From Science & Research The polymer of squares Iron-catalyzed [2+2] oligomerization of butadiene produces (1,n’-divinyl)oligocyclobutane, a new polymer that can be chemically recycled. Figure by Jon Darmon As the planet’s burden of rubber and plastic rises unabated, scientists look to the promise of closedloop recycling to reduce trash. Researchers in the Chirik Lab, Department of Chemistry at Princeton University (Princeton, NJ, USA), have discovered a potentially game-changing new molecule, with vast implications for fulfiling that promise through depolymerization. It was found in a material called polybutadiene, which has been known for over 100 years and is used to make rubber and plastic products. Butadiene, its monomer, is an abundant organic compound and a major byproduct of fossil fuel development. The Chirik lab explores sustainable chemistry by investigating the use of iron – another abundant natural material – as a catalyst to synthesize new molecules. In this particular research, the iron catalyst clicks the monomers together to make oligocyclobutane. Normally, enchainment occurs with an S-shaped structure that is often described as looking like spaghetti. The lab reports in Nature Chemistry (Jan 2021) that during polymerization the molecule, more specifically named (1,n’-divinyl)oligocyclobutane enchains in a repeating sequence of squares, a previously unrealized microstructure that enables the process to go backwards, or depolymerize, under certain conditions. In other words, it can be zipped up to make a new polymer; that polymer can then be unzipped back to a pristine monomer to be used again. To bring about depolymerization, oligocyclobutane is exposed to a vacuum in the presence of the iron catalyst, which reverses the process and recovers the monomer. The lab identifies this as a rare example of closed-loop chemical recycling. The chemical industry uses a small number of building blocks to make most commodity plastic and rubber. Three such examples are ethylene, propylene, and butadiene. A major challenge of recycling these materials is that they often need to be compounded with additives to make plastics and rubbers. These additives all have to be separated again in the recycling process. But the chemical steps involved in that separation and the input of energy required to bring this about make recycling prohibitively expensive in many cases, particularly for common consumer products. Plastic is cheap, lightweight, and convenient, but it was not designed with disposal in mind. Chemists liken the process of producing a product from a raw material to rolling a boulder up a hill, with the peak of the hill as the transition state. From that state, you roll the boulder down the other side and end up with a product. But with most plastics, the energy and cost to roll that boulder back up the hill to recover its raw monomer are staggering, and thus unrealistic. So, still too many plastic or rubber products end up in incineration or landfills. The Chirik research demonstrates this butadiene polymer as a possible alternative, as it’s almost energetically equal to the monomer, which makes it a candidate for closed-loop chemical recycling. In the past, depolymerization has been accomplished with expensive, specialized polymers and only after a multitude of steps, but never from a raw material as common as this one. “The interesting thing about this reaction of hooking one unit of butadiene onto the next is that the destination is only very slightly lower in energy than the starting material”, said C. Rose Kennedy Assistant Professor of Chemistry at the University of Rochester, (and former postdoc in the Chirik lab). “That’s what makes it possible to go back in the other direction”. In the next stage of research, Paul Chirik, the Edwards S. Sanford Professor of Chemistry, said his lab will focus on the enchainment, which at this point chemists have only achieved on average up to 17 units. At that 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 that”, said Chirik. “We’re limited by its own strength. I would like to see a higher molecular weight”. The material also has intriguing properties as characterized by Megan Mohadjer Beromi, a postdoctoral fellow in the Chirik lab, together with chemists at ExxonMobil’s polymer research centre. For instance, it is telechelic, meaning the chain is functionalized on both ends. This property could enable it to be used as a building block in its own right, serving as a bridge between other molecules in a polymeric chain. In addition, it is thermally stable, meaning it can be heated to above 250 °C without rapid decomposition. 18 bioplastics MAGAZINE [02/21] Vol. 16
Finally, it exhibits high crystallinity, even at a low molecular weight of 1,000 g / mol. This could indicate that desirable physical properties – like crystallinity and material strength – can be achieved at lower weights than generally assumed. The polyethylene used in the average plastic shopping 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 really tough materials out of this monomer”, said Chirik. “The energy between polymer and monomer can be close, and you can go back and forth, but that doesn’t mean the polymer has to be weak. The polymer itself is strong. “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. That’s not optimized. But it’s there. The chemistry’s clean”. The research is still at an early stage and the material’s performance attributes have yet to be thoroughly explored. But under Chirik, the lab has provided a conceptual precedent for a chemical transformation not generally thought practical for certain commodity materials. Still, researchers are excited about the prospects for oligocyclobutane, and many investigations are planned in this continuing collaboration towards chemically recyclable materials. “The current set of materials that we have nowadays doesn’t allow us to have adequate solutions to all the problems we’re trying to solve”, said Alex Carpenter, a collaborator on the research and a former staff chemist with ExxonMobil Chemical. “The belief is that, if you do good science and you publish in peer-reviewed journals and you work with worldclass scientists like Paul, then that’s going to enable our company to solve important problems in a constructive way. “This is about understanding really cool chemistry”, he added, “and trying to do something good with it”. AT Read the full paper here: https://www.nature.com/articles/s41557-020- 00614-w Based on an article by Wendy Plump. https://chemistry.princeton.edu/news/chirik-lab-discovers-transformativeroute-chemically-recyclable-plastics From Science & Research The only conference dealing exclusively with cellulose fibres – Solutions instead of pollution Cellulose fibres are bio-based and biodegradable, even in marine-environments, where their degrading does not cause any microplastic. 300 participants and 30 exhibitors are expected in Cologne to discuss the following topics: CELLULOSE FIBRE INNOVATION OF THE YEAR 2023 I N N O V AT B Y N O V A - I N S T I T U T E I O N A W A R D • Strategies, Policy Framework of Textiles and Market Trends • New Opportunities for Cellulose Fibres in Replacing Plastics • Sustainability and Environmental Impacts • Circular Economy and Recyclability of Fibres • Alternative Feedstocks and Supply Chains • New Technologies for Pulps, Fibres and Yarns • New Technologies and Applications beyond Textiles Call for Innovation Apply for the “Cellulose Fibre Innovation of the Year 2023” Organiser Contact Dr Asta Partanen Program asta.partanen@nova-institut.de Dominik Vogt Conference Manager dominik.vogt@nova-institut.de cellulose-fibres.eu bioplastics MAGAZINE [02/21] Vol. 16 19
