Researchers promote new path to chemically recyclable plastics

As the planet’s load of rubber and plastic rises steadily, scientists are increasingly looking at the promise of closed – loop recycling to reduce waste. A team of researchers from the Department of Chemistry in Princeton announces the discovery of a new polybutadiene molecule – from a material that has been known for more than a century and is used to make ordinary products such as tires and shoes – which will one day achieve this goal. can promote through depolymerization.

The Chirik Laboratory reported in Natural chemistry that the molecule, called (1, n’-divinyl) oligocyclobutane, anchors during polymerization in a repeating sequence of squares, a previously unrealized microstructure that enables the process to deteriorate or depolymerize under certain conditions.

In other words, the butadiene can be ‘zipped’ to make a new polymer; that polymer can then be packed back to a pristine monomer for reuse.

The research is still at an early stage and the performance characteristics of the material have yet to be thoroughly investigated. But the Chirik laboratory has set a conceptual precedent for a chemical transformation that is not usually considered practical for certain commodity materials.

In the past, depolymerization was accomplished with expensive niche or specialized polymers and only after a multitude of steps, but never from such a common raw material as polybutadiene, one of the seven leading petrochemicals in the world. Butadiene is an abundant organic compound and an important by-product of the development of fossil fuels. It is used to make synthetic rubber and plastic products.

“To take a very common chemical that people have been studying and polymerizing for decades and to make a fundamentally new material out of it – let alone to say that the material has interesting innate properties – is not only unexpected, it really is.” “You would not necessarily expect more fruit from that tree,” said Alex E. Carpenter, a chemist at ExxonMobil Chemical, a research fellow.

“The focus of this collaboration for us was on developing new materials that benefit society by focusing on some new molecules that [Princeton chemist] “Paul Chirk has discovered that it is quite transformative,” Carpenter added.

“Humans are good at making butadiene. It’s very nice if you can find other useful applications for this molecule, because we have a lot of them.”

Catalysis with iron

The Chirik Laboratory investigates sustainable chemistry by investigating the use of iron – another abundance of natural materials – as a catalyst to synthesize new molecules. In this particular research, the iron catalyst clicks the butadiene monomers to make oligocyclobutane. But it does so in a very unusual square structural motif. Normally, anchoring takes place with an S-shaped structure that is often described as a spaghetti.

To influence the polymerization, oligocyclobutane is then exposed to a vacuum in the presence of the iron catalyst, which reverses the process and recycles the monomer. The paper from the Chirik Laboratory, “Iron Catalyzed Synthesis and Chemical Recycling of Telechelic, 1,3-Enchained Oligocyclobutanes”, identifies it as a rare example of closed loop chemical recycling.

The material also has intriguing properties as characterized by Megan Mohadjer Beromi, a postdoctoral fellow in the Chirik laboratory, along with chemists at ExxonMobil’s polymer research center. It is, for example, telechelic, which means that the chain functions on both sides. This property allows it to be used as a building block in its own right, and serves as a bridge between other molecules in a polymer chain. In addition, it is thermally stable, which means it can be heated above 250 degrees C without rapid decomposition.

Finally, it exhibits high crystallinity, even with a low molecular weight of 1000 grams per mole (g / mol). This may indicate that desired physical properties – such as crystallinity and material strength – can be achieved with lower weights than are generally accepted. The polyethylene used in the average plastic bag, for example, has a molecular weight of 500,000 g / mol.

“One of the things we demonstrate in the paper is that you can make very difficult materials from this monomer,” said Chirik, a professor of chemistry at Edwards S. Sanford, Princeton. “The energy between polymer and monomer can be close, and you can go back and forth, but that does not mean that the polymer should be weak. The polymer itself is strong.

“What people tend to assume is that if you have a chemically recyclable polymer, it must somehow be naturally weak or not durable. We’ve made something that’s really, really tough, but also chemically recyclable.We can extract pure monomer again.from it.and it surprised me.It’s not optimized.But it’s there.The chemistry is clean.

“I honestly think this work is one of the most important things that ever came out of my lab,” Chirik said.

