N,O-Amino-phenolate Mg(II) and Zn(II) Schiff base complexes: Synthesis and application in ring-opening polymerization of ε-caprolactone and lactides

Abstract

Solventless grinding of 2-aminophenol and the corresponding aldehyde furnished compounds 2-(pyridin-2-ylmethylene)aminophenol (L1), 2-(pyridin-4-ylmethylene)aminophenol (L2), 2-(thiophen-2-ylmethylene)aminophenol (L3) and 2-(furan-2-ylmethylene)aminophenol (L4), in excellent yields of 97–99%. Reaction of ligands L1–L4 with zinc or magnesium alkyl solutions produced complexes [Mg(L1)2] (1), [Mg(L2)2] (2), [Mg(L3)2] (3) and [Mg(L4)2] (4) or [Zn(L1)2] (5), [Zn(L2)2] (6), [Zn(L3)2] (7) and [Zn(L4)2] (8). The complexes were characterized by 1H NMR, 13C NMR and IR spectroscopies, elemental analysis and mass spectrometry. Complexes 1 was further analysed by single-crystal X-ray crystallography. The complexes were investigated as catalysts/initiators in the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL), ʟ-lactide (ʟ-LA) and rac-lactide (rac-LA). Mg(II) complexes 1–4 (kapp = 0.12–0.58 h−1) were more efficient catalyst as compared to the Zn(II) complexes 5–8 (kapp = 0.04–0.24 h−1). The catalytic activity was enhanced by adding of alcohol co-initiators. Comprehensive kinetic and mechanistic investigations revealed that the polymerization reaction follows pseudo-first-order kinetics. Moderate molecular weights increased from 1234 to 4567 g mol−1 with an increase in catalyst concentration with polydispersity (PDIs) ranging from 1.4 to 2.0. The enthalpy ΔHp were found to be 23.5 and 63.9 kJ mol−1 while the entropy of activation (ΔSp) values were −157 and −33 JK−1 mol−1 for ε-CL and ʟ-LA monomers, respectively.