Polyethylene is one of the world’s most commonly used plastics, found in bottles and packaging films, but also one of the most difficult to degrade. By itself, polyethylene would take hundreds of years to fully decompose. Scientists have been working to address this problem as polyethylene waste clogs landfills and pollutes beaches and oceans.
A major obstacle to the degradation of polyethylene is a feature of its molecular structure. It contains nonreactive carbon chains, covalent bonds that hold atoms together so tightly that it takes a great deal of force and energy to pull them apart. However, scientists have struggled to find a solution to the degradation of polyethylene. A new to learn appeared in the magazine on Thursday Science proposes a method for effectively converting polyethylene to propylene, a chemical that is easier to use for future chemical reactions.
Converting polyethylene into usable polymers can increase the value of the plastic and offers an alternative to throwing away, he explains Suzanne Scott, professor of chemistry at the University of California, Santa Barbara, and author of a separate new study that used a similar technique to achieve the same goal. In this preprint study, Scott and her co-authors, published Tuesday, also described an approach in which they removed hydrogen from the polyethylene chains, creating reactive bonds that are easier to break.
Polyethylene has been around since 1930s, but it took scientists 20 years to refine the plastic to make it stiffer, harder and more heat-resistant. Fast forward to the present and polyethylene’s carbon-carbon and carbon-hydrogen bonds are nearly unbreakable, which has helped create a variety of materials from plastic water bottles to wire insulation. However, those same chemical bonds have made them difficult to break without incurring high energy costs.
[Related: Will we ever be able to recycle all our plastic?]
Normally, in order to break the carbon-carbon single bonds in polyethylene, you would have to heat them up under special conditions: without oxygen or with a catalyst and the addition of hydrogen. However, no approach can completely eliminate the unreactive bonds. John HartwigProfessor of Chemistry at the University of California, Berkeley and senior author of the study in Sciencelooked for a way to break these carbon chains by incorporating a more reactive bond between each pair of atoms.
The bond, known as a double bond, is formed when two pairs of electrons are shared between atoms. While a double bond is stronger than a single bond, it is less stable, making it easier to break. Like Scott’s team, they sequentially removed hydrogen from progressively smaller carbon chains, creating tiny pieces of reactive material. When the double bond was cleaved, the scientists were left with products that also had reactive bonds, allowing the pieces to be reused. Then they connected the small pieces together in a different arrangement to make propylene, another polymer.
About 80 percent of the end products produced by this reaction were propylene. “Other methods have formed mixtures of products” that may only have niche applications, explains Hartwig. Mainly producing propylene is important, he says, because it’s in high demand. Propylene is an important building block for a number of other chemicals used in processes to make rubbing alcohol and polyesters.
It is said that the propylene yield is “pretty exciting”. Mahdi Abu OmarProfessor of green chemistry at the University of California, Santa Barbara, who was not involved in the study because in the final product, all two of the three carbon atoms come from waste materials.
[Related: A close look at the Great Pacific Garbage Patch reveals a common culprit]
In the new preprint study, Scott and her colleagues’ method produced 94 percent propylene. Although similar to the procedure published in Science, she says, in the preprint study, the method had a more continuous flow and used less of an organic chemical called ethylene. This is because they simultaneously added ethylene while removing the propylene formed. “We call this circularity, where you take the polymer apart, get the pieces back, and put them back together in a new way to make a polymer,” says Scott.
These studies are consistent with previous efforts to degrade polyethylene. A 2020 paper proposed a model for an organic reaction in which carbon-carbon double bonds could be redistributed using a metal catalyst to speed up the process.
While polyethylene is not toxic by itself, it can interact with other molecules that are potentially toxic and can contaminate the plastic surface. In addition, when polyethylene degrades in the environment, there is a possibility that animals ingest tiny bits of polymer called microplastics, which can also be absorbed in the soil. While we still don’t know the true ecological impact of microplastics, research suggests that human ingestion of microplastics can lead to cellular damage, developmental toxicity, and an increased risk of cancer.
“It’s an interesting scientific and societal challenge,” says Scott. “We are all aware of that [plastic degradation] is a problem we urgently need to solve, and it’s exciting because people are pushing this whole field forward at a pace not typically seen in research.”