Vinyl polymers (VPs), one of the most widely used plastic materials, can be found in products ranging from poly(vinyl chloride) pipes and surgical gloves to disposable polystyrene plates. Chemical recycling of VPs is considered a key technology required for realizing a sustainable society.
Recently, a team of researchers led by associate professor Yasuhiro Kohsaka from the Faculty of Textile Science and Technology (FTST) and Research Initiative for Supra-Materials (RISM), both at Shinshu University, Ueda, Japan, presented a new strategy for effectively depolymerizing the VPs of cyclic styrene derivatives to retrieve a monomer precursor.
Traditionally, chemical recycling of VPs has always been challenging. The conventional approach reverses the polymerization process, breaking a single large molecule made up of repeating monomeric units down to its parent monomeric components.
Depolymerizing VPs is difficult because the covalent carbon-carbon bonds holding together the monomer units are very stable and tough to break. Studies have proposed ways to break the bond, but most fail to ensure quantitative and selective scission of its main chain, which is crucial to the effective recovery of monomers.
"Polymers that are stable have poor recyclability, and the ones that are easily recyclable are unstable in nature," said Kohsaka in a press release. "We overcame this trade-off by forgoing conventional strategies that try to reverse the polymerization reaction to recover monomers and developing a two-step recycling process. In the first step, degradation of the polymer to a monomeric precursor was achieved, which was followed by the recovery of the monomer by chemical modification."
In their recent breakthrough published online in ACS Macro Letters, the team chose VPs made of cyclic α-substituted styrene derivatives, such as 3-methylene phthalide, as their molecule for testing chemical recyclability and investigated the ring-opening reaction of the pendant groups in the presence of a base like sodium hydroxide. They found that the opening of the rings due to saponification increased the steric hindrance around the pendant groups, which led to main-chain scission and depolymerization of the VP into monomer precursors.
These recovered precursors were then converted to monomers via single-step chlorination and spontaneous intramolecular esterification. The researchers further that the same cyclic monomer structure that facilitated depolymerization was also responsible for promoting polymerization owing to reduced steric hindrance around the vinylidene group. These findings led the researchers to conclude that cyclic α-substituted styrene derivatives can potentially recycle chemicals.
The researchers believe their findings can provide useful fodder for further research on not just the depolymerization of plastic materials but also the development of new recyclable plastics.
"The aim of our research was to aid the mission of developing efficient plastic recycling technology, which is a tool that humanity desperately needs against the backdrop of environmental pollution caused by plastics. While we cannot remove all the plastic that already exists on this planet, we can at least make the best use of plastic resources available to us with our new chemical recycling strategy," concluded Kohsaka.