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.