysis that previous methods demanded. The result was a dark, conductive ink that could be used immediately.

To prove that this method could work on a larger scale, the team scaled up the process from a small test tube to a 500-milliliter batch, a volume suitable for industrial production. Even at this larger size, the material retained excellent conductivity, averaging over 1,300 Siemens per centimeter. The ink remained stable in the air for two months, losing very little of its performance, which is a critical requirement for manufacturing. The team then used this ink to build working electronic devices, including thermoelectric generators that convert heat into electricity and organic electrochemical transistors that can sense biological signals. These devices performed at the highest levels reported for this class of materials, demonstrating that the new synthesis method does not just create a theoretical curiosity but a practical, high-performance material.

The implications of this work extend beyond just making a better plastic. By replacing toxic catalysts and eliminating the need for energy-intensive purification, the researchers have opened a path for the mass production of high-performance negative-charge electronics. The use of elemental sulfur, a material that is cheap and safe, suggests that the future of flexible electronics could be built on a foundation that is both environmentally friendly and industrially viable. This simple yet powerful adjustment to the manufacturing process resolves a long-standing bottleneck, bringing the promise of advanced, balanced electronic circuits closer to reality.