Investigations into the well-controlled stereoselective ring-opening polymerisation of lactide
Polylactide (PLA) is a biodegradable and biocompatible alternative to traditional petrochemicalbased polymers. Synthesised by the ring-opening polymerisation of lactide (LA), the dehydrated form of lactic acid, PLA sits within a renewable cycle, and can be used in many commodity and biomedical applications. The intrinsic stereochemistry of LA can lead to a variety of polymer microstructures, and current industrially used initiators allow no control over this. Within this thesis a series of investigations into the use of amine tris(phenolate) metal complexes as stereoselective initiators for the ROP of LA are discussed. Chapter 1 introduces the field of ring-opening polymerisation (ROP) via a coordination insertion mechanism, presents previously reported initiators, and examines the influence of stereocomplexation on the physical properties of PLA chains. This introductory chapter also includes an in-depth review of recent developments in poly(phenolate) complexes of Group 4 and rare earth metals. Chapter 2 describes the synthesis of a series of isopropoxide and zwitterionic Group 4 complexes featuring the amine tris(phenolate) ligand motif, with emphasis on the effect of ligand variation on complex structure. The potential of the resulting complexes as initiators for the ROP of LA is also investigated, with in depth kinetic studies allowing for a proposed diastereoselective mechanism of stereocontrol. Chapter 3 details the synthesis of a series of borohydride and amide Group 4 amine tris(phenolate) complexes, and investigates their potential as initiators for the ROP of rac-LA. Chain-end analysis and kinetic studies are included, providing the basis for mechanistic discussions. Chapter 4 focuses on the synthesis of stereoblock PLA materials in a stereospecific manner, and co-block PLA-PEG materials by the reinitiation of short-chain macroinitiators. The effect of stereocomplexation on the thermal properties of these materials is investigated. Chapter 5 concerns the synthesis of a series of isopropanol and zwitterionic rare earth complexes featuring the amine tris(phenolate) ligand motif. The potential of these complexes as initiators for the ROP of rac-LA is also investigated, but only slight stereocontrol was observed in selected cases. Chapter 6 provides details of procedures employed in the synthesis of ligands, complexes and polymers within this thesis, as well as details of the analytical techniques used in their characterisation.