Abstract
Whilst plastics have played an instrumental role in the development of modern-day society, current practices within industry are unsustainable and require urgent revision. Central to the pursuit of a sustainable and circular plastics economy is the development of renewable alternatives and diversification of existing waste management strategies. Chapter 1 aims to introduce and expand on these concepts, principally focusing on the catalytic chemical recycling of two commercial polyesters, namely poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET).In Chapter 2, a range of Al(III)-complexes based on a novel ligand framework, namely the catalen, were prepared and characterised using a range of techniques. These Al(III)-catalen complexes were subsequently trialed in the ring-opening polymerisation (ROP) of rac-lactide (rac-LA) for the production of PLA, a bio-based polymer. Such initiators were found to exhibit unprecedented activity in the melt (TOFs up to 45,300 h^-1), whilst an unusual stereoselectivity switch dependent on the nature of the ligand substituents employed was observed under solution conditions {Al(1)Me, Pr = 0.30; Al(3)Me, Pr = 0.72}.
In Chapter 3, metal diversification of these complexes was explored in pursuit of catalysts active for PLA degradation. Metal exchange was found to adversely impact polymerisation activity under both melt and solution conditions. The first example of PLA methanolysis mediated by a discrete Mg(II)-complex was demonstrated under relatively mild conditions (80 °C). Preliminary work extended the scope of such catalysts to the chemical recycling of PET.
In Chapter 4, Zn(II)- and Mg(II)-complexes based on a simple monoanionic tridentate {ONN} ligand (1H) were prepared and characterised. Zn(1)2 and Mg(1)2 were found to facilitate rapid PLA methanolysis under mild conditions (50 °C) and are among the fastest reported to date. Catalyst versatility and selectivity was further explored through the chemical upcycling of various commercial polyesters and the use of mixed plastic waste feeds. Metal exchange was found to dramatically influence catalyst activity, which appeared system dependent, whilst elevated temperatures were found to adversely impact PLA methanolysis.
In Chapter 5, a series of well-defined Zn(II)-half-salan complexes were synthesised and characterised. These complexes were shown to mediate the mild and selective degradation of various commercial polyesters and polycarbonates. The first example of discrete metal-mediated poly(bisphenol A carbonate) (BPA-PC) methanolysis being appreciably active at room temperature was demonstrated, with further kinetic analysis confirming Zn(2)2 to be the fastest catalyst reported to date. A completely circular upcycling approach for bottle-grade PET waste was demonstrated through the production of several renewable poly(ester-amide)s (PEAs).
| Date of Award | 22 Feb 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Matthew Jones (Supervisor) & Emma Emanuelsson Patterson (Supervisor) |