Ring-opening polymerization of lactides and ε-caprolactone catalyzed by Zn(II) aryl carboxylate complexes supported by 4-pyridinyl schiff base ligands

Synthesis and catalytic studies of aryl carboxylate Zn (II) complexes is reported. Reaction of substituted (E)-N-phenyl-1-(pyridin-4-yl)methanimine with a methanolic solution of Zn(CH(3)COO)(2) and substituted aryl carboxylate co-ligands gave heteroleptic Zn(II) complexes; [Zn(C(6)H(5)COO)(2)(L1)](2) (1), [Zn(C(7)H(7)COO)(2)(L1)](2) (2), [Zn (4-F-C(6)H(4)COO)(2)(L1)](2) (3), [Zn(C(6)H(5)COO)(2)(L2)](2) (4), [Zn(C(7)H(7)COO)(2)(L2)](2) (5), [Zn (4-F-C(6)H(4)COO)(2)(L2)](2) (6), [Zn(C(6)H(5)COO)(2)(L3)](2) (7), [Zn(C(7)H(7)COO)(2)(L3)](2) (8), [Zn (4-F-C(6)H(4)COO)(2)(L3)](2) (9). The molecular structures of complexes 1 and 4 are dinuclear with the zinc atom in complex 1 adopting a distorted trigonal bipyramidal geometry in a bi-metallacycle while complex 4 is square pyramidal where all four benzoate ligands bridge the zinc metals in a paddle wheel arrangement. All complexes successfully initiated mass/bulk ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL) and lactides (LAs) monomers with or without alcohol co-initiators at elevated temperatures. Complexes 1, 4 and 6 containing the unsubstituted benzoate co-ligands were the most active in their triad; with complex 4 being the most active (k(app)) of 0.3450 h(−1). The physicochemical properties of the polymerization products of (l)-lactide and rac-lactide in toluene revealed melting temperatures (T(m)) between 116.58 °C and 188.03 °C, and decomposition temperatures between 278.78 °C and 331.32 °C suggestive of an isotactic PLA with a metal capped end.