Applications of Rare Earth Complexes in the Ring- Opening Copolymerisation of Epoxides and Anhydrides

Abstract

This thesis describes the synthesis of a series of new Group 3/Lanthanide complexes bearing a series of Salen-type ligands, and related complexes bearing pyridyl donor which have not been previously reported for Group 3/Lanthanide metals. These complexes were studied for their effectiveness as catalysts for polymerisation reactions, with a particular focus on the co- polymerisation of epoxides and cyclic anhydrides. Chapter 1 provides an overview of polymer chemistry and highlights the most significant environmental issues affected by the use of unsustainable plastics. It discusses how ring- opening polymerisation of cyclic monomers can be used to produce hydrolysable polyesters. This chapter also discusses the role of bio-degradable polyesters. This chapter includes a discussion of previously reported metal-based catalysts for the production of biodegradable polyesters. Discussion is also provided, relating to the chemistry of complexes bearing pyridyl donors and, why rare earth metals were chosen as catalysts for these studies. Chapter 2 discusses the synthesis and characterisation of Salen- type ligands and the preparation of the monometallic and bimetallic rare earth complexes, as well as paramagnetic lanthanide metal complexes. Salen ligand and Salen-type ligands which bear an additional pyridyl donor (named as Salpy ) were prepared and some of their identities have been supplemented by solid state analysis. Three types of rare earth complexes have been prepared: chloride, alkyl, and alkoxide. These complexes were evaluated using NMR spectroscopy, which includes a discussion of the coordination of Salen and Salpy ligands to the metal center through the imine nitrogens and the phenolic oxygens and thus offer a (usually) planar N2O2 core, as well as via coordination of the pyridyl donor. Chloride complexes were well-defined, alkoxide less so, and alkyl complexes were well-defined for smaller radii metals. Chapter 3 Investigates the efficacy of Group 3/Lanthanide Salen-type complexes which contain C2,C3,C6 and a pyridine backbone (backbone between two imines) as catalysts for the ring-opening copolymerisation of epoxides and anhydrides. Salen complexes are able to perform copolymerisation with high conversions, selectivity, and moderate molecular weight under various conditions. A range of monomers were employed, thereby diversifying the range of available polymers (and associated properties). Bimetallic complexes were active catalysts for the copolymerization of polyester without cocatalyst with moderate selectivity. The copolymerization reaction was evaluated under various conditions for example: including with and without Lewis base, different types of Lewis base, and the polymers were investigated using 1H NMR spectroscopy, MALDI-ToF-MS, IR spectroscopy, and gel permeation chromatography. Chapter 4 describes Salpy complexes bearing a pyridyl ring ligands described in Chapter 2 as catalysts for epoxide/anhydride copolymerisation under various conditions. This chapter probes the ring opening copolymerisation with various metals, coligands, and phenoxide substituents. Studying the effect of the steric and electronic environment imposed by Salpy ligand on the polymerization system. Comparisons are made between the data obtained for Salen complexes in terms of reactivity, substrate scope, and GPC data. Polymers were evaluated by 1H NMR spectroscopy, IR and by MALDI-ToF-MS.