Katja Loos from the University of Groningen. Photo: University of Groningen.
Katja Loos from the University of Groningen. Photo: University of Groningen.

Like most plastics, polyethylene terephthalate (PET), which is widely used for making bottles and fibers for clothing, is derived from petroleum-based building blocks. An alternative to PET can be made from bio-based furan molecules, but to polymerize these furans you need toxic catalysts and high temperatures. Now, in a paper in ChemSusChem, polymer chemists from the University of Groningen in the Netherlands, led by Katja Loos, professor of polymer chemistry, describe an alternative polymerization method that utilizes enzymes.

PET is used to make fizzy drink bottles because it has excellent barrier properties, which keeps the fizz inside. “But furan-based polymers are a good alternative,” says Loos. Furans, which comprise an aromatic ring with four carbon atoms and one oxygen atom, can be made from biomass-derived sugars and then polymerized into polyethylene 2,5-furandicarboxylate (PEF). Other co-polyesters can be created from furans as well, resulting in plastics with various different properties.

“Furans are mainly produced with enzymes,” explains Loos. “But for the polymerization, the same processes are used as have been used for PET production for the last 70 years.” These processes require toxic metal-based catalysts and high temperatures, which means they’re not very environmentally friendly.

That is why Loos and her colleagues decided to look for an alternative polymerization method that uses enzymes. “We eventually found a commercially available enzyme that would do this,” says Loos.

The polymers are produced by combining furans with linear monomers, either aliphatic diols or diacidic ethyl esters. The enzyme Candida antarctica lipase B (CALB) normally breaks down ester bonds, but the polymerization requires the creation of these bonds. This may seem counter-intuitive, but it is not, explains Loos: “Enzymes catalyze equilibrium reactions, and we simply pushed the equilibrium towards the formation of ester bonds.”

In the paper, the scientists describe how CALB and a number of furans and linear monomers are used to form different co-polyesters. By increasing the content of aromatic units, they were able to produce a polyester that exceeded the properties of PET, which suggests that this enzymatic polymerization process could be a viable alternative to current catalytic polymerization.

“In our experiments, we used ether as a solvent, which you don't want in a factory setting. But as the melting point of furans is quite low, we are confident that enzymatic polymerization will work in liquid monomers as well,” says Loos.

As the CALB enzyme is commercially available, Loos thinks it’s surprising that no one had tried using it before to avoid the need for toxic catalysts and high temperatures. The only explanation that Loos can offer is that most polyester production lines are geared to using these classical reactions, rather than the enzymatic alternative. And changing a production line is expensive. “However, our enzymatic polymerization process would be ideal for new companies working on green alternatives to PET,” she says.

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.