By editing the polymers of discarded plastics, chemists at the Department of Energy’s Oak Ridge National Laboratory have found a way to generate new macromolecules with more valuable properties than those of the starting material. Upcycling may help remedy the roughly 450 million tons of plastic discarded worldwide annually, of which only 9% gets recycled; the rest is incinerated or winds up in landfills, oceans or elsewhere.
ORNL’s invention may change plastic’s environmental fate by rearranging polymeric building blocks to customize the properties of plastics. Molecular subunits link to produce polymer chains that can connect through their backbones and cross-linked molecules to form multipurpose plastics. The makeup of polymer chains determines how strong, rigid or heat-resistant those plastics will be.
Molecular editing is so promising that it has been the basis of two Nobel Prizes in Chemistry. In 2005, the prize went to developers of the metathesis reaction, which breaks and makes double bonds between carbon atoms in rings and chains so their subunits can swap to create new molecules limited only by imagination. Similarly, in 2020, the prize went to developers of CRISPR, “genetic scissors” for editing DNA strands, biopolymers made of nucleotide subunits that carry the code of life.
”This is CRISPR for editing polymers,” said ORNL’s Jeffrey Foster, who led a study that was published in Journal of the American Chemical Society. “However, instead of editing strands of genes, we are editing polymer chains. This isn’t the typical plastic recycling ‘melt and hope for the best’ scenario.”
The ORNL researchers precisely edited commodity polymers that significantly contribute to plastic waste. In some experiments, the researchers worked with soft polybutadiene, which is common in rubber tires. In other experiments, they worked with tough acrylonitrile butadiene styrene, the stuff of plastic toys, computer keyboards, ventilation pipes, protective headgear, vehicle trim and molding, and kitchen appliances.
“This is a waste stream that’s really not recycled at all,” Foster said. “We’re addressing a significant component of the waste stream with this technology. That’d make a pretty big impact just from conservation of mass and energy from materials that are now going into landfills.”
Dissolving the waste polymers is the first step in creating drop-in additives for polymer synthesis. The researchers shredded synthetic or commercial polybutadiene and acrylonitrile butadiene styrene and immersed the material in a solvent, dichloromethane, to conduct a chemical reaction at a low temperature (40 degrees Celsius) for less than two hours.
A ruthenium catalyst facilitated the polymerization, or polymer addition. Industrial firms have used this catalyst to make robust plastics and to convert biomass such as plant oils into fuels and other high-value organic compounds with no difficulty, highlighting the potential for its use in chemical upcycling.
The molecular building blocks of the polymer backbone contain functional groups, or clusters of atoms that serve as reactive sites for modification. Notably, the double bonds between carbons increase the chances for chemical reactions that enable polymerization. A carbon ring opens at a double bond to create a polymer chain that grows as each functional polymer unit directly slips in, conserving the material. The plastic additive also helps control the molecular weight of the synthesized material and, in turn, its properties and performance.
If this material synthesis strategy could be expanded to a broader range of industrially important polymers, then it could prove an economically viable path for reusing manufacturing materials that today can only be used in a single product. The upcycled materials might be, for instance, softer and stretchier than the original polymers or, perhaps, easier to shape and harden into durable thermoset products.
This Oak Ridge National Laboratory news article "Polymer editing can upcycle waste into higher-performance plastics" was originally found on https://www.ornl.gov/news
