Polystyrene is everywhere, from takeaway food containers to test tubes in labs. Once discarded, this familiar plastic becomes one of the most persistent forms of waste. To transform this waste into industrially relevant chemicals, existing upcycling approaches can derive simple molecules such as benzoic acid. But within its very structure, polystyrene carries a yet untapped—or rather, blocked—potential for generating other higher-value products.
Polystyrene’s phenyl rings, which consist of carbon and hydrogen bound together in a hexagonal structure, offer several molecular sites where atoms can be added to create new chemicals. “However, these rings are partially blocked by their attachment to the polymer backbone, making some positions difficult for chemical reactions to reach and hindering the production of more valuable multi-substituted chemicals,” explained Jason Y. C. Lim, a Principal Scientist at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
Imagine a series of cars facing the wall, with little space between each vehicle. If the hood is the attachment point to the wall, like the first carbon position in the phenyl ring bound to the backbone, then the driver’s seat is the adjacent position in the hexagon that ends up physically blocked. This often leaves only the car’s trunk accessible for reactions through conventional methods.
Together with Research Scientist Albert Ong and A*STAR IMRE colleagues, the team devised a strategy to unlock a more valuable class of compounds by altering the way polystyrene’s phenyl groups are linked to the polymer backbone. The team’s approach was inspired by previous work that showed how to reorganise the polymer structure into one where the rings are fused into indane units.
“In this polyindane arrangement, the rings are linked to the backbone at two adjacent positions, rather than a single point,” Lim said. “This structural change effectively circumvents the problem of site accessibility on the aromatic ring previously blocked by the polymer backbone.” The two attachment sites create a geometric shift, like parking the cars at a new angle that affords greater accessibility.
The researchers could then introduce an oxidation reaction to free up the phenyl rings from the backbone. Oxygenating not one but two positions led to the production of 1,2-disubstituted compounds such as phthalic acid and phthalic anhydride, which are used in pharmaceuticals, energy storage devices, dyes and other advanced materials.
“The global market for phthalic anhydride is worth billions of dollars annually and is several times larger than that of benzoic acid,” Lim said, adding that their strategy can also contribute to supply chain resilience by offering an alternative production route for these high-value chemicals.
After filing a patent application, the researchers next aim to improve the efficiency and scalability of their approach as well as expand its use cases. “We believe that applying this strategy to different polystyrene derivatives can yield an even larger family of high-value aromatic specialty chemicals, further enhancing the value of polystyrene as a feedstock,” said Lim.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
