Ring-opening polymerization of cyclic esters by 3- and 4-pyridinyl Schiff base Zn(II) and Cu(II) paddlewheel complexes: kinetic, mechanistic and tacticity studies

Abstract

We have previously reported a series of highly active N,O-Amino-phenolate Mg(II) and Zn(II) Schiff base complexes for caprolactone and lactide polymerization (Munzeiwa, Nyamori, and Omondi, 2019). Herein we report carboxylate Zn(II) and Cu(II) pyridinyl Schiff base complexes and in particular, those whose ligands have substituents slightly situated away from the metal center. The complexes were fully characterized and for some fully by single-crystal X-ray diffraction studies. The ligands were confirmed to be monodentate and their complexes have the famous paddlewheel conformation. The carboxylate complexes were found to initiate the polymerization of ɛ-caprolactone and lactides in the presence or absence of alcohol, yielding low molecular weight polymers between 1.27 and 5.65 kg mol ‐1 , with broad Ð between 1.7 and 2.0. The Zn(II) complexes were more reactive than their Cu(II) analogues. Activity of the complexes seems to have been influenced by the electronic properties of the substituents with