Towards Benign Initiators for the Production of Sustainable Plastics

Abstract

The environmental persistence of petrochemical-derived polymers has created an increasingly urgent need to develop inexpensive, compostable alternatives, with thermal and mechanical properties compatible with use across a variety of sectors. Various compostable aliphatic polyesters can be prepared via the metal-catalysed ring-opening polymerisation of cyclic esters. In particular, the solvent-free polymerisation of lactide offers an industrially relevant route to the large scale delivery of an inexpensive, bio derived material suitable for a broad range of applications. It is desirable to develop initiators for the polymerisation of lactide and other cyclic esters that are highly active, biochemically benign, robust, and inexpensive, to maximise the relevance of the resulting material to food-contact and biomedical applications. Currently, it is appropriate both to deliver protocols for the non-stereoselective polymerisation of stereopure L-lactide, directly applicable to contemporary industrial processes, and to undertake more fundamental mechanistic studies of the polymerisation of cyclic esters. The latter area of work may inform the design of stereoselective initiators in the future, for the efficient production of aliphatic polyesters with enhanced properties.

Chapter 1 of the current work introduces the various monomers relevant to the polymerisation studies described herein, as well as reviewing the application of biochemically benign and earth-abundant metals to the polymerisation of cyclic esters. In particular the structure and catalytic activity of metal amine tris(phenolate) complexes will be discussed.

Chapter 2 is concerned with the preparation and reactivity of several amine tris(phenolate) supported cationic niobium(V) species, including the synthesis of an unprecedented lactone adduct of a metal alkoxide complex. The application of those systems to the polymerisation of E caprolactone is also discussed, considering their general relevance to the coordination-insertion mechanism of ring-opening polymerisation.

In Chapter 3, the scope of synthetic and catalytic studies described in Chapter 2 is expanded, to encompass both tantalum(V)-based systems, and the polymerisation of other lactones. In particular, the mechanism of initiation by lactone adducts of metal alkoxide complexes is considered.

Chapter 4 describes the development of a liquid formulation of a robust and highly-active, zwitterionic zirconium(IV) amine tris(phenolate) catalyst, suitable for the industrial solvent-free polymerisation of lactide. Kinetic studies were carried out, monitored in-situ via ATR-FT-IR spectroscopy, to assess the activity of the catalyst formulation and determine its industrial relevance.

Chapter 5 describes further optimisation of the catalyst formulation method described in Chapter 4, and application of the resulting systems to the ring-opening polymerisation of lactide on a large-scale, under industrially relevant conditions.

Date of Award	1 May 2020
Original language	English
Awarding Institution	University of Bath
Sponsors	Total-Corbion
Supervisor	Matthew Jones (Supervisor), Matthew Davidson (Supervisor) & Antoine Buchard (Supervisor)