Scientists from the University of Groningen in the Netherlands and the East China University of Science and Technology (ECUST) in Shanghai have found a way to produce polymers from lipoic acid, a natural molecule. As an added benefit, these polymers can be easily depolymerized under mild conditions: some 87 per cent of the monomers can be recovered in their pure form and re-used to make new polymers of virgin quality. The researchers report their work in a paper in Matter.
A problem with recycling plastics is that it usually results in a lower-quality product. The best results are obtained by chemical recycling, in which the polymers are broken down into their component monomers, but this depolymerization is often very difficult to achieve. At the Feringa Nobel Prize Scientist Joint Research Center, a collaboration between the University of Groningen and ECUST, scientists developed a polymer that can be created and fully depolymerized under mild conditions.
"We found a way to produce polymers from the natural molecule lipoic acid in a very controlled way," explains Ben Feringa, professor of organic chemistry at the University of Groningen. "It is a beautiful molecule and a perfect building block that was created by nature."
Lipoic acid has a ring structure that includes a sulphur-sulphur bond. When this bond is broken, the sulphur atoms can react with those of another monomer. "This process was known before, but we managed to find a way to control it and to create long polymers," Feringa says.
The molecule also has a carboxyl group, which readily reacts with metal ions. These can crosslink the polymers, producing an elastic material. By dissolving the molecule in water with sodium hydroxide and then evaporating the water, a firmer polymer film can be produced through ionic bonds.
As the polymerization is achieved through reversible bonds, the material is also self-healing. "When it is cut, you can simply press the ends together and they will reconnect in a few minutes," says Feringa.
Most of the work was carried out by Qi Zhang, first as a PhD student at ECUST in Shanghai and later as a postdoctoral researcher at the University of Groningen. "Lipoic acid is a natural small molecule with an elegant structure," says Zhang. "We didn't have to do any tedious re-designing of the monomer to achieve a fully reversible polymerization."
Simply exposing the polymers to sodium hydroxide dissolves the polymers back into their monomers. "By adding a little acid, the monomers precipitate and can be recovered. The quality of these recycled monomers is identical to that of the original material."
"Our experiments show what is possible with these monomers," adds Feringa. "We can even recycle the material into monomers several times, without loss of quality."
Industrial applications of this new polymer are still a long way off. "This is a proof of principle," says Feringa. "We are conducting experiments now to create polymers with new functionalities and to better understand the polymerization and depolymerization processes."
Although 87% of the monomers can already be recovered, the scientists want to get as close to 100% as possible. "Our experiments show that we can produce, in a controlled fashion, hard and soft elastic polymers that can be fully depolymerized," Feringa concludes. "This molecule is really very promising."