Exploring cooperativity in multimetallic main group catalysts for polyester synthesis via ring opening (co)polymerisation
Creators: name: Lowy, Phoebe Alice
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: biodegradable plastics
polylactic acid
PCL
cyclic esters
metal catalysts
cyclic ester polymerisation
bimetallic catalysts

description: Plastics are essential to present society. However, due to depleting resources and the substantial impact of plastic pollution, there has been a significant drive to develop sustainable alternatives to fossil fuel derived commodity plastics. This sustainability drive has led to the development of new commercial production processes, including the production of polyesters via cyclic ester ring-opening polymerisation (ROP) and polycarbonates via ring-opening (co)polymerisation (ROCOP) of carbon dioxide and epoxides. Derived from biomass, poly(lactic acid) (PLA) is an attractive biodegradable polymer with the potential to replace some petrochemical-based plastics. PLA is typically prepared through the ROP of lactide (LA), which requires the use of a catalyst. Salen ligand-supported aluminium complexes are recognised as efficient and versatile catalysts for LA ROP, displaying some of the best stereochemical control over the PLA microstructure reported to date. However, the catalytic activity of Al-salen complexes is often relatively slow compared to other organometallic catalysts. The use of multinuclear catalysts is emerging as a method of enhancing organometallic catalyst performance in LA ROP, where multinuclear complexes (with two or more metals held in close proximity) often outperform the mononuclear analogues in terms of reactivity, whilst maintaining good polymerisation control. However, this concept is underexplored in Al-salen catalyst development. In contrast to the extensive library of symmetrical salen ligands, the preparation of asymmetric salen ligands is limited due to multiple, complex synthetic steps. Salen-type ligands have been used extensively to complex transition and main group metals and applied as catalysts in both cyclic ester ROP and epoxide ROCOP with CO₂ or anhydrides. Within Chapter 2, a variety of synthetic routes are investigated for the preparation of mono-and multi-nuclear salen ligand scaffolds. The stability of intermediate imine bonds, and their tolerance to hydrolysis, seems to be key for the isolation of salen ‘half-units’, required for the preparation of pure asymmetric salen ligands. The design, synthesis, and characterisation of a salen-hybrid ligand, combining a phloroglucinol bridging unit with three salenlike coordination environments, is described. Notably, the tris-salen ligands display keto-enamine tautomerisation, resulting in the loss of aromaticity. NMR studies of all species as well as characterisation by single crystal X-ray diffraction of the salophen ligand provide evidence of this tautomerisation in the solution and solid state respectively. Multimetallic catalysts are attractive due to their enhanced catalyst performance in cyclic ester ROP, often outperforming monometallic analogues in terms of reactivity and/or polymerisation control. Such enhancements are generally attributed to “multimetallic cooperativity”, yet the origin of such cooperativity often remains unexplored. In Chapter 3, a trinuclear Al-salen complex has been synthesised, characterised and tested as a catalyst for LA ROP. The analogous mononuclear Al-salen complex was also prepared for benchmarking purposes. The trinuclear complex contains three Alsalen subunits in close proximity, and displays good LA ROP rates (kₒ₈ₛ = 9.7 x 10⁻³ min⁻¹, [cat]:[BnOH]:[rac-LA] = 1:1:100, toluene, 70 ℃). These detailed kinetic studies show that the monoand trimetallic catalysts display similar rates ‘per-activated-metal’ centre but the trimetallic L1(AlMe)3 outperforms the monometallic analogue L2(AlMe) when compared ‘per-catalyst’. Detailed kinetic and spectroscopic studies into the origins of cooperativity revealed that neighbouring metals play a key role in the initiation step resulting in the observed cooperativity. Overall, these studies highlight the importance of understanding the reaction conditions in order to accurately define true multimetallic cooperativity in a system. Chapter 4 builds upon the work in Chapter 3, leading to the synthesis and characterisation of two new trimetallic aluminium and zinc complexes based on the same ligand scaffold. Their monometallic analogues were also prepared for benchmarking purposes. The Al complexes, bearing an ethyl group are expected to be more BrØnsted basic and thus more prone to alcoholysis than their Al-Me counterparts - a key step in LA ROP highlighted in Chapter 3. Although under some conditions both the mono- and tri-metallic AlEt complexes outperformed their AlMe analogues, LA ROP under optimised conditions (mentioned above) showed that the rates of polymerisation were similar for L1(AlEt)₃ and L1(AlMe)₃ (kₒ₈ₛ = 11.0 x 10⁻³ and 9.4 x 10⁻³ min⁻¹, respectively). These studies highlight that the Al-Et or Al-Me units have little impact the polymerisation rate. Furthermore, this chapter explores the use of tri-metallic complexes as catalysts in 𝜀�-CL ROP as well as epoxide/anhydride ROCOP. Heterometallic cooperativity can enhance the activity and selectivity of homometallic catalysts and has been exploited in a range of applications. The concept of accessing heterometallic cooperativity via incorporation of Group 2 inorganic salts into Group 1 homometallic ROP catalysts based on a ProPhenol scaffold has recently been reported. The combination of simple salts and mono- or trimetallic complexes previously reported (L1(AlMe)₃, L1(Zn)₃, L2(AlMe) and L2(Zn), is explored in Chapter 5 to confirm the general applicability of this approach. Additionally, simple salts exploited as catalysts in LA ROP and ROCOP have displayed reasonable activities on their own. Therefore, in this chapter, these simple, cheap non-toxic salts are investigated as catalysts in cyclic ester ROP. These salts, synthesised via simple synthetic routes, are more industrially relevant than typical multimetallic, highly air- and moisture- sensitive catalysts. Therefore, within Chapter 5 a range of conditions are explored which target industrially relevant settings; high monomer loadings, non-anhydrous conditions using technical grade LA and bulk conditions.