The development of switchable initiators for rac-Lactide ring-opening polymerisation

This thesis describes various systems to achieve switchability for the ring-opening polymerisation of lactide by means of catalyst design. Heterobimetallic cooperativity is reported whereby a combination of hard/soft metals have been shown to enhance the catalytic activity compared to each independent species. A traditional hard titanium salen bis(isopropoxide) complex in combination with a soft, non-oxophilic silver salt induces catalytic activity at room temperature, where the presence of cation- interactions between the silver cation and the phenolate ring of the ligand have been supported by experimental work and theoretical studies. The nature of the silver salt is important, in particular the anion, where unprecedented salt metathesis reactions occurred for coordinating anions, leading to the isolation and characterisation of a new family of titanium salen complexes. Dinuclear and mononuclear aluminium salen complexes featuring a disulfide or thiomethyl motif respectively were isolated and fully characterised by multinuclear NMR spectroscopy, elemental analysis, and in some cases X-ray diffraction. All complexes could successfully initiate the polymerisation with a high activity and good control, and they all proved to be active under immortal conditions. Homolytic cleavage of the disulfide bond was not achieved, but little differences were observed between the disulfide and thiomethyl complexes, with respect to their catalytic capability. Investigations into azo-functionalised ligands have commenced, as an alternative way to achieve photoswitching between the cis and trans isomers. Novel homoleptic zinc complexes featuring a Schiff base ligand scaffold functionalised with ferrocene were isolated and fully characterised using multinuclear NMR, high-resolution mass spectrometry and elemental analysis. Cyclic voltammetry, spectroelectrochemistry and NMR analysis evidenced oxidation of the ferrocene could occur using stoichiometric amounts of chemical oxidant (AgOTf). The catalyst exhibited good activity ...