Polyethylene (PE) and polypropylene (PP) are the two most widely produced plastics today, but they rank among the least recycled [1]. Objects made from PE and PP, such as bags, detergent bottles, cosmetics containers, or milk bottles, are difficult to rapidly sort at recycling facilities. It would thus be ideal to directly reuse blends of PE and PP. Unfortunately, attempts to mechanically recycle blends of these two plastics generally result in low-value materials. Despite having similar chemical structures, PE and PP are incompatible with each another, which causes the resulting material to have poor mechanical properties. Although numerous strategies are known to address this issue, virtually none of them are practiced on an industrial scale because of a combination of high costs and impractical processing requirements. Thus, the vast majority of polyolefin waste in Europe today is incinerated for energy recovery. In this context, our laboratory has discovered a promising new strategy for upcycling polyolefin blends into high-performance materials [2]. As schematically depicted in Fig. 1, this process is based on the fascinating chemistry of azidotriazines, which are molecules that are able to form new chemical bonds with polyolefins at high temperature via a highly reactive nitrene species. We implemented this chemistry into a protocol based on reactive extrusion, which is a solvent-free process used in industry for transforming the structure of polymers. The resulting upcycled PE and PP blends are dynamic polymer networks that exhibit excellent tensile properties, creep resistance, and mechanical recyclability. We propose that this unique behavior is derived from the thermomechanically activated reversibility of the nitrogen- nitrogen bonds that make up the cross- linking structures. This process was also found to be applicable to objects recovered from everyday life. We envision this approach as a valuable route to producing high-performing engineering materials that can be further recycled.

References

1. A. H. Westlie, E. Y. Chen, C. M. Holland, S. S. Stahl, M. Doyle, S. R. Trenor, K. M. Knauer, Macromol. Rapid Commun. 2022, 43, e2200492.
2. T. Vialon, H. Sun, G. J. M. Formon, P. Galanopoulo, C. Guibert, F. Averseng, M. N. Rager, A. Percot, Y. Guillaneuf, N. J. Van Zee, R. Nicolay, J. Am. Chem. Soc. 2024, 146, 2673-2684.

Acknowledgments

The authors thank the Ville de Paris (Emergence 2018 program), the Université Paris Sciences et Lettres (Prematuration program UNICYCLE), the Institut Carnot IPGG Microfluidique (C.A.R.N.O.T. program), the SOLEIL Synchrotron (proposal no. 20220576), the Ile-de-France Region (SESAME equipment project no. 16016326), and the Centre Régional de Compétences en Modélisation Moléculaire de Marseille (DFT calculations).