Thermo-chemical upgrading of waste polyethylene and polypropylene plastics

Polyolefin (polyethylene/polypropylene) plastics are widely used due to their low cost, durability, broad processability, and favorable performance profiles. Nonetheless, their chemical upcycling remains highly challenging owing to the inert nature of the C(sp3)–C(sp3) bonds, resulting in most polyolefins being discarded as waste. Effective upcycling of waste polyolefins not only mitigates plastic pollution but also enables value enhancement and expanded functionality. This review summarizes recent advances in chemical upcycling strategies that convert waste polyolefins into value-added functional materials, small-molecule chemicals, and new polymeric structures. We discuss a diverse range of approaches, including functionalization; catalytic cracking, hydrogenolysis/hydrocracking, and tandem catalysis; as well as depolymerization–repolymerization routes. Collectively, these strategies allow the production of high-quality, high-value products from waste polyolefins under relatively mild conditions, offering critical environmental and economic opportunities aligned with carbon-neutral and sustainable development goals.