Structurally well-defined group 4 metal complexes as initiators for the ring-opening polymerization of lactide monomers

Abstract

Polylactide (PLA) is an attractive polymeric material due to its origin from annually renewable resources and its biodegradability. The ring-opening polymerization (ROP) of lactide initiated by Lewis acidic and oxophilic metal-based catalysts constitutes the method of choice to access PLA in a controlled and stereoselective manner. The design and synthesis of ligand-supported metal complexes to act as effective ROP initiators of lactide monomers have been the subject of numerous investigations over the past decades. In view of their oxophilic nature, well-defined group 4 metal complexes supported by polydentate supporting ligands have appeared as active initiators for lactide ROP. This perspective summarizes various classes of structurally well-defined group 4 metal initiators developed for lactide ROP. It also provides observed trends regarding their catalytic performance. Whenever appropriate and possible, catalyst structure-ROP performance (i.e. activity, control and stereoselectivity) relationships are rationalized.

Computational Studies on the Selective Polymerization of Lactide Catalyzed by Bifunctional Yttrium NHC Catalyst

“…Particularly, divalent metal catalysts bearing a monoanionic auxiliary skeleton are advantageous due to their low cost, and group 3 metal complexes are also exquisite initiators in the synthesis of PLAs by the ROP of rac-lactide [18,19]. The explicit group 4 metal complexes bearing polydentate ligands have also been proved to be active initiators for the synthesis of PLA [20]. As compared to the other metal-based catalysts for the synthesis of PLAs by ROP, titanium complexes are not high in terms of activity and stereoselectivity, but their low toxicity is an attractive property for the synthesis of PLAs [21].…”

Group 4 metal complexes of Trost's semi-crown ligand: synthesis, structural characterization and studies on the ring-opening polymerization of lactides and ε-caprolactone

“…The zirconium and hafnium complexes (2 and 3) have shown stereocontrol in the ring-opening polymerization of racLA, producing PLA with isotacticenriched stereoblock microstructures (P m up to 0.78) ( Table 2, 10 Entries 2 and 3). 28,39 So, the results disclosed that the ligand architecture and the nature of metal centers could be accounted for the stereocontrol in ring-opening polymerization of rac-LA. This lack of stereocontrol by the Ti complexes can be explained by Rzepa et al's analysis of the origin of stereocontrol from the coordination of the growing polymer chain to the metal center 15 and Okuda et al's similar observation on the group 4 metals based on Salan complexes.…”

“…Indubitably, the most important breakthrough was 50 made in 1996, when Spassky et al 17 used enantiopure {(R)-SalBinap}Al(OR) (R)-1 for the ring-opening polymerization of racLA and highly isotactic and crystalline PLA was obtained. 28 Recent advances in catalyst design inspired us to develop a new catalytic system for the ring-opening polymerization of lactide. AlO i Pr catalyst which exhibited the highest stereoselectivity and afforded substantially isotactic PLA with a P m of 0.90.…”

Structurally diverse dysprosium and yttrium complexes containing an amine-bis(benzotriazole phenolate) ligand: Synthesis, characterization and catalysis for lactide polymerization

“…Such biodegradable polyesters also have been regarded as a promising replacement of traditional petrochemical based polymers. Consequently, ROP using various catalytic systems with the single-site metal catalysts modified by diverse ancillary ligands has been extensively investigated, and several of them are able to achieve great catalytic activities in a controlled manner as well as high stereoselectivity for the utilization of racemic monomers [3][4][5][6][7][8][9][10].…”