Zirconium vs Aluminum Salalen Initiators for the Production of Biopolymers

Abstract

Herein we report the synthesis and full characterization (NMR and solid-state structures) for a series of Zr(IV), Hf(IV), and Al(III) salalen complexes, together with salen bimetallic counterparts. With the salalen ligand, 1H 2 , monometallic complexes were observed in solution and solid state. Complex Zr(1)(O i Pr) 2 was able to produce isotactic polylactide (PLA) from rac-lactide (P m up to 0.85) in solution at 50 °C; in the melt (130 °C) this reduced to ca. 0.75. Al(1)Me was significantly less active and produced PLA with only a very modest isotactic enchainment (P m ≈ 0.6). Zr(1)(O i Pr) 2 was also able to produce copolymers with lactide and ε-caprolactone, producing copolymers of a "blocky" nature.

Highly active Mg(ii) and Zn(ii) complexes for the ring opening polymerisation of lactide

“…42 A range of lanthanide initiators have also been applied to the ROP of rac-LA; [43][44][45][46][47][48][49][50][51][52][53] in particular Williams and co-workers have demonstrated highly active Y(III) and Lu(III) systems amenable to the preparation of isotactic and heterotactic PLA. 43,44,47 Group IV metal complexes have also garnered interest, due to their high activity and robustness, [54][55][56][57][58][59][60][61][62][63] with both isotactic and heterotactic PLA being achieved with Zr(IV). 64,65 Whilst the majority of these are solution based studies there is a need for catalysts with earth abundant and benign metals that work under challenging melt conditions as these are the industrial conditions.…”

Group 4 Metal Complexes of Phenylene–Salalen Ligands in rac‐Lactide Polymerization Giving High Molecular Weight Stereoblock Poly(lactic acid)

“…Non-symmetrics equential tetradentate ligandsc an wrap aroundo ctahedral metal centers in four different geometries: fac-fac helicalw rapping, mer-mer planar wrapping, and fac-mer/mer-fac semi-helicalw rappings. [31] rac-Lactide polymerization activity Indeed, previously reported salalen ligands based on the (non-chiral and chiral) aliphatic two-carbon bridges, [18][19][20]24] were found to wrap in this mode, as supported by crystallographic studies.…”

“…[27] Chromiumc omplexes of ap henylene-salalen ligand featuring tert-butyl phenolate substituents (Lig 1 H 2 of the current work) were previously introducedb yN ozaki and were found to be active in epoxide/carbon dioxide copolymerization. [31] Results and Discussion Group 4m etal complexes of such phenylene-salalen ligands were previously reporteda sc atalysts for olefin polymerization by our group.…”

“…[18,20] However,s uch at endency was not found for the complexes reported herein (nor for (Lig 1 Zr(O-isopropyl) 2 reported by Jones and co-workers). [31] rac-Lactide polymerization activity…”

“…[30] During the preparation of this manuscript, az irconium complex of Nozaki's phenylene-salalen ligand was published by Jones and co-workersa nd was reported to lead to isoselective polymerization of rac-LA. [31]…”