Thick the ethylene

The project extends over several years until 2017, when C. Rose Kennedy, then a postdoctoral fellow in the Chirik laboratory, noticed a viscous liquid accumulating at the bottom of a bottle during a reaction. Kennedy said she expected something volatile to form, and the result spurred her curiosity. Closer to the reaction, she discovers a proliferation of oligomers – or low molecular weight non-volatile products – indicating that polymerization has taken place.

“Because we knew what we already knew about the mechanism, it was immediately clear how it would be possible to click it together in a different or continuous way. We immediately realized that it could be something very valuable, says Kennedy, now an assistant. professor of chemistry at the University of Rochester.

At that early stage, Kennedy was improving butadiene and ethylene. It was Mohadjer Beromi who later assumed that it would be possible to remove the ethylene completely and use neat butadiene at elevated temperatures. Mohadjer Beromi ‘gave’ the four-carbon butadiene to the iron catalyst, which yielded the new polymer of squares.

“We knew the motive tended to be chemically recycled,” Mohadjer Beromi said. ‘But I think one of the new and very interesting features of the iron catalyst is that it can do [2+2] cycloadditions between two services, and this is what this reaction is essentially: it is a cycloaddition where you link two olefins together to make a square molecule each time.

“It’s the coolest thing I’ve ever worked on in my life.”

To further characterize oligocyclobutane and understand its performance characteristics, the molecule had to be scaled and studied at a larger facility with expertise in new materials.

“How do you know what you made?” Vra Chirik. “We used some normal tools here at Frick. But what really matters is the physical properties of this material and ultimately what the chain looks like from it.”

For that, Chirik traveled to Baytown, Texas last year to present the findings of the lab to ExxonMobil, which decided to support the work. An integrated team of Baytown scientists was involved in computer modeling, X-ray distribution work to validate the structure, and additional characterization studies.

Recycling 101

The chemical industry uses a small number of building blocks to make most products plastic and rubber. Three such examples are ethylene, propylene and butadiene. A major challenge in recycling these materials is that they often need to be combined and then reinforced with other additives to make plastics and rubbers: additives offer the performance properties we want – the hardness of a toothpaste. shell, for example, or the lightness of a grocery bag. These ‘ingredients’ must be separated again in the recycling process.

But the chemical steps involved in the separation and supply of energy needed to accomplish this make recycling excessively expensive, especially for disposable plastics. Plastic is cheap, lightweight and convenient, but it is not designed for disposal. That, Chirik said, is the biggest snowball problem with it.

As a possible alternative, the Chirik research shows that the butadiene polymer is almost energetically equal to the monomer, making it a candidate for closed loop chemical recycling.

Chemists compare the process of making a product from a raw material with the role of a rock on a hill, with the peak of the hill as the transition state. From the condition, you roll the rock to the other side and end up with a product. But with most plastics, the energy and cost of rolling up that boulder at the back of the hill to recycle its raw monomer is staggering and therefore unrealistic. Most plastic bags and rubber products and car buffers therefore end up in landfills.

“The interesting thing about this reaction of linking one unit of butadiene to the next is that the ‘destination’ just has a lot less energy than the source material,” Kennedy said. “That’s what makes it possible to go back in the other direction.”

In the next phase of research, Chirik said his laboratory would focus on the decoration, which at the time only chemists had achieved up to 17 units. With the chain length, the material becomes crystalline and so insoluble that it falls out of the reaction mixture.

“We need to learn what to do with it,” Chirik said. “We are limited by its own power. I would like to see a higher molecular weight.”

Yet researchers are excited about the prospects for oligocyclobutane, and many investigations are planned in this ongoing collaboration on chemically recyclable materials.

“The current set of materials we have these days does not allow us to find adequate solutions to all the problems we are trying to solve,” Carpenter said. “The belief is that doing good science and judging peer-reviewed journals and working with world-class scientists like Paul will enable our company to solve important problems in a constructive way.

“It’s about understanding really cool chemistry,” he added, “and trying to do something good with it.”