As the pollution of the planet from rubber and plastic waste continues unabated, scientists are increasingly relying on the promise of closed recycling to reduce waste. A team of researchers from the Princeton Department of Chemistry announces the discovery of a new polybutadiene molecule – made from a material known for over a century that is used to make common products such as tires and shoes – that could one day advance this goal through depolymerization.
The Chirik laboratory reports in Natural chemistry that during polymerization, the molecule called (1, n & # 39; -Divinyl) oligocyclobutane is chained together in a repeating sequence of squares, a previously unrealized microstructure that allows the process to go backwards or forwards under certain conditions depolymerize.
In other words, the butadiene can be "zipped" to make a new polymer; This polymer can then be unpacked back into an untouched monomer for reuse.
Research is still at an early stage and the performance characteristics of the material have yet to be thoroughly investigated. However, the Chirik Laboratory has set a conceptual precedent for a chemical conversion that is generally not considered practical for certain raw materials.
In the past, depolymerization was done with expensive niche or specialty polymers and only after a multitude of steps, but never from a raw material as common as that from which polybutadiene was made, one of the seven most important primary petrochemicals in the world. Butadiene is an abundant organic compound and a major by-product of fossil fuel development. It is used to make synthetic rubber and plastic products.
"A really common chemical that humans have studied and polymerized for many decades, taking and making a fundamentally new material from it – let alone having that material have interesting innate properties – is not only unexpected, it really is a huge step you would don't necessarily expect there to be fruit left on that tree, "said Alex E. Carpenter, a chemist at ExxonMobil Chemical, a researcher.
"The focus of this collaboration has been for us to develop new materials that will benefit society by focusing on some new molecules that Paul Chirk (Princeton chemist) discovered that are quite transformative," added Carpenter.
"Humankind is good at making butadiene. It is very nice to find other useful uses for this molecule because we have plenty of it."
Catalysis with iron
The Chirik Laboratory studies sustainable chemistry by studying the use of iron – another natural material – as a catalyst for the synthesis of new molecules. In this particular research, the iron catalyst clicks the butadiene monomers together to make oligocyclobutane. However, this happens in a highly unusual square structural motif. Usually the chaining is done with an S-shaped structure, often described as spaghetti.
To influence the depolymerization, oligocyclobutane is exposed to a vacuum in the presence of the iron catalyst, which reverses the process and recovers the monomer. In the work of the Chirik laboratory "Iron-catalyzed synthesis and chemical recycling of telechelic 1,3-linked oligocyclobutanes" this is identified as a rare example of chemical recycling in a closed cycle.
The material also has fascinating properties, as characterized by Megan Mohadjer Beromi, a postdoctoral fellow in the Chirik laboratory, together with chemists from the ExxonMobil Polymer Research Center. For example, it is telechelic, which means that the chain is functionalized at both ends. This property could make it possible to use it as a stand-alone building block that acts as a bridge between other molecules in a polymer chain. In addition, it is thermally stable, which means it can be heated to over 250 ° C without rapid decomposition.
Finally, it shows high crystallinity even at a low molecular weight of 1000 g / mol (g / mol). This could indicate that desirable physical properties – such as crystallinity and material strength – can be achieved at lower weights than commonly believed. For example, the polyethylene used in the average plastic bag has a molecular weight of 500,000 g / mol.
"One of the things that we demonstrate in this article is that you can make really tough materials from this monomer," said Chirik, Edwards S. Sanford Professor of Chemistry at Princeton. "The energy between polymer and monomer can be tight and you can go back and forth, but that doesn't mean the polymer has to be weak. The polymer itself is strong.
"People tend to assume that a chemically recyclable polymer has to be inherently weak or not durable. We made something that is really, really tough, but it's also chemically recyclable. We can get pure monomer back out." And that surprised me. That is not optimized. But it is there. The chemistry is clean.
"I honestly think this work is one of the most important things my lab will ever do," said Chirik.
