Polymetallic Group 4 Complexes: Catalysts for the Ring Opening Polymerisation of rac-Lactide

Abstract

Five novel air- and moisture-stable polymetallic Ti and Zr amino acid-derived amine bis(phenolate) (ABP) complexes were synthesised and fully characterised, including X-ray crystallographic studies. The reaction of the ABP proligands with Ti or Zr alkoxides has resulted in the formation of polymetallic aggregates of different nuclearity. The steric bulk on the pendant arm of the ligand was found to play a critical role in establishing the nuclearity of the aggregated complex. Sterically, less-demanding groups, such as H or Me, facilitated the formation of tetrametallic Ti clusters, bridged by carboxylate groups, while increased steric bulk (tBu) led to the formation of binuclear μ-oxo-bridged species. The isolated complexes were employed as catalysts for the ring opening polymerisation (ROP) of rac-lactide. Overall, the Ti catalysts were all active with the smaller, bimetallic Ti aggregates exhibiting relatively faster rates. A monometallic, bis(ABP) Zr complex was found to exert remarkable ROP activity, albeit with limited control over the tacticity and molecular weight distribution of the polymer. A further oxo-bridged Zr cluster was shown to display a previously unprecedented trimetallic structure and achieved a moderate rate in the ROP of rac-lactide.

Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers

“…While multimetallic catalyst development has largely focused on bimetallic catalysts (section ), there are also some reports of tri- and tetrametallic systems, most of which are based on Mg, Al, ,,, Ga, Ti, , Zn, − Zr, and rare earth metals. , Rather than providing a comprehensive review, this section focuses on examples that display multimetallic cooperativity or have delivered insight to the polymerization mechanism and are based on tri- and tetranucleating ligand scaffolds, whereas aggregates are discussed in section .…”

Mononuclear Zn(ii) compounds supported by iminophenolate proligands binding in the bidentate (N, O) and tridentate (N, O, S) coordination mode: synthesis, characterization and polymerization studies

“…10 In this regard, the stereocontrolled ROP of rac-LA with single-site discrete metal catalysts has received considerable attention over the last decade. [11][12][13][14][15][16][17] The zinc-based metal compounds, due to their low toxicity, ample availability of zinc metal, and excellent Lewis acidity of zinc(II), have gained considerable attention from researchers worldwide. 18,19 The earliest example of Zn(II) compounds for the stereoselective ROP of rac-LA was reported by Coates and coworkers.…”

Titanium and Vanadium Complexes of Tridentate Phenoxy-Imine and Phenoxy-Amine Ligands and Their Application in the Ring-Opening Polymerization of Cyclic Esters

“…The air- and moisture-stable Ti amino acid-derived amine bis(phenolate) complexes, which were obtained as polymetallic aggregates of different nuclearity, showed low activity in the ROP of rac -lactide. However, smaller binuclear Ti aggregates exhibited relatively better efficiency in that process compared to bigger Ti tetramers [ 39 ]. Upitak et al compared the literature data on a five-coordinated titanium complex with a tridentate bis(phenolate)-amine ligand and six-coordinated analogs with tetradentate ligands.…”