Switchable catalysis to make multi-block copolymers

Abstract

This thesis describes the development of methods to selectively generate block copolymers using a single catalyst and multiple different polymerisation cycles referred to as “switchable’’ catalysis. It exploits the ability of various dinuclear zinc catalysts to selectively catalyse both ring opening copolymerisation of epoxides and carbon dioxide or anhydrides and ring opening polymerisation of lactones and lactide. The catalyst selectivity is proposed to arise from different rates of insertion into the key intermediate, a zinc alkoxide species, and from the different stabilities of the linkages formed. Chapters 2 and 3 describe the new switchable catalysis to link the ring opening copolymerisation of epoxides and carbon dioxide with the subsequent ring opening polymerisation of lactones (ε-caprolactone, ε-decalactone, δ-valerolactone). This method allows the formation of well-defined block copolymers. The block copolymers are characterised using in-situ infrared spectroscopy, nuclear magnetic resonance spectroscopy including diffusion ordered spectroscopy analysis and size exclusion chromatography analysis. Block copolymers are synthesised with a range of compositions from 1:0.5 – 1:10 carbonate:ester. The thermal properties of the copolymers are highly dependent on the proportion of polyester. The block copolymers containing ε-caprolactone and ε-decalactone were found to act as flexible plastics by tensile mechanical measurements. In contrast, the block copolymer containing δ˗valerolactone was a soft, inflexible plastic. Chapter 4 describes the use of the switchable catalysis method to form multiblock copolymers compromising of up to seven blocks. The dizinc catalyst is able to switch between ring opening copolymerisation of cyclohexene oxide and carbon dioxide and ring opening polymerisation of ε-caprolactone reversibly and exclusively forms a multiblock copolymer. Chapter 5 describes the application of the method to a mixture of four monomers: epoxides, carbon dioxide, anhydrides and lactones or lactide. The dizinc catalyst selectively forms pentablock copolymers with predictable compositions and block sequences. The structure of the pentablock copolymer depends on whether lactone or lactide is applied as the monomer. Overall, this thesis presents a novel type of catalyst selectivity and a means to control it. The switchable catalysis is demonstrated with dizinc catalysts, and is used to form a variety of multiblock copolymers from mixtures of monomers.