Post‐Synthetic Amide Insertion Into Polyethylenes Augments Diverse Physical Properties and Enables Branching‐Dependent Recyclability

Abstract Polyethylenes (PEs) possess unreactive saturated hydrocarbon backbones, which make chemical recycling highly challenging, even at high temperatures. The insertion of strongly‐hydrogen‐bonding amide linkages into the PE backbone can potentially confer recyclability via amide cleavage and reformation, whilst simultaneously bringing about new material properties for functional upcycling. Herein, it is demonstrated that in‐chain amide insertion can be performed under a variety of reaction conditions, including the usage of p ‐cymene as a bioderived green solvent, with common and low‐cost reagents, on both high‐density (HDPE) and low‐density PEs (LDPE). Approximately 1 amide bond/100 ethylene repeating units can enhance not only the polymers’ tensile properties, but also elicit an unexpected increase in surface hydrophobicity and solid‐state clusteroluminescence that is absent in the parent PEs. Additionally, the hitherto‐overlooked importance of PE structure is uncovered in its chemical recyclability. While low‐branching amide‐containing HDPE could be hydrolysed and reformed for three cycles at ≤150 °C without compromising their tensile properties, attempted chemical recycling of highly‐branched LDPE formed a thermoset‐like crosslinked polymer with greatly enhanced ductility. These findings showcase amide‐insertion as a versatile means to access new functional applications that are originally inaccessible to PEs without requiring additives, with branching‐dependent influences on chemical recyclability for circular polymeric materials design.