Benign metal Initiators for the production of biopolymers and their subsequent depolymerisation

Abstract

Our modern society has been built on the exploitation of fossil resources for fuels, materials and chemicals. However, it has long been apparent that there are serious and potentially existential issues associated with our continued reliance on these finite resources. Innovative, green chemistry is a crucial component in solving these issues and moving towards a more sustainable society. One important example is the use of hydrocarbon polymers that are made from fossil resources and have a devastating impact on ecosystems when released into the environment. Chapter 1 gives an introduction to the issues associated with plastic production and waste and to the use of poly(lactic acid) (PLA) as an
established and viable alternative to these unsustainable materials.

In Chapter 2, five tetradentate {ONSO} ‘thiolen’ ligands were synthesised and complexed with iron (III) chloride. The resulting complexes were applied to the ring opening polymerisation (ROP) of rac-lactide, and Fe(A)Cl gave highly isotactic PLA with an elevated melting temperature. The kinetics and mechanism of the reaction were studied and competition between activated monomer and coordination insertion mechanisms was identified at low temperature.
Iron (III) thiolen complexes were further applied to the coupling of CO2 and epoxides in a comparative study with analogous iron (III) salalen complexes. The five ligands were also coordinated to aluminium and applied to rac-lactide ROP.

In Chapter 3, a series of nine, monophenolate {ONS} ligands were prepared with a range of phenolate, amine and thioether substituents. All ligands were successfully coordinated with diethyl zinc to give homoleptic complexes of the form Zn(Lig)2. The activity of the initiators for lactide ROP varied significantly, which was rationalised through considering the Zn – S bond lengths and the geometrical preference for tetrahedral geometry (τ4’). Zn(J)2 produced PLA rapidly with a turn over frequency (TOF) around 250,000 h-1. The kinetics of the two most active initiators were studied with in-situ Raman spectroscopy. Unfortunately, the high activities seen in small scale reactions could not be replicated but some valuable kinetic insights were obtained. Five {ONS} ligands were coordinated to aluminium and applied to lactide ROP. Relatively high
activity was observed in the melt, and Al(M)2Me was active up to ratios of [LA]/[Al]/[BnOH] = 10000 : 1 : 1.

The chemistry of catalytic PLA degradation was discussed, and all zinc complexes were able to degrade PLA to methyl lactate (Me-LA) in varying amounts over eight hours. Results were consistent with literature values and the degradation of PET was also demonstrated.

In Chapter 4, a series of iron complexes were characterised through cyclic voltammetry (CV) towards the goal of achieving electrochemically switchable co-polymerisation. Iron (III) thiolen and salalen complexes were initially tested and the salalen analogues showed quasireversible redox peaks, suggesting the potential to switch from Fe3+ to Fe2+. The redox potential of the peaks was shown to correlate with polymerisation activity. Three iron (II) {NNNN} complexes were prepared, analysed by CV and applied to lactide polymerisation. Fe(O)Cl2 showed reversible redox behaviour and good activity for lactide ROP. The activity of Fe(O)Cl2 was significantly reduced upon the addition of a chemical oxidant and so it was identified as a good candidate for switchable polymerisation.

Date of Award	12 Dec 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Matthew Jones (Supervisor) & Antoine Buchard (Supervisor)