A new recycling method breaks mixed plastics into their building blocks and selectively rebuilds each one, turning multilayer packaging and complex waste into usable materials again.
(Nanowerk Spotlight) At the edge of most recycling facilities, there is a pile no one wants to deal with. It is full of food wrappers with multiple polymer layers, textiles mixed from different fibers, and household packaging that defies separation. These are not the easy items that can be chopped and melted into pellets. They are the misfits: products made from materials chosen for performance rather than recyclability.
Removing each polymer by force is expensive and often impossible. The result is predictable. These objects are sent to incinerators, landfills, or discarded into the environment. The chemistry that created them becomes a barrier to their second life.
A group of researchers at ETH Zurich have asked a different question. What if the problem is not the plastics themselves, but the assumption that they must be recycled one at a time? Their work, published in Advanced Functional Materials (“Chemoselective Sequential Polymerization: An Approach Toward Mixed Plastic Waste Recycling”), presents an approach called chemoselective sequential polymerization. It borrows an idea from biology. Human cells take in proteins as mixtures, break them into amino acids, and rebuild new proteins with precise structures. Nature does not separate its feedstock. It uses selective reactions to sort it at the molecular level.
The researchers designed a similar strategy for synthetic polymers. They focused on three materials that can be broken back into the ring-shaped molecules they start from. Poly(lactic acid) is the first, widely used in packaging and biomedical products. A polycarbonate known as poly(2,2-dimethyltrimethylene carbonate) is the second. A polyester named poly(pentadecalactone) is the third. All three can undergo a process called ring closing depolymerization. When heated in the presence of a catalyst, their long chains fold back into the cyclic monomers from which they were made.
Schematic illustration of chemoselective
sequential polymerization strategy and its application in chemical recycling of a multi-material object. (Image: Reproduced from DOI:10.1002/adfm.202518059, CC BY)
In the study, the team mixed these plastics and depolymerized them at once. Instead of trying to unwind each chain in isolation, they treated them as one feedstock. Under heat and vacuum, the monomers evaporated and were collected together. They were not separated by hand or filtration. They were simply distilled. The yields were high, with about 90 % lactide, about 82 % carbonate monomer, and about 89 % pentadecalactone
recovered.
The chemistry held even when pieces of polypropylene, polyethylene, and Nylon-6 were present. These common packaging polymers stayed intact and did not interfere. In a world where waste streams are rarely pure, tolerance to contamination matters as much as efficiency.
The next step is where selectivity enters. Imagine three different keys floating in a pool of water. Only one will fit the door you are trying to unlock, and that door only opens at a specific temperature. The team relied on that kind of difference. Some monomers open their rings and connect into chains because doing so releases stored energy. They are like springs snapping into a relaxed position.
These reactions can happen at low temperature. Others polymerize only when warmed. Higher heat triggers them because the resulting long chains increase the randomness of the system, which is favored at elevated temperature.
Using this logic, the researchers rebuilt the polymers from the mixed monomer pool in sequence. The first target was poly(lactic acid). At room temperature and in the presence of a specific catalyst, lactide assembled rapidly. The other monomers remained inactive. The polymer was removed through precipitation, leaving the rest behind. The recovered material reached high molecular weight and was nearly pure, with only small traces of the carbonate monomer.
Next, the carbonate monomer was activated with its own catalyst. It too responded at room temperature, forming long chains. Pentadecalactone did not react. The resulting polycarbonate was comparable to one prepared from a single monomer feed. Small remnants of the lactide monomer were detected but did not meaningfully alter the polymer.
The final material required a different approach. Pentadecalactone assembles only when heated. At 80 °C, it became the only monomer that favored chain growth. The reaction progressed until the mixture gelled, a sign of long polymer chains. After precipitation, the third polymer was isolated. It showed a modest degree of impurity from the carbonate monomer, a consequence of being last in the sequence, but retained the essential properties of the control polymer.
To see whether the method holds outside the flask, the researchers created a three-layer film. One polymer formed the outer layer. The second supported mechanical strength. The third sat in the middle. It is the kind of structure found in food packaging that resists recycling because cutting through layers destroys the very traits that make the material useful.
The team subjected the film to the same depolymerization conditions. It came apart into its monomers and was rebuilt through the same stepwise process. The regenerated polymers displayed similar molecular weights and mechanical traits to those formed from new monomers.
This chemistry is not limited to the three polymers in the study. The researchers tested additional recyclable monomers and saw no disruption of selectivity. The monomer used to make polycaprolactone did not interfere with lactide or carbonate polymerization. Nylon-6 monomer, which can be recovered by depolymerizing fabrics or films, remained inert under the conditions used for the first two rebuilding steps and did not hinder pentadecalactone polymerization.
They also tried a monomer used to form polymers by ring opening metathesis, a fundamentally different mechanism. It assembled cleanly without triggering unwanted reactions. These results suggest that the strategy may extend to broader polymer systems if catalysts and conditions are chosen carefully.
There are limitations. The lactide monomer partly converts into a different stereochemical form during depolymerization. That reduces crystallinity when it is repolymerized, which affects the stiffness of the final material. Impurities accumulate for the last polymer in the sequence, lowering its thermal stability. Selectivity is strong, not perfect.
The authors note that adjusting catalysts and the composition of the starting mixture could reduce carryover effects. They also propose that a continuous system, in which each polymer precipitates as soon as it forms, could improve purity while lowering solvent demand.
Chemoselective sequential polymerization reframes mixed plastics not as trash but as a shared molecular reservoir. It transforms waste into its original ingredients and rebuilds each polymer without sorting. That design principle aligns closer to biological recycling than to industrial processing.
By taking advantage of how molecules prefer to react, rather than forcing them to behave, the work opens a route to reuse that does not punish complexity or composite materials. If optimized and scaled, it could allow multi-material products to return to manufacturing without dismantling them first.
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