Reviewed by Lexie CornerMar 4 2025
According to a study published in Science, polymer scientists at ETH Zurich discovered an unexpected method for nearly completely breaking down PMMA plastic, also known as Plexiglas, into its monomer building blocks. The process is not affected by the presence of additives.
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Currently, plastic recycling is mostly limited to the collection of sorted PET or polyethylene beverage bottles, where the plastic has a uniform chemical composition and similar polymer molecule lengths. The additives used to enhance qualities like color, softness, and solar resistance are also similar across these materials. This uniformity makes it easier to recycle and reform the plastic into new bottles.
In contrast, mixed plastics—those with varying types, qualities, and additives—are typically incinerated to generate heat for cement factories, as recycling them is much more challenging.
A team of scientists led by Athina Anastasaki from ETH Zurich's Laboratory of Polymeric Materials has developed a method that addresses this challenge. Their technique allows for the nearly complete breakdown of Plexiglas (PMMA) into its monomer building blocks. By adding additives, these building blocks can be purified through distillation into virgin-grade starting materials, which can then be used to produce new Plexiglas polymers. Importantly, this process is not affected by the presence of additives.
The implications are significant: Plexiglas (PMMA) is a durable and lightweight acrylic material with a global annual production of about 3.9 million tons. It is increasingly used in the aerospace, automotive, and screen manufacturing industries.
The method developed by the ETH researchers is highly robust, working with polymer chains containing up to 10,000 monomer units. It also remains effective even when using multicolored Plexiglas from the DIY market, yielding 94 to 98 % purity despite the presence of additives like copolymers, plasticizers, and dyes.
Surprisingly Simple Process
Our process is extremely simple. All we need is a chlorine-based solvent and to heat the dissolved recycling mixture to a temperature of between 90 and 150 °C to start the depolymerization reaction with the aid of UV or visible light.
Athina Anastasaki, Professor, Laboratory of Polymeric Materials, ETH Zurich
The ETH professor was surprised at how simple the procedure was. Plexiglas polymers, like many other common plastics such as polyethylene or polypropylene, consist of a polymer chain of carbon atoms with various side groups that branch off depending on the type of plastic.
Until now, these homogeneous carbon chains have posed a significant challenge for targeted monomer separation because they lack specific points for splitting.
Currently, pyrolysis is the only technology used in industry to completely break down homogeneous carbon chains. This process involves the thermal breakdown of carbon chains at around 400 °C. However, pyrolysis reactions are non-specific, producing a variety of cleavage products. Additionally, the high energy requirements for pyrolysis and the costs associated with purifying the resulting mixture significantly reduce its economic efficiency.
For many years, different research groups have focused on modifying polymers by adding easily detachable molecular groups to the ends of polymer chains, causing the polymer to break apart from the chain's end. This approach has allowed researchers to achieve yields of more than 90 %.
However, these modified polymers have notable drawbacks. They must be integrated into existing plastic production processes, and the reactive end groups severely limit the heat stability of the polymers, reducing their potential applications. Furthermore, many commonly used plastic additives interfere with the reaction, limiting the effectiveness of depolymerization, even for long polymer chains found in commercial plastics.
The Solvent Determines the Reaction
As is common in chemistry, the method was discovered by coincidence.
“We were actually looking for specific catalysts that would promote the targeted breakdown into monomers. But a control experiment led to the surprising revelation that the catalyst was not even necessary,” Anastasaki added.
The chlorinated solvent used to dissolve the crushed Plexiglas sample was effective in nearly completely breaking down the polymer when exposed to ultraviolet (UV) light.
Upon closer examination of the splitting reaction, the researchers discovered an unexpected mechanism. They found that the key particle involved in the process was a chlorine radical, which detaches from the chlorinated solvent when exposed to UV light.
It was surprising that high-wavelength light could break the chlorine's bond with the solvent molecule. This happens due to a relatively obscure photochemical process in which only a small fraction of the solvent molecules absorb high-wavelength ultraviolet light.
Anastasaki collaborated with experts from various ETH research groups to investigate the mechanism behind the splitting reaction. Tae-Lim Choi from the Laboratory of Polymer Chemistry calculated the theoretical electronic states of the molecules, while Gunnar Jeschke from the Institute of Molecular Physical Science used electron paramagnetic resonance measurements to experimentally confirm the theoretical predictions.
The Chlorine Must Go
In the future, the ETH researcher hopes to eliminate the chlorinated solvent from her recycling method.
“Chlorinated chemical compounds harm the environment. Our next goal is, therefore, to modify the reactions to enable them to work without the chlorinated solvent,” Anastasaki stated.
It is unclear when or how the ETH technique will be applied in practice. However, Anastasaki and her team have opened the door for new recycling methods that could target the breakdown of carbon chains in plastics that were previously difficult to break down chemically.
Journal Reference:
Wang, H. S. et. al. (2025) Visible light-triggered depolymerization of commercial polymethacrylates. Science. doi.org/10.1126/science.adr1637