Polymers as a Blank Canvas: Introducing Polarity via Post-Polymerisation Modification

Abstract

Over more than a century of polymer science, lightweight, durable, and versatile synthetic polymers have been produced on a million-ton scale and have become indispensable to modern society. Their widespread use, however, has led to significant environmental challenges. The accumulation of plastic waste in oceans and soils has damaged ecosystems, while the reliance on fossil feedstocks and the CO₂ emissions associated with polymer production and disposal have contributed to climate change. These benefits and drawbacks are particularly evident for commodity hydrocarbon polymers such as polyethylene (PE), polypropylene (PP), polybutadiene (PBD), and polystyrene (PS), which represent the largest share of global polymer production by volume. The properties of these polymers arise from their chemical structure, consisting solely of carbon–carbon and carbon–hydrogen bonds. While these strong, nonpolar, and chemically inert bonds provide durability and stability, they also limit compatibility with polar materials. This incompatibility complicates the incorporation of additives, colourants, and adhesives, as well as the production of polymer blends. Furthermore, the chemical inertness of hydrocarbon polymers results in poor end-of-life performance, as these materials resist degradation and are difficult to recycle. Introducing small structural modifications that increase polymer polarity offers a promising strategy to retain the advantageous properties of hydrocarbon polymers while mitigating their drawbacks. In particular, post-polymerisation modification (PPM) enables the installation of functional groups onto existing polymer backbones, allowing access to new properties without altering polymerisation processes. Previous studies have shown that such modifications can enhance tensile strength, adhesion, and compatibility, and may improve recyclability or degradability. This thesis investigates the introduction of polar oxygen- and nitrogen-based functional groups into hydrocarbon polymers via PPM. Chapter 1 provides a general overview of current strategies to functionalise polymer backbones, with a focus on state-of-the-art post-polymerisation modification methods. Chapter 2 describes a catalyst- and solvent-free photochemical functionalisation of polyethylene. Using tert-butyl nitrite as an inexpensive and easily handled nitric oxide radical source and UVA light (370 nm), various grades of polyethylene, including LDPE and HDPE with different molecular weights, were successfully decorated with oxime, ketone, and nitro functional groups. In Chapter 3, tert-butyl nitrite was further explored as a nitration reagent for functionalising unsaturated bonds in polybutadienes in the presence of TEMPO. The resulting nitro groups could be readily reduced to primary amines, a functional group that remains challenging to install on polymer backbones. Notably, when the amine-functionalised polybutadienes were applied as thin films on copper electrodes, they enabled the electrochemical conversion of CO₂ with high selectivity toward C₂+ products, particularly ethylene and ethanol. Chapter 4 presents a metal-catalysed oxidation strategy for styrenic and rubbery (co-)polymers. Using a robust manganese-based homogeneous catalyst (MnTACN) and hydrogen peroxide, polystyrene backbones were functionalised with alcohol and ketone groups, while polybutadienes were modified to introduce alcohols and epoxides. Finally, Chapter 5 demonstrates C–H bond activation and subsequent oxidation of polyethylene and polypropylene using frustrated radical pairs and mCPBA, respectively.