Drop the ethylene
The project dates back a few years to 2017 when C. Rose Kennedy, then a postdoc in the Chirik laboratory, noticed that a viscous liquid built up at the bottom of a flask during a reaction. Kennedy said she expected something fleeting to form, and the result piqued her curiosity. As she studied the reaction, she discovered a distribution of oligomers – or low molecular weight non-volatile products – that indicated that polymerization had occurred.
"When we knew what we already knew about the mechanism, it was immediately clear how this would be possible to click them together in different or continuous ways. We immediately realized that this could potentially be extremely valuable," said Kennedy, now Assistant Professor of Chemistry at the University of Rochester.
It was at this early stage that Kennedy was captivating butadiene and ethylene. It was Mohadjer Beromi who later suspected that it would be possible to completely remove the ethylene and use only pure butadiene at elevated temperatures. Mohadjer Beromi "gave" the four-carbon butadiene to the iron catalyst, and that made the new polymer of squares.
"We knew that the motif tends to be chemically recycled," said Mohadjer Beromi. "But I think one of the new and really interesting features of the iron catalyst is that it can do (2 + 2) cycloadditions between two dienes, and that is exactly this reaction: it's a cycloaddition where you always join two olefins together form a square molecule again.
"It's the coolest thing I've ever worked on in my life."
To further characterize oligocyclobutane and understand its performance properties, the molecule had to be scaled up and studied in a larger facility with expertise in new materials.
"How do you know what you've been doing?" Asked Chirik. "We used some of the normal tools that we have here at Frick. But what really matters is the physical properties of the material and, ultimately, what the chain looks like."
For this purpose, Chirik traveled to Baytown, Texas last year to present the results of the laboratory to ExxonMobil, which decided to support the work. An integrated team of scientists from Baytown was involved in computer modeling, X-ray diffraction work to validate the structure, and additional characterization studies.
The chemical industry uses a small number of building blocks to make most of the raw materials from plastic and rubber. Three such examples are ethylene, propylene and butadiene. A major challenge in recycling these materials is that they often have to be combined and then reinforced with other additives in order to manufacture plastics and rubbers: Additives offer the desired performance properties – for example the hardness of a toothpaste cap or the lightness of a shopping bag. These "ingredients" all have to be separated again in the recycling process.
However, the chemical steps involved in this separation and the energy required to do so make recycling prohibitively expensive, especially for single-use plastics. Plastic is cheap, light, and practical, but it wasn't designed for disposal. That, said Chirik, is the main problem with snowballing.
As a possible alternative, the Chirik research shows that the butadiene polymer has almost the same energy as the monomer, which makes it a candidate for closed-loop chemical recycling.
Chemists compare the process of making a product from a raw material to rolling a rock onto a hill, with the top of the hill being the transition state. From this state, you roll the boulder over to the other side and get a product. But with most plastics, the energy and cost to roll that boulder backwards up the hill to reclaim its raw monomer is amazing and therefore unrealistic. Most plastic bags, rubber products and bumpers end up in landfills.
"The interesting thing about this reaction of adding one unit of butadiene to the next is that the 'target' is only slightly less energetic than the starting material," said Kennedy. "That makes it possible to go back in the other direction."
In the next phase of research, Chirik said his lab would focus on concatenation, which chemists only averaged up to 17 units at this point. At this chain length, the material becomes crystalline and so insoluble that it falls out of the reaction mixture.
"We have to learn what to do with it," said Chirik. "We are limited by its own strength. I would like a higher molecular weight."
Nonetheless, the researchers are excited about the prospects for oligocyclobutane, and much research is planned in this continuing collaboration towards chemically recyclable materials.
"The current materials we have today do not allow us to come up with adequate solutions to all of the problems we are trying to solve," said Carpenter. “We believe that if you do good science and publish in peer-reviewed journals, and work with world-class scientists like Paul, our company can solve important problems in a constructive manner.
"This is about understanding really cool chemistry," he added, "and trying to make something good out of it."