Initiators for the production of PLA and terpene derived bioplastics

Abstract

As we experience the detrimental effects of plastics on the environment, preparing new bio-derived alternatives has become an important challenge in both academia and industry. Bio-derived polylactide (PLA) is a promising candidate, being biodegradable and biocompatible, although challenges include the synthesis of isotactically enriched PLA from racemic monomer feeds through utilisation of stereoselective initiators, and copolymerisation with other bio-derived monomers to prepare fully bio-based copolymers. In Chapter 1 the synthesis of PLA and current research into stereoselective initiators for the polymerisation of rac-lactide (LA) are discussed, with a focus on lithium, magnesium, zinc, aluminium and zirconium complexes. Routes to novel substituted ε-caprolactone and ε-caprolactam monomers from abundant terpene feedstocks are also introduced, in the context of preparing fully bio-based copolymers.In Chapter 2, the synthesis and characterisation of multinuclear zirconium, lithium, magnesium and zinc complexes with amine bis(phenolate) ligands is discussed. These complexes were applied as initiators in the ring-opening polymerisation (ROP) of rac-LA. Most initiators resulted in atactic PLA, although in some cases PLA with a slight heterotactic bias (Pr <0.58) was obtained. Two different lithium tetrametallic structures were characterised by X-ray crystallography. Dimeric magnesium complexes were active for ROP under solvent-free and solution conditions, with undesirable transesterification occurring and atactic PLA produced in all cases. Magnesium complexes were also trialled for ring-opening copolymerisation of epoxides and anhydrides. Interesting tri- and tetranuclear species were observed with zinc, which exhibited different levels of aggregation in solution. Finally, attempts to prepare a new bis(phenolate) ligand with a bi-isoindolene backbone are reported.In Chapter 3, the synthesis of salen ligands and their complexation to aluminium to prepare mononuclear and dinuclear complexes is discussed. Complexes were characterised in the solid-state and in solution. A dinuclear complex with a 1,8-naphthalene backbone was found to be highly active for ROP of rac-LA even at room temperature, producing isotactically enriched PLA (Pm <0.82). This is a rare example of a LAlMe2 complex imparting stereoselectivity to this process. DFT calculations support potential cooperativity occurring between metal centres, and results are compared with analogous mononuclear and dinuclear complexes where cooperativity is not possible. The preparation of aluminium and lithium complexes bearing imidazolidine bis(phenolate) ligands is also reported, with some interesting structures isolated and characterised.In Chapter 4 the preparation of a substituted ε-caprolactone monomer from a terpene feedstock, β-pinene, and its polymerisation to a novel, low Tg, aliphatic polyester is reported. The copolymerisation of this monomer with lactide is discussed. The preparation of a substituted ε-caprolactam from β-pinene is also described. Chemoselective synthetic routes to novel unsaturated substituted ε-caprolactone monomers are discussed, and the synthesis of a polyester bearing pendant isopropenyl moieties is reported. Initial attempts at post-polymerisation modification of unsaturated polymers highlights their potential for post-polymerisation modification in the development of new functional materials.Synthetic procedures and characterisation for ligands, complexes, monomers and polymers are given in Chapter 6, along with details of general experimental methodologies and techniques.

Date of Award	17 Aug 2018
Original language	English
Awarding Institution	University of Bath
Supervisor	Matthew Jones (Supervisor) & Davide Mattia (Supervisor)