Multimetallic main group prophenol catalysts for homo- and multi-block polyester synthesis and depolymerisation

Abstract

Cyclic ester ring-opening polymerisation (ROP) is an efficient route to prepare biodegradable polyesters, such as poly(lactide) (PLA), poly(ε-caprolactone) (PCL), poly(3-hydroxybutyrate) (PHB) and poly(δ-valerolactone) (PVL). The material properties of these polyesters can be further expanded via the formation of block copolymers such as PCL-block-PLA. Suitable catalysis is key to controlling the (co)polymer microstructure and thus the material properties and applications. Bimetallic complexes have been especially efficient at combining high catalytic activities with exquisite polymerisation control, most of which are homometallic with dimeric structures. Both homobimetallic systems based on dinucleating ligands and cooperative heterometallic complexes can offer equally or even more promising activity and selectivity enhancements in cyclic ester ROP, yet remain underexplored. Based on a dinucleating ProPhenol ligand, this work investigates the influence of the (hetero)metal upon the activity and selectivity in cyclic ester ROP and polyester depolymerisation.

The in situ generation of metal-alkoxide ROP catalysts from metal-alkyl precursors is often a “black box”, where the species so generated are assumed to be the same as the isolated alkoxide complexes. The work described in Chapter 2 shows that this is not always the case. These studies show that the catalyst aggregation state can significantly impact the activity, with the isolated bis-Zn benzoxide complex displaying a 10-fold activity enhancement in rac-lactide (rac-LA) ROP compared to the in situ generated analogue. The bis-Zn ProPhenol catalyst also offers excellent activities and control over the homopolymerisation of ε-caprolactone (ε-CL) and rac-β-butyrolactone (rac-β-BL). The stability of this isolated catalyst controls chain exchange and back-biting, enabling the one-pot synthesis of multi-block polyesters and the first selective preparation of a PCL-block-PLA-block-PHB terpolymer.

The synthesis, characterisation and polymerisation studies of the first heterotrimetallic Na/Zn2 and K/Zn2 ProPhenol complexes are reported in Chapter 3. These complexes deliver “best of both” in cyclic ester ROP, combining the outstanding activity (Na/K) and good control (Zn2) of homometallic analogues. Detailed NMR studies and density-functional theory (DFT) calculations show that the Na/Zn2 and K/Zn2 complexes retain their heterometallic structures in the solution-state. The K/Zn2 analogue displays exceptional activity in rac-LA ROP (kobs = 1.7 x 10-2 s-1), giving activities five times greater than the Na/Zn2 complex. These versatile catalysts also display outstanding performance in ε-CL and δ-valerolactone (δ-VL) ROP.

Chapter 4 builds upon the work in Chapter 3 and extends heterometallic cooperativity in ROP to unexplored divalent metal heterocombinations. The Mg/Zn and Ca/Zn ProPhenol complexes outperform their homobimetallic counterparts in rac-LA and ε-CL, with the Ca/Zn analogue exhibiting the highest activity, converting 48 equiv. rac-LA in 5 s and 380 equiv. ε-CL in 30 s at room temperature. The activity enhancements are credited to “ate”-type activation, concurrently amplifying the polarity of the Zn-ethyl/benzoxide bond and Lewis acidity of the Ca centre, thus accelerating the nucleophilic attack and monomer coordination in ROP. These heterometallic complexes also display unusual two-step kinetic plots in rac-LA ROP, which are attributed to catalyst preference for coordination and insertion of L-LA and D-LA stereoisomers of rac-LA, based on detailed kinetic and DFT studies.

The concept of accessing heterometallic cooperativity via incorporation of Group 2 inorganic salts into Group 1 homometallic ROP catalysts is demonstrated for the first time in Chapter 5. This methodology provides a simple yet effective strategy for accessing heterometallic cooperativity in ROP. The activity enhancements obtained with salt additives are compared to isolated heterometallic ProPhenol catalysts. Both approaches display similar activity trends, with the K/Mg and K/Ca heterocombinations converting > 85 equiv. rac-LA in just 5 s at room temperature. To the best of our knowledge, these systems are the most active heterometallic rac-LA ROP catalysts reported to date. NMR spectroscopy is employed to probe the catalyst solution-state structures, highlighting the complexity of heterometallic cooperativity in solution.

Chapter 6 describes the preliminary results for the selective depolymerisation of polyesters using the Na/Zn2, Ca/Zn, Mg/Zn and bis-Mg ProPhenol complexes. These systems depolymerise PCL and PVL to reform monomers in the presence of just one equivalent of rac-LA or rac-β-BL. The selective depolymerisation is attributed to the combined effect of ligand-mediated transesterification of the inserted PLA or PHB, and the presence of highly Lewis acidic metals (Na, Ca, Mg) that can coordinate the propagating PCL or PVL chains. A depolymerisation mechanism is tentatively proposed based on experimental and NMR spectroscopic studies.