Abstract
Plastics are found everywhere, from electronics to materials, and while these are a revolutionary material, they go hand in hand with significant levels of environmental pollution. To this end, renewable plastics such as aliphatic polyesters, sourced from natural feedstocks with the potential to degrade into non-toxic components, are necessary to replace the non-degradable, fossil fuel-derived plastics; for this reason, poly(lactic acid) (PLA) alone is now produced on a multi-tonne scale each year.Despite this, the synthesis of polymers such as PLA are plagued with their own challenges, including residual cytotoxic metals from the catalyst which could render the plastic unsuitable for use in biomedical or food packaging applications. Throughout this thesis, renewed efforts to create robust, biocompatible heterogeneous catalysts are explored, leading to plastic with low levels of metal content and offering new strategies for the recovery and reuse of these catalysts, in an effort to contribute to a circular plastic economy.To begin, Chapter 1 will explore the structure, properties and synthesis of PLA and the current homogeneous catalysts which offer rapid, controlled ring-opening polymerisation (ROP), and discuss their shortcomings. The benefits of heterogeneous catalysis are outlined, along with a summary of heterogeneous catalysts for ROP, and how these can be implemented into a flow reactor for continuous production of polymer. Chapter 2 offers an overall summary of how these topics feed into the thesis work.Chapter 3 covers the published work (I. C. Howard, C. Hammond, A. Buchard, Polym. Chem., 2019, 10, 5894-5904), describing the development of a series of metal complexes, immobilised onto an inert poly(styrene) (PS) support. Excellent, rapid conversions within 55 minutes are observed in the solvent-free ROP of L-Lactide (LA), with Mn up to 35 000 Da and minimal leaching of the metal into the crude polymer (335 ppm). Catalyst reuse is also explored, with up to 7 reuse cycles, suggesting these systems as promising reusable catalysts for the industrial production of metal-free renewable polymers.These catalysts are used in Chapter 4, covering the ROP of other lactones such as ε-caprolactone and ε-decalactone and extending to the copolymerisation of these with L-LA. Investigations into copolymer microstructure reveal the formation of amorphous copolymers with random distribution of the monomers throughout the copolymer when a one-pot method is used, while block copolymers form when a sequential monomer addition method was utilised. ABC and ABA triblock co-polymers can also be accessed, highlighting the potential to create a thermoplastic elastomers (TPEs) with these heterogeneous catalysts.Chapters 5 and 6 examine the ROP with heterogeneous organocatalysts. Chapter 5 first looks at the ROP with commercially available PS-immobilised organocatalysts in the melt ROP, with comparisons to their homogeneous analogues. Coupling these with an immobilised urea (PS-U) to improve the activity of the bases is investigated: compared to other bases, an improvement in conversion is possible when PS-U is combined with imidazole (from 64 to 73% conversion). Proximity of the base to the PS-U is necessary to enable cooperativity, with larger bases struggling to access the PS-U active site for bifunctional ROP. This indicates that correct selection of the base and urea combination is needed to improve conversion; the nature of the urea catalyst and the base must be well matched to achieve cooperation between the two components.
Chapter 6 considers the solution-phase ROP using the immobilised (thio)urea catalysts coupled with KOEt, creating a semi-heterogeneous bi-component system. Formation of the active thioimidate ion is confirmed and evidence of modification in base activity by the PS-(T)U is presented, achieving increased levels of ROP control due to the better coordination between the two components in solution-phase. Combination of KOEt with PS-U improved the ROP control compared to the base on its own, with a more reliable increase in conversion over time, reaching 87% in 30 minutes, with a monomodal SEC trace.
Finally, Chapters 7-8 explore the beginnings of an effort to implement these heterogeneous systems into a continuous flow reactor. Chapter 7 concerns the solution-phase ROP in a microreactor is explored, using homogeneous organocatalysts, working towards the rapid generation of molecular weight libraries to en- able information encryption in unique molecular weight distributions; this work was conducted in collaboration with Professor Tanja Junkers at Monash University, Australia. Judicious choice of base, solvent, concentration and temperature (among other parameters) are all explored, achieving 97% conversion of LA in 2 minutes at 35 °C (ÐM 1.25, [LA]:[DBU]:[I] = 50:1:1, [LA] = 1 mol L−1 in DCM). This is then followed by an increase in reactor scale in Chapter 8, built to accommodate heterogeneous catalysts in a packed-bed; reactor development and initial results from the homogeneous ROP in solution phase – used as a model for future heterogeneous work – are outlined.
| Date of Award | 11 Oct 2021 |
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| Original language | English |
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| Supervisor | Antoine Buchard (Supervisor) |