Effect of initiator on the catalytic performance of zinc(II) complexes supported by aminomethylquinoline and aminomethylpyridine derived ligands in stereoselective ring opening polymerization of rac-lactide

Abstract

Zinc(II) complexes, namely [LnZnCl2] (Ln = LA–LD) supported with N,N'-bidentate aminomethylquinoline and aminomethylpyridine derived ligands, such as 2-(piperidin-1-ylmethyl)quinoline (LA), 4-(quinolin-2-ylmethyl)morpholine (LB), 2-(piperidin-1-ylmethyl)pyridine (LC), and 4-(pyridin-2-ylmethyl)morpholine (LD), were synthesized and structurally characterized. The structural data revealed that distorted tetrahedral geometries are adopted by Zn(II) metal ions by coordinating with corresponding ligands in a bidentate fashion. Catalytic studies conducted with these complexes in the presence of initiating reagents, i.e., LiMe, LiOCHMe2, and LiCl, showed that they can initiate ring-opening polymerization (ROP) of rac-lactide (rac-LA). All the tested initiators were active, with the in situ catalytic systems of [LnZnCl2] and [LiMe] displaying marked differences in the initiation, polymerization rate and control toward ROP of rac-LA compared to the in situ catalytic systems of [LnZnCl2] and [LiOiPr], giving an 87% conversion of rac-LA within 30 sec using [rac-LA]:[LBZnCl2]:[LiMe] = 100:1:2 at 0 ℃. Polylactides (PLAs) obtained were of low molecular weights with slightly higher polydispersity indices (PDI = 1.21–1.25) in all cases regardless of the initiating Zn(II) system and variations in ancillary ligands. Overall, the higher hetero-enriched PLAs were provided by the in situ catalytic systems of [LnZnCl2] and [LiMe] (the highest Pr up to 0.86 at 0 ℃) compared to that with the in situ system of [LnZnCl2] and [LiOiPr].