One of the most successful plastics is polyethylene terephthalate (PET), a material used to make bottles and clothing fibers. However, PET is manufactured from petroleum building blocks. An alternative to PET can be made from biological furan molecules, but for the polymerization of these furans, toxic catalysts and high temperatures are required. Now, chemistry chemists from the University of Groningen, under the guidance of prof. Katy Loos, described the enzyme-based polymerization method. Their results are published in the journal ChemSusChem January 29th.
PET is used to make a bottle of aerated drinks because it has excellent barrier properties, which retains its physicality in the interior. "But furan-based polymers are a good alternative," says Katja Loos. Furanes, which are characterized by an aromatic ring of four carbon atoms and one oxygen atom, may be made of biomass-derived sugars and polymerized in polyethylene 2,5-furandicarboxylate (PEF). Other copolyesters can also be produced from furan, resulting in plastic with different properties.
Furans are mainly produced with enzymes. However, polymerization uses the same processes that have been used for PET production over the last 70 years, "says Loos. Toxic catalysts based on metals and high temperatures required for this process mean that it is not environmentally friendly.
That is why Loos and her colleagues sought an alternative polymerization method that uses enzymes. "In the end, we found a commercially available enzyme that would do it," says Loos. Polymers are obtained by combining furan with linear monomers, either aliphatic diols or diciselinic ethyl esters. Enzyme Candida antarctica lipase B (CALB) is a lipase that breaks down the ester linkages, but polymerization requires the formation of these bonds. This may seem counter-intuitive, but not, explains Loos: "Enzymes catalyze the equilibrium reactions, and we simply pushed the balance toward the formation of ester bonds."
In his paper, scientists describe how CALB and numerous furans and linear monomers use to create different copolyester. They have succeeded in increasing the content of aromatic units in the polyester to a point exceeding the PET properties. Enzyme polymerization seems to be a viable alternative to current catalytic polymerization. In our experiments we used ether as a solvent, which you do not want in the factory environment. But as the furnace content is fairly low, we are convinced that enzymatic polymerization will also act in liquid monomers, "says Loos.
Since the CALB enzyme is commercially available, it is surprising that no one used it before to avoid the toxic catalyst and high temperature process. The only explanation Loos can offer is that most polyester production lines are directed to the use of these classic reactions instead of the enzyme alternatives. Changing the production line is expensive. "However, our enzyme polymerization process would be ideal for new companies working on green PET alternatives."
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