Synthesis of N,N,O-chelate zinc and aluminum complexes and their catalysis in the ring-opening polymerization of ε-caprolactone and rac-lactide

Abstract

Zinc and aluminum complexes supported by N,N,O-chelate ligands were synthesized and characterized. The zinc complexes [Zn(Et){2-{OC(R¹)CH}-6-(3,5-Me₂C₃HN₂)C₅H₃N}]₂ (R¹ = Ph, 1a; R¹ = Bu^t, 1b) were synthesized by reaction of ligand precursors 2-{R¹C(O)CH₂}-6-(3,5-Me₂C₃HN₂)C₅H₃N (R¹ = Ph, HL¹; R¹ = Bu^t, HL²) with ZnEt₂. The aluminum complexes [Al(R)₂{2-{OC(Ph)CH}-6-(3,5-Me₂C₃HN₂)C₅H₃N}] (R = Me, 2a; R = Et, 2b) were synthesized by reaction of HL¹ with AlMe₃ or AlEt₃. Similar treatment of the ligand precursor 2-{Ph₂C(OH)CH₂}-6-(3,5-Me₂C₃HN₂)C₅H₃N (HL³) with AlMe₃ or AlEt₃ afforded aluminum complexes [Al(R)₂{2-{OC(Ph)₂CH₂}-6-(3,5-Me₂C₃HN₂)C₅H₃N}] (R = Me, 3a; R = Et, 3b). The complexes were characterized by ¹H and ¹³C{¹H} NMR spectroscopy, elemental analyses and single crystal X-ray diffraction (for 1a, 1b, 2b and 3a). All the complexes are active to catalyze the ring-opening polymerization of ε-caprolactone in the presence of BnOH, leading to polycaprolactone with good molecular weight control and relatively narrow molecular weight distribution. The zinc complexes/BnOH showed good catalytic activity for the ring-opening polymerization of rac-lactide, displaying good molecular weight control and very narrow molecular weight distributions. The PLA catalyzed by complex 1a/BnOH showed somewhat hetero-stereoselectivity with Pr up to 0.73 when the polymerization was performed in THF at 0 °C. Complex 1a/BnOH also catalyzed block copolymerization of ε-CL and rac-LA with good molecular weight control of the polymer. Kinetic studies of the polymerization reactions were performed.