Page 51 - Plastics News February 2021

teChnoLogy

Degradable, bio-based polymers Molecule from nature provides

offer options for chemical recycling fully recyclable polymers

egradable, cargo-bearing polymers are important cientists from the University of Groningen (The
Dfor medical and sensor applications, says the study SNetherlands) and the East China University of
published in the journal Angewandte Chemie. Scientists Science and Technology (ECUST) in Shanghai produced
have developed a class of sugar-based polymers that are different polymers from lipoic acid, a natural molecule.
degradable through acid hydrolysis. The researchers also These polymers are easily depolymerized under mild
integrated "cargo" molecules in the polymer, which are conditions. Some 87 per cent of the monomers can be
designed to split off after polymer degradation. recovered in their pure form and re-used to make new
polymers of virgin quality. The process is described
Tae-Lim Choi and
colleagues from Seoul in an article that was published in the journal Matter
National University, on 4 February. At the Feringa Nobel Prize Scientist
South Korea, have Joint Research Center, a collaboration between the
found a way to produce University of Groningen and ECUST, scientists developed
high-quality polymers a polymer that can be created and fully depolymerized
from xylose-based under mild conditions. 'We found a way to produce
monomers. Xylose is polymers from the natural molecule lipoic acid in a
a sugar found in plant very controlled way,' explains Ben Feringa, Professor
cell walls. The method of Organic Chemistry at the University of Groningen.
they use involves the 'It is a beautiful molecule and a perfect building block
preparation of the that was created by nature.' The molecule has a ring
xylose-based monomers, including the attachment of linker structure that includes a sulphur-sulphur bond. When
groups, and reacting the monomers in a polymerization this bond is broken, the sulphur atoms can react with
process called cascade metathesis polymerization. To test those of another monomer. 'This process was known
if such plastic materials are degradable, the researchers before, but we managed to find a way to control it
treated the xylose-based polymers with hydrochloric and to create long polymers.' The molecule also has a
acid, a treatment commonly found in chemical recycling carboxyl group, which readily reacts with metal ions.
procedures. The researchers found that the degradability
depended on the linkage type. If the polymer contained
a linkage made of a carbon atom, the polymer resisted
hydrolysis, but linkages made with nitrogen or oxygen
atoms led to immediate degradation. Polymers with a
nitrogen-based linkage resulted in compounds called
pyrroles, whereas those made with oxygen produced
furans. Pyrroles and furans are both abundant, naturally
occurring compounds. However, the researchers advise
care: "Furan derivatives are known to have a broad
range of biological activity, which should be taken into
consideration when identifying applications for these
polymeric materials," they said. In block copolymers, These can crosslink the polymers, which results in an
different "blocks" of shorter polymer strands are attached elastic material. By dissolving the molecule in water
to each other. Accordingly, block copolymers have with sodium hydroxide and then evaporating the water,
properties arising from those of the single blocks. As many a firmer polymer film is produced through ionic bonds.
functional materials can be made with block copolymers, As the polymerization is achieved through reversible
the authors tested whether xylose-based block copolymers bonds, the material is also self-healing, explains
containing blocks with non-degradable linkages would also Feringa: 'When it is cut, you can simply press the ends
disintegrate by acid treatment. They did. together and they will reconnect in a few minutes.'

51 Februar y 2021 Plastics News