Most plastic persists in the environment. A recently developed polymer degrades in a week and doesn’t leave microplastics behind. Image credit: Shutterstock/Larina Marina
Plastic trash chokes shorelines and oceans, in part because plastic polymers do not easily decompose. But a new kind of environmentally degradable plastic could help change that: It breaks down in about a week in sunlight and air, according to a recent study in the Journal of the American Chemical Society (JACS). Chemical characterization using nuclear magnetic resonance (NMR) and mass spectroscopy, among other techniques, revealed that the plastic decomposed rapidly in sunlight from a petroleum-based polymer into succinic acid, a naturally occurring nontoxic small molecule that doesn’t leave microplastic fragments in the environment.
Although a sun-sensitive plastic might not be a good choice for bottles or bags that need to last more than a week on shelves, integrating the environmentally degradable polymer as a minor ingredient, or with other biodegradable polymers, could help speed breakdown of these materials in landfills, says coauthor Liang Luo, an organic materials scientist at Huazhong University of Science and Technology in Wuhan, China. The flexible and degradable material would be potentially useful inside electronics, he says. Sealed inside a cell phone or other flexible electronic device, the polymer could last for years isolated from light and oxygen, Luo notes, while making smartphones easier to dispose of at the end of their service life. And the byproduct succinic acid could be upcycled for commercial uses in the pharmaceutical and food industries, Luo adds.
When Luo first developed the plastic in 2020, he intended it to change color with pH, for use as a chemical sensor. But then he noticed that the plastic’s natural deep red color faded quickly and the plastic film broke apart over several days in sunlight. In conjugated polymers such as this, which have a long backbone chain of alternating double and single bonds, the material’s color comes from its molecular structure—long chains of monomers—rather than a dye. Loss of that color means the chains have broken down into their monomer units. Cleaving polymer chains is often a challenging step in breaking down plastics Luo says. “But for our polymer, we can simply use sunlight.”
The red plastic film breaks apart in sunlight and air over 7 days. Here, each vial represents a day from 0 to 7, left to right. Image credit: Qiang Yue
Luo and coauthors sought to understand the chemistry of how the plastic breaks down. They used NMR to characterize the polymer’s structure based on its magnetic field. The intact polymer has two broad NMR peaks corresponding to chains of polymers. But after exposure to sunlight, the material just gives a single sharp peak, “like a needle,” Luo says, indicating its molecular composition has changed. The sharp NMR peak corresponds to the structure of succinic acid, which forms during the degradation reaction. The process seems to use photo-oxidative degradation, in which sunlight irradiation breaks the polymer’s double- and triple-bonded carbon backbone.
The postulated degradation mechanism is totally different from the breakdown of other degradable plastics, for instance by hydrolysis of ester or amide bonds, says materials chemist Zhibin Guan at the University of California, Irvine, who was not involved in the new study. The mechanism clearly doesn’t occur in the consumer plastics that litter sunny beaches. It’s possible the degradation could occur in other conjugated polymer plastics, but “it will take more work to demonstrate the generality of this mechanism,” Guan says. Ultimately, the work is “an exciting example of degradable conjugated polymers,” he adds, which could be valuable for a variety of applications, such as electronics, in the future. Polymer chemist Eugene Chen at Colorado State University in Fort Collins calls the recent work a “tour de force” that addresses several key challenges in plastic design and achieves “nearly ideal plastic degradation.” In particular, using sunlight and oxygen rather than focusing on microbial activity to break down the plastic is an advance for the field, he says.
“We will continue to explore the degradation of plastics,” Luo says, looking ahead. Though he doesn’t have a timeline for commercialization, he says the ballpark “could be 5 or 10 years.”