A team of ETH researchers headed by Athina Anastasaki were successful in breaking down plastic into its molecular building blocks and recovering over 90% of them. This is the first step towards genuine plastic recycling.
Like the pearls of a pearl necklace, molecular building blocks of polymer chains can be recovered and fully reused. Image Credit: Adobe Stock.
Polymer production has a long history in the chemical industry. This entails combining small molecular building blocks to form long chains of molecules. All types of everyday plastics, such as polyurethane and PET, are made up of polymers.
While the formation of polymers is well understood and researched, little attention has been paid to how polymer chains are broken down (a process known as depolymerization) to retrieve their individual building blocks — monomers. A major reason is that disintegrating polymers is a complex process. The type of polymer manufacturing process used determines whether a polymer can be broken down into its constituent parts.
Another reason is that current depolymerization processes are energy-intensive, making them economically unviable. This is compounded by the fact that recycled polymers are typically only used in the production of low-value goods.
Breaking Down Polymers is the Goal
Athina Anastasaki, Professor of Polymeric Materials at ETH Zurich, wants to change that. She has set a goal for herself to create polymers that can be effortlessly broken down into their building blocks and completely recycled.
A first significant step in this direction has been made by the materials scientist. Anastasaki and her co-workers demonstrated that certain polymers can be broken down into their basic building blocks — monomers — and recycled for use in materials for further applications. The study was published in the Journal of the American Chemical Society.
The broken-down polymers were polymethacrylates (e.g., Plexi Glass) that were developed by a specific polymerization technique known as reversible addition-fragmentation chain-transfer polymerization, also called RAFT. Polymer chains of uniform length are produced using this relatively new technique, which is now attracting the industry’s attention.
First Success
Without using a catalyst to enable or accelerate the reaction, ETH Zurich researchers were able to recover up to 92% of the building blocks of polymethacrylates.
Our method could conceivably be developed even further to involve the use of a catalyst. This could increase the amount recovered even more.
Athina Anastasaki, Professor, Polymeric Materials, ETH Zurich
The chemical group at the end of a polymer chain is critical for its breakdown. The researchers created “radicals” at the end of a polymethacrylate chain by heating the polymer solvent mixture to 120 °C, which induced the depolymerization. The results were mathematically confirmed by scientists at the Australian National University in Canberra.
Producing the Same or a Different Product
The building blocks retrieved this way, according to Anastasaki, can be used to make the same polymer or an entirely different product — an insoluble hydrogel that can be broken down into its monomers. The quality of the newly created products is comparable to that of the earlier ones. This is in contrast to earlier recycled polymer-based products.
However, there is indeed a catch.
Products made with RAFT polymerization are more expensive than conventional polymers.
Athina Anastasaki, Professor, Polymeric Materials, ETH Zurich
To address this shortcoming, she and her colleagues are already working to expand the technique for large-scale applications, making it more competitive and the resulting products less expensive. The researchers also want to increase the amount obtained and recover all of a polymer’s building blocks.
Other polymers that can be depolymerized are also being investigated by the materials scientist. Anastasaki is specifically interested in polystyrene, a common, low-cost plastic used in different areas of day-to-day life (Styrofoam).
Method Will Not Resolve the Plastics Problem Short Term
Even if this new approach offers hope for solving humanity’s plastic waste problem, Anastasaki brushes aside the idea for the time being. The problem does not have a quick fix. Anastasaki states, “It will take a lot of time and research before the process is established in the chemical industry.”
The method will not get rid of plastic waste either, as today’s polymers cannot be broken down in this manner. Before their building blocks can be recovered, some new, suitable polymers must be introduced into circulation. However, the method has one advantage — it does not necessitate the establishment of new chemical plants for its implementation and use.
We are only at the beginning of our research into depolymerization. There are over 30,000 studies on developing new polymerization strategies, with only a handful of them addressing the subject of monomer recovery.
Athina Anastasaki, Professor, Polymeric Materials, ETH Zurich
Journal Reference:
Wang, H. S., et al. (2022) Reversing RAFT Polymerization: Near-Quantitative Monomer Generation Via a Catalyst-Free Depolymerization Approach. Journal of the American Chemical Society. doi.org/10.1021/jacs.2c00963.
Source: https://ethz.ch/en.html