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

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-Me-2-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 (H-1, C-13) 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 degrees C within 20 minutes.