Exploring ligand substituent effects on stereoselective polymerization of racemic lactide using aluminium salen-type complexes

• Published in: Polymer chemistry Vol. 14; no. 18; pp. 2174 - 218
• Main Authors: Peng, Zengping, Ahmed, Hassan, Xu, Guangqiang, Guo, Xuanhua, Yang, Rulin, Sun, Hongguang, Wang, Qinggang
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 09.05.2023
• Subjects: Aluminum; Asymmetry; Catalysts; Ligands; Polymer chemistry; Polymerization; Stereoselectivity
• ISSN: 1759-9954; 1759-9962
• DOI: 10.1039/d3py00228d

A number of unreported aluminium complexes bearing chiral cyclohexylsalen ligands were prepared and utilized for the asymmetric kinetic resolution polymerization (AKRP) of rac -lactide (LA). In this contribution, these chiral complexes displayed an interesting stereoselectivity-switch behavior toward rac -LA polymerization. ( R , R )-Salen[6-Cl] displayed the highest activity among these complexes for the ROP of rac -LA, and ( R , R )-salen[4,6-Br, t Bu] demonstrated excellent stereoselectivity for the ROP of rac -LA with enriched isotactic PLA ( P m = 0.88, CEC; P m = 0.93, ESC). Kinetic study employing the complex ( R , R )-salen[4,6-Br, t Bu] as an initiator revealed first-order dependence on monomer concentration with a rate constant ratio k L / k D = 30. Asymmetric kinetic resolution polymerization provides evidence that the enantiomorphic site control mechanism plays a major role in the ROP of rac -LA mediated by aluminium catalysts with a bulkier electron donor located near the metal center, whereas the ROP of rac -LA mediated by Al(salen) catalysts bearing electron-withdrawing groups mainly occurs via the chain end control mechanism. A number of unreported aluminum complexes were utilized for the asymmetric kinetic resolution polymerization of rac -lactide.