In this animation, polymers of discarded plastics are edited to generate macromolecules that may be reused in other products. The makeup of the polymer chains determines the properties of resulting plastics. Credit: Jacquelyn DeMink/ORNL, U.S. Dept. of Energy
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.
The scientists upcycled plastic waste by employing two processes in tandem. Both are types of metathesis, which means a change of places. Double bonds break and form between carbon atoms, allowing polymer subunits to swap.
One process, called ring-opening metathesis polymerization, opens carbon rings and elongates them into chains. The other process, called cross metathesis, inserts chains of polymer subunits from one polymer chain into another.
Traditional recycling fails to capture the value in discarded plastics because it reuses polymers that become less valuable through degradation with each melt and reuse. By contrast, ORNL's innovative upcycling utilizes the existing building blocks to incorporate the mass and characteristics of the waste material and provide added functionality and value.
"The new process has high atom economy," Foster said. "That means that we can pretty much recover all the material that we put in."
The ORNL scientists demonstrated that the process, which uses less energy and produces fewer emissions than traditional recycling, efficiently integrates waste materials without compromising polymer quality. Foster, Ilja Popovs and Tomonori Saito conceptualized the paper's ideas. Nicholas Galan, Isaiah Dishner and Foster synthesized monomer subunits and optimized their polymerization. Joshua Damron performed nuclear magnetic resonance spectroscopy experiments to analyze reaction kinetics. Jackie Zheng, Chao Guan and Anisur Rahman characterized mechanical and thermal properties of final materials.
"The vision is that this concept could be extended to any polymer that has some sort of backbone functional group to react with," Foster said. If scaled up and expanded to employ other additives, broader classes of waste could be mined for molecular building blocks, dramatically reducing the environmental impact of other difficult-to-process plastics. The circular economy - in which waste materials are repurposed rather than discarded - then becomes a more realistic goal.
Next, the researchers are interested in changing the types of subunits in the polymer chain and rearranging them to see whether they can create high-performance thermoset materials. Examples are epoxy resins, vulcanized rubber, polyurethane and silicone. Once cured, thermoset materials cannot be remelted or reshaped because their molecular structure is cross-linked. That makes their recycling a challenge.
The researchers are also interested in optimizing solvents for environmental sustainability during industrial processing.
"Some preprocessing is going to be required on these waste plastics that we still have to figure out," Foster said.
The DOE Office of Science Materials Science and Engineering program and ORNL Laboratory Directed Research and Development program funded the research.
UT-Battelle manages ORNL for DOE's Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science . - Dawn Levy
