Post-Polymerisation Modification of Polyethylene through Photochemical Oximation

Post-polymerisation modification (PPM) can provide access to functional polymers not readily available through bottom-up synthesis. Incorporating polar functional groups via PPM on the carbon-carbon backbone of polyolefins is a powerful strategy to tailor the polymer structure and enlarge the material property window. Here, we demonstrate the clean and efficient modification of polyethylene (PE) through the solvent- and catalyst-free photochemical oximation, using t-butyl nitrite (t-BuONO) as inexpensive and easy to handle NO radical source. The photochemical oximation uses LED long wave ultraviolet light and is run neat under aerobic conditions. The photochemical reaction conditions were optimised using linear small- to large-chain alkanes as model substrates. Using four different types of polyethylene (self synthesised low-molecular weight, commercially available low molecular weight and high molecular weight as well as post consumer material), we successfully incorporated oxime, ketone and nitro functional groups on the polymer backbone, without radical cleavage or crosslinking. The functionalisation degree of the three different functional groups is tuneable depending on the reaction atmosphere and system pressure, and the total functionalisation degree on the backbone can reach up to 2.9%. Detailed analysis, including 2D NMR spectroscopy with 15N-isotopically labelled modified PE produced with t-BuO15NO, provided insight into functional group distribution and regiochemistry, revealing that the photochemical oximation preferentially functionalises the pre-terminal carbon position and that the functional groups are randomly spaced apart on the polymer backbone rather than positioned adjacent to each other. Additionally, reactivity studies together with electron paramagnetic resonance (EPR) spectroscopy showed that the oxime is converted to the ketone via an iminoxyl radical species, confirming that it solely arises from the further reactivity of the oxime functional group via an iminoxyl radical pathway. These results demonstrate tuneable functionalisation, enabling the production of modified polyolefins with tailored properties that potentially broaden their application range and provide opportunities for improved recyclability.