Synthesis, Structures, and Catalytic Activity of Homo‐ and Heteroleptic Ketoiminate Zinc Complexes in Lactide Polymerization

Abstract

The heteroleptic complex LZnEt [(1); L = MeNacac = MeNC(Me)CHC(Me)O] containing the sterically less demanding N,O‐chelating ketoiminate (MeNacac) ligand was synthesized by reaction of ZnEt2 with an equimolar amount of MeNacacH, whereas the reaction with two equivalents MeNacacH gave the homoleptic complex L2Zn 5. 1 reacts with ArOH (Ar = 2,6‐Me2‐Ph) with ethane elimination and formation of LZnOAr (2). Addition of the Lewis base dmap (dmap = 4‐dimethylamino pyridine) to 1 and 2 yielded the corresponding Lewis acid‐base adducts dmap‐Zn(L)Et 3 and dmap‐Zn(L)OAr 4. Compounds 1–5 were characterized by heteronuclear NMR (1H, 13C) and IR spectroscopy, elemental analysis, and single‐crystal X‐ray diffraction (1–3, 5). Their technical application as catalysts in ring‐opening polymerization (ROP) of rac‐lactide in CD2Cl2 or CDCl3 solutions at ambient temperature was investigated. 2 and 3 show the highest activities, whereas those of 1 and 5 are substantially lower. 2 and 3 polymerize rac‐lactide (90 % conversion) within 140 (3) and 180 (2) minutes, respectively. In contrast, 4 could only be investigated in bulk polymerization reactions due to its very poor solubility in common organic solvents. These studies proved that 4 is active in bulk polymerization, converting 93 % lactide to polylactide at 140 °C within 20 minutes.