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.
Highlights
The ring-opening polymerisation (ROP) of cyclic esters has been intensely studied in recent years as attention has moved towards creating biodegradable plastics that can be produced from renewable sources [1,2]
To provide stability to we investigated metal complexes, we investigated the extra incorporation of metal aminocomplexes, acids into amine bis(phenothe incorporation of amino acids into amine bis(phenolate)
Results and Discussion ethyl of esters of aZrseries of amino acids were used in the synthesis of amine bis(phenolate) (ABP) proligands
Summary
The ring-opening polymerisation (ROP) of cyclic esters has been intensely studied in recent years as attention has moved towards creating biodegradable plastics that can be produced from renewable sources [1,2]. The most commonly studied sustainable polymer is poly(lactic acid) (PLA), which is typically produced through the ROP of lactide (LA). The catalytic ROP of lactide, commonly carried out using homogenous Lewis acidic metal complexes, can be selectively controlled via two possible mechanisms; chain-end control and site control. Chain-end control occurs when the coordination of the incoming monomer is influenced by the chirality of the terminal monomer unit, if the structure of the catalyst/initiator controls the reaction, it is termed site control [4,5]. Induced chirality, from the unique coordination mode of ligand to the metal centre, results in complexes that exhibit chain-end control, with some examples working through a proposed dynamic enantiomorphic site control mechanism [6,7,8]
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