Plastic broken down into molecular building blocks

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The chemical industry has a long tradition of producing polymers. It involves turning small molecular building blocks into long chains of molecules that bind together. Polymers are the basis of all kinds of common plastics, such as PET and polyurethane.

However, while polymer formation is well established and well studied, scientists have paid little attention to how polymer chains are broken down (a process called depolymerization) to recover their individual building blocks – monomers. One of the reasons for this is that the breakdown of polymers is a complex process. Whether a polymer can be broken down into its constituent parts depends on the different polymer manufacturing processes that have been used. Another reason is that the depolymerization processes used so far require a lot of energy, which makes them economically unviable. Added to this is the fact that recycled polymers are generally only used in the manufacture of low-value products.

The goal is to break down the polymers

Athina Anastasaki, professor of polymer materials at ETH Zurich, wants to change that. It has set itself the goal of producing polymers that can be easily broken down into their constituent parts so that they can be fully recycled.

The materials scientist was able to take an important first step in this direction: a study by her group has just been published in the journal Journal of the American Chemical Society. In it, Anastasaki and his colleagues show that they can break down certain polymers into their basic building blocks – monomers – and recycle them for use in materials for other applications.

The decomposed polymers are polymethacrylates (e.g. Plexi Glass) that have been produced using a specific polymerization technique called reversible addition-​fragmentation-​chain transfer polymerization – otherwise known as RAFT. This relatively new method, which is now also attracting industrial interest, produces polymer chains of uniform length.

First success

ETH Zurich researchers were able to recover up to 92% of the building blocks of polymethacrylates without adding a catalyst that would enable or accelerate the reaction. “Our method could possibly be developed even further to involve the use of a catalyst. This could increase the amount recovered even more,” says Anastasaki.

The chemical group present at the end of a polymer chain is crucial for the degradation of the polymer. By heating the polymer solvent mixture to 120°C, the researchers created so-called “radicals” at the end of the polymethacrylate chain, which triggered depolymerization. Researchers from the Australian National University in Canberra were able to confirm the results mathematically.

Produce the same product or a different product

According to Anastasaki, the building blocks recovered this way can be used to produce the same polymer or a completely different product – an insoluble hydrogel that can also be broken down into its monomers. The newly created products are similar in quality to the original ones. This contrasts with previous products made from recycled polymers.

But there’s a catch: “Products made with RAFT polymerization are more expensive than conventional polymers,” says Anastasaki. To overcome this drawback, she and her group are already working to extend the method to large-scale applications, which will make it more competitive and the resulting products cheaper. The researchers also aim to increase the amount recovered and to recover all of the building blocks of a polymer.

The materials scientist is also investigating whether other polymers can be depolymerized. She is particularly interested in polystyrene, a widespread and inexpensive plastic that is used in many areas of daily life (Styrofoam).

The method will not solve the problem of plastics in the short term

Even though this new method holds out hope of solving humanity’s plastic waste problem, Anastasaki dismisses the idea for now. There is no quick fix to the problem. She goes on to say. “It will take a lot of time and research before the process is established in the chemical industry.” Nor will it get rid of plastic waste: today’s polymers cannot be broken down in this way. Some suitable new polymers must come into circulation before their building blocks can be recovered. But the method has an advantage: no new chemical plant is needed for its introduction and use.

“We are only at the beginning of our research on depolymerization. There are over 30,000 studies on the development of new polymerization strategies, of which only a handful address the topic of monomer recovery,” says Anastasaki.

Reference: Wang HS, Truong NP, Pei Z, Coote ML, Anastasaki A. Reversal of RAFT polymerization: Quasi-quantitative monomer generation via a catalyst-free depolymerization approach. J Am Chem Soc. 2022;144(10):4678-4684. do I: 10.1021/jacs.2c00963

This article was republished from the following materials. Note: Material may have been edited for length and content. For more information, please contact the quoted source.

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