Breaking down plastic into its constituent parts

Date:
March 24, 2022
Source:
ETH Zurich
Summary:
A team of ETH researchers led by Athina Anastasaki have succeeded
in breaking down plastic into its molecular building blocks and in
recovering over 90 percent of them. A first step towards genuine
plastic recycling.



FULL STORY ==========================================================================
The chemical industry has a long tradition of producing polymers. This
involves turning small molecular building blocks into long chains of
molecules that bond together. Polymers are the basis of all kinds of
everyday plastics, such as PET and polyurethane.


========================================================================== However, while the formation of polymers is well established and well researched, scientists have given little attention to how polymer chains
are broken down (a process called depolymerisation) to recover their
individual building blocks -- monomers. One reason for this is that
breaking down polymers is a complex process. Whether a polymer can be
broken back down at all into its constituent parts depends on which of
the different polymer manufacturing processes were used. Another reason
is that the depolymerisation processes used to date require a lot of
energy, which has made them economically unviable.

Added to this is the fact that recycled polymers are usually only used
in the manufacture of low-value products.

Breaking down polymers is the goal Athina Anastasaki, Professor of
Polymeric Materials at ETH Zurich, wants to change this. She has set
herself the goal of producing polymers that can be easily broken down
into their building blocks so that they can be fully recycled.

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

The polymers broken down are polymethacrylates (e.g. Plexi Glass) that
were produced using a specific polymerisation technique called reversible addition- fragmentation chain-transfer polymerisation -- otherwise known
as RAFT. This relatively new method, which is now also attracting the
interest of industry, produces polymer chains of uniform length.



========================================================================== First success The researchers at ETH Zurich have succeeded in recovering
up to 92 percent of the building blocks of polymethacrylates without
adding a catalyst that would enable or accelerate the reaction. "Our
method could conceivably 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 polymer's breakdown. By heating the polymer solvent
mixture to 120DEGC, the researchers created what are referred to as
"radicals" at the end of a polymethacrylate chain, which triggered the depolymerisation. Researchers at the Australian National University in
Canberra were able to confirm the results mathematically.

Producing the same or a different product According to Anastasaki, the
building blocks recovered in 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 of similar quality to the original ones. This is in contrast to
previous products made from recycled polymers.



==========================================================================
But there is a catch: "Products made with RAFT polymerisation are more expensive than conventional polymers," says Anastasaki. To address this drawback, she and her group are already working on expanding the method
for large-scale applications, which will make it more competitive and
the resulting products cheaper. The researchers also aim to increase
the amount retrieved and recover all the building blocks of a polymer.

The materials scientist is also researching whether other polymers
can be depolymerised. She is particularly interested in polystyrene, a widespread, low-cost plastic that is used in many areas of everyday life (Styrofoam).

Method will not resolve the plastics problem in the short term Even if
this new method raises hopes of solving humankind's plastic waste problem, Anastasaki dismisses the idea for the time being. 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 new, suitable polymers have to come into circulation
before their building blocks can be recovered. But the method has one advantage: no new chemical plants are needed for its introduction and use.

"We are only at the beginning of our research into depolymerisation. There
are over 30,000 studies on developing new polymerisation strategies,
with only a handful of them addressing the subject of monomer recovery,"
says Anastasaki.


========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
Peter Rueegg. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Hyun Suk Wang, Nghia P. Truong, Zhipeng Pei, Michelle L. Coote, and
Athina Anastasaki. Reversing RAFT Polymerization: Near-Quantitative
Monomer Generation Via a Catalyst-Free Depolymerization
Approach. Journal of the American Chemical Society, 2022 DOI:
10.1021/jacs.2c00963 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220324104451.htm

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