Researchers from Princeton University’s Department of Chemistry have discovered a potentially game-changing new molecule with vast implications for fulfilling that promise.

A team of scientists led by Paul Chirik report that this molecule connects in a very unusual way: as a repeating sequence of squares, which allows the process to go backwards under certain conditions. In other words, the molecule can be “zipped up” to make a new polymer for use in plastic, and then unzipped — depolymerized — back to its pristine state, ready to be used again.

Molecule Enabling Use of Plastic Many Times Over
The molecule — formally named oligocyclobutane could one day enable the use of plastic products many times over, unlike most of plastics that can only be recycled once, if at all.

The research is still at an early stage, and the material’s performance attributes have yet to be thoroughly explored. But the Chirik lab has provided a conceptual precedent for a chemical transformation not generally thought practical for certain materials.

In the past, turning a material like this back into its pristine state has been accomplished with expensive, specialized polymers and only after a multitude of steps — and never with a raw material as common as this one. The molecule is a form of polybutadiene, which has been known for over a hundred years and is used to make rubber and plastic products like tires and shoe soles. Butadiene, its parent material, is an abundant organic compound and a major byproduct of fossil fuel development.

The molecule can be “zipped up” to make a polymer of squares for use in plastics, and then unzipped — depolymerized — back to its pristine state, ready to be used again.

“To take a really common chemical that people have been studying and polymerizing for many decades and make a fundamentally new material out of it — let alone have that material have interesting innate properties — not only is that unexpected, it’s really a big step forward,” said collaborator Alex Carpenter, a former staff chemist with ExxonMobil Chemical.

Using Iron to Persuade Monomer Shape
The Chirik lab explores sustainable chemistry by investigating the use of iron to build new molecules. In this particular research, the iron “persuades” the monomers to click together to make the polymer of squares. Normally, monomers connect to form a polymer — which is simply a repeating unit of monomers — through an S-shaped structure often described as looking like spaghetti. But with this new molecule, the monomers zip together in a chain of squares.

To unzip them, the molecule is exposed to a vacuum in the presence of the iron catalyst, which reverses the process and recovers the original material, making this a rare example of closed-loop chemical recycling.

Chirik added, “You can make really tough materials out of this monomer. What people tend to assume is that when you have a chemically recyclable polymer, it has to be somehow inherently weak or not durable. We’ve made something that’s really, really tough but is also chemically recyclable. We can get pure monomer back out of it. And that surprised me.”

Megan Mohadjer Beromi, a postdoctoral research fellow said the material has other intriguing properties that will form the basis of future research on chemically recyclable materials.

» Publication Date: 05/02/2021

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