Multi- and heterometallic ProPhenol catalysts for cyclic ester ring-opening polymerisation

Abstract

Plastic is an integral part of daily life, utilised in many sectors spanning from domestic to industrial purposes. However, high societal reliance on plastic products as well as the durability of plastic have led to rising environmental concerns. This has prompted urgent research into sustainable and biodegradable alternatives. Polymers such as aliphatic polyesters (i.e. poly(lactic acid) (PLA), poly(ε-caprolactone) (PCL) and poly(δ-valerolactone) (PVL)) are attractive candidates owing to their desirable properties, biodegradability and biocompatibility. The ring-opening polymerisation (ROP) of cyclic esters is an efficient route for producing well-controlled biodegradable polyesters, typically performed using organometallic catalysts. Heterometallic cooperativity is of key interest in polymerisation catalysis, with many heterometallic complexes outperforming the homometallic counterparts in terms of activity and control. In heterometallic systems, synergistic interactions between different metals depend on the position of proximate metals within a ligand scaffold; this plays a significant role in monomer coordination and the subsequent ring-opening process. The work in this thesis aims to exploit and understand the synergistic effects stemming from heterometal interactions in the ring-opening (co)polymerisation of cyclic esters, for multimetallic complexes based on the ProPhenol ligand scaffold.

While heterometallic cooperativity has delivered enhanced catalyst performance in the ROP of cyclic esters, so far this has been almost exclusively limited to their homopolymerisation. The work in Chapter 2 outlines heterobimetallic cooperativity and trade-offs in the synthesis of PCL-PLA block copolymers, comparing the catalyst performance and control to the homobimetallic analogue. Specifically, this study shows that heterometallic Mg/Zn and Ca/Zn catalysts, based on the ProPhenol ligand, show significant activity enhancements in the synthesis of PCL-b-PLA diblock copolymers, outperforming the Zn/Zn analogue with the activity order Ca/Zn > Mg/Zn > Zn/Zn. The excellent activity accompanies good (co)polymerisation control, generating well-defined PCL-b-PLA block copolymer structures. Interestingly, the catalyst activity order is completely reversed upon the sequential addition of ε-caprolactone as the third monomer in an attempt to generate PCL-PLA-PCL triblock copolymers. This demonstrates that the features of heterometallic “ate” catalysts that boost polymerisation activity can also enhance competitive undesired transesterification processes. Overall, this chapter highlights the importance of the choice of the metal combination to generate well-defined block copolymer microstructures, especially when targeting multi-block copolymer structures.

Heterometallic complexes can be accessed through several different synthetic pathways, including transmetallation and sequential deprotonation routes. Many s-block metals exhibit high catalyst activities in polymerisation owing to the Lewis acidity of the metal (facilitating monomer coordination), yet often give poor control. In contrast, catalysts based on earth-abundant aluminium often show good polymerisation control yet lower activities. Heterometallic catalysts have the potential to combine these features to deliver improved performance. Chapter 3 describes synthetic approaches to prepare novel heterotrimetallic ProPhenol complexes based on inexpensive and low-toxicity metals, by combining an alkali metal (Na or K) with two aluminium centres (AlMe₃, AlEt₃ or Al(iBu)₃). These complexes were characterised in the solution-state by NMR spectroscopy, and in the solid-state via single crystal X-ray diffraction. The molecular structures provide insight into the metal coordination geometry and different bonding modes for the two Al centres; where one Al centre sits in a ‘pocket’ within the ligand scaffold, while the other is coordinated through a dative bond to AlR₃ from a benzylic oxygen of the ProPhenol ligand.

Chapter 4 builds upon the work in Chapter 3, and probes the reactivity of these Na/Al₂ complexes through NMR studies. The abstraction of the datively bound AlR₃ unit was investigated as a route to form bimetallic mono/trivalent Na/Al analogues of the di/divalent Mg/Zn and Ca/Zn complexes reported in Chapter 2. However, the AlR₃ group persisted in the presence of a range of Lewis donors. Therefore, heterotrimetallic Na/Al₂ complexes were directly tested for the ROP of cyclic esters using benzyl alcohol as an initiator, as aluminium analogues of a previously reported Na/Zn₂ ProPhenol complex. PCL, PLA and PVL were successfully generated with good control, albeit at relatively low activity rates (0.068 h⁻¹, 0.053 h⁻¹ and 0.195 h⁻¹). Notably, reactivity studies of the heterotrimetallic Na/Al₂ complex with Lewis base additives provided insight into the different roles of sodium and aluminium metals in cyclic ester polymerisation.