Polythioenones: a step toward a circular economy for synthetic polymers
The possibilities for material design and production of
plastic components are being expanded through 3D printing technology.
However, there is a shortage of recyclable polymers that meet the
performance requirements. In the journal Angewandte Chemie, a
research team has introduced a new class of polymers called
polythioenones, which are mechanically and chemically recyclable and
suitable for 3D printing. They also demonstrate better mechanical
properties than conventional polyolefins—thanks to a special,
ring-shaped building block.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Using digital modeling, complex structures can be precisely
constructed layer by layer with 3D printers. Extensive customization and
rapid prototyping open new possibilities in fields such as biomedical
engineering, automotive manufacturing, and consumer product design. In
3D printing with the fused filament fabrication (FFF) process, a
threadlike thermoplastic material is pressed through a hot nozzle, where
it melts. It is then applied in layers until the desired
three-dimensional component is produced—with a minimal waste of
material. The downside to this method is the lack of suitable polymers
that can be recycled. Chemically recyclable polymers, which can be split
apart into their building blocks (monomers) and repolymerized, would help reduce
environmental problems resulting from long-lasting plastic waste and
help conserve fossil-derived feedstocks.
The synthesis of novel thermoplastic polymers
with improved recyclability begins and ends with the design of suitable
monomers. A team led by Will R. Gutekunst and H. Jerry Qi at the Georgia
Institute of Technology (Atlanta, USA) has now developed a novel family
of monomers: cyclic thioenones (CTE), rings made of seven carbon atoms
and one sulfur atom. The rings contain one C=C double bond and one
carbonyl group (C=O) and can readily be modified by the addition of
different side groups. The monomers can be polymerized through a
ring-opening reaction in which monomers are added one by one to the end
of the growing chain. Known as a thia-Michael addition, this reaction is
reversible, meaning that the resulting polythioenones (PCTE) can be
depolymerized back to the starting monomer.
One of the synthesized polymers, PCTE-Ph proved to be
particularly interesting. It is made from a CTE-monomer with an aromatic
six-membered carbon ring (phenyl ring) as its side group. PCTE-Ph is a
thermally stable thermoplastic with outstanding mechanical properties.
Colorants and fillers can be incorporated, and it can be processed by
customary methods. This new material is especially well suited for 3D
printing. Components printed with this material can be mechanically
recycled by simply melting the material and processing it again while
maintaining its advantageous properties, such as tensile strength and
thermal stability. In addition, it can be catalytically depolymerized
back to the initial monomer in 90 % yield. This recovered monomer is
then available for additional rounds of polymerization.
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About the Author
Dr. Will Gutekunst is an Associate Professor in the School
of Chemistry and Biochemistry at the Georgia Institute of Technology.
His research focuses on the invention of new monomer families and
polymerization concepts to address modern challenges in materials
sustainability.
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