Chemically Recyclable Polyethylene-like Sulfur-Containing Plastics from Sustainable Feedstocks

Plastics plays a major role in our daily life, from packaging to healthcare, transportation to construction. However, the widespread use and disposal of plastic products have led to a global environmental crisis in the form of plastic pollution. Plastic waste has reached alarming proportions globally, with over 350 million tons of plastic produced annually. Unfortunately, more than 80% of this plastic ends up as waste in landfills or the ocean, creating a severe environmental crisis. Only a fraction, less than 20%, is recycled, primarily through incineration or mechanical recycling, both of which have their limitations. Chemically recyclable polymers offer a promising solution to mitigate plastic waste accumulation and make better use of resources. Chemically recyclable polymers, also known as chemically recyclable plastics, are plastic materials that can be broken down into their original monomer units through chemical processes. This is in contrast to traditional plastics, which are typically not easily recyclable and often end up in landfills or as forever pollution in the environment. A new study conducted by Dr. Yanni Xia, Dr. Xinchen Yue, Dr.  Yue Sun, Dr.  Chengjian Zhang, and led by Professor Xinghong Zhang from Zhejiang University introduces an innovative approach to synthesize chemically recyclable polymers with favorable mechanical properties using water (H₂O), carbonyl sulfide (COS), and diacrylates. The work is currently published in the Journal Angewandte Chemie International Edition.

The researchers hypothesize a cascade reaction mechanism, involving an O/S exchange between COS and H₂O, releasing CO₂ and highly reactive H₂S, which is then coupled with diacrylates through thiol Michael additions. This mechanism is inspired by similar processes found in nature and utilizes COS, a gas produced on a large scale in industries, which is abundant and considered a waste product. Additionally, the method aims to incorporate in-chain ester groups as breaking points, enhancing the potential for chemical recycling. The authors successfully synthesized a series of polymers (P1 to P16) containing in-chain thioether and ester groups, demonstrating air tolerance and high efficiency under mild conditions. Notably, polymers P7 to P11, which are structurally similar to linear polyethylene with low densities of in-chain polar groups, exhibited remarkable mechanical properties, making them comparable to commercial high-density polyethylene (HDPE). The ability to produce these high-performance polymers from easily accessible monomers under simple conditions is a significant achievement in the field of polymer science.

The research team demonstrated the versatility of the new method through its applicability to a wide range of diacrylates, resulting in various polymers with different properties. The ease of obtaining high molecular weight polymers through this process is a promising feature, and the method can be scaled up for large-scale production with little impact on the properties of the resulting polymers. The inclusion of COS as a raw material introduces some safety concerns due to its flammability and toxicity. However, the other advantages of the method, such as the synthesis of deuterated polymers, offer diverse applications in fields like organic light-emitting diodes.

One important aspect of the new polymers is chemical recycling. The authors outlined two distinct recycling cycles: Cycle 1 involves hydrolysis, esterification, and step-growth polymerization, while Cycle 2 utilizes methanolysis and polycondensation. Both methods efficiently recover monomers and facilitate the regeneration of the polymers. This aspect of the research provides a practical solution to address the issue of plastic waste by closing the recycling loop for these polymers. Furthermore, the incorporation of sulfur atoms in these polymers gives them unique properties, including a high refractive index, making them suitable for optical applications, and oxidative responsiveness. The presence of in-chain ester groups also suggests potential biodegradability, making these polymers environmentally friendly.

In conclusion, the new study by Zhejiang University scientists represents an important advancement in the field of chemically recyclable polymers. Their innovative approach to synthesizing these polymers using easily accessible monomers, efficient processes, and the ability to recover monomers through chemical recycling is highly promising. The exceptional mechanical properties demonstrated by some of the polymers, similar to commercial HDPE, open up a wide range of applications. Additionally, the incorporation of sulfur atoms in these polymers gives them unique properties and functionality. Overall, this study offers a solution to the pressing problem of plastic waste and provides a sustainable pathway for the future of polymer science.