Lithium Half‐Salen Complexes: Synthesis, Structural Characterization and Studies as Catalysts for rac‐Lactide Ring‐Opening Polymerization

Abstract

Seven lithium complexes supported by sterically and electronically diverse phenoxyimine ligands were synthesized and characterized by X-ray diffraction, NMR spectroscopy and elemental microanalysis. These complexes show high activity (k obs � 7.43 × 10 À 2 s À 1 ) for rac-lactide ring-opening polymerization (ROP) in the presence of co-initiator benzyl alcohol (BnOH), with the exception of Li4 which features an unusual polymeric ladder structure. Overall, the catalyst activity correlates to the aggregation state; the catalysts with low aggregation states display increased propagation rates attributed to improved metal accessibility and kinetic mobility. The nature of the ligand substituents and solvent influence the catalyst aggregation in both the solid-and solution-state. While the lithium complexes can initiate rac-lactide ROP without BnOH, the addition of this co-initiator significantly increases the polymerization rate by switching the mechanism from a coordination-insertion to an activated monomer pathway, changing the resultant poly(lactic acid) architecture from cyclic to linear.

Incorporating Sodium to Boost the Activity of Aluminium TrenSal Complexes towards rac‐Lactide Polymerisation

“…Sodium (and indeed other alkali metal) complexes typically display very high catalytic activities in rac ‐LA ROP, yet often give poor control over the molecular weight and dispersity [17] . Previous studies have suggested that alkali metal complexes can simultaneously operate through two different LA ROP mechanisms (coordination‐insertion and activated monomer) in the presence of an exogeneous alcohol, a factor which may be partially responsible for the poor polymerisation control [15,16] . Indeed, poor control occurs with [LNa 3 ] ; a bimodal distribution and broad dispersities are observed after 50 s (Table 1, entry 4; Table S5, entries 9–15).…”

“…To determine whether initiation with [LNaAlMe] /BnOH occurred via an activated monomer mechanism (where nucleophilic attack of LA occurs from an exogeneous alcohol) or a coordination insertion mechanism (where nucleophilic attack occurs from a metal‐alkoxide unit), [15] the reaction between [LNaAlMe] and BnOH (1 : 1 ratio) was investigated. For consistency with the polymerisation conditions, the stoichiometric reaction was performed in toluene solvent at room temperature and then analysed in CDCl 3 solvent due to the poor solubility of [LNaAlMe] in d 8 ‐toluene.…”

“…Alkali metals and aluminium are attractive choices in LA ROP due to their low cost, low toxicity, lack of colour and Earth abundance, yet both currently have limitations. Alkali metal complexes tend to display exceptionally high catalyst activities yet often give poor polymerisation control [15–17] . In contrast, aluminium complexes often display good (stereo)control but with relatively low catalytic activities [18] .…”

Homoleptic and Heteroleptic Substituted Amidomethylpyridine Iron Complexes: Synthesis, Structure and Polymerization of rac‐Lactide

“…The presence of pyridine within the heteroleptic complexes appeared to increase the rate of polymerization while giving rise to the formation of a broad mixture of polymer structures including low M n of cyclic PLA. At this stage, it is difficult to propose a mechanism because the polymerization of lactide with these systems could be based on a coordinationinsertion or activated monomer process, which could also involve one of the two ligands (e. g. Li, [37] Na [38] or Zn [36] systems). Also, we cannot exclude that the mechanism could occur by zwitterionic ROP (e. g. N-heterocycle carbene, [39] Ce(III)À NHC complexes, [40] ) or a ring-expansion polymerization process (e. g. Ln, [41] Bi, [42] Sn [43] complexes).…”

“…At this stage, it is difficult to propose a mechanism because the polymerization of lactide with these systems could be based on a coordination‐insertion or activated monomer process, which could also involve one of the two ligands ( e. g . Li, [37] Na [38] or Zn [36] systems). Also, we cannot exclude that the mechanism could occur by zwitterionic ROP ( e. g .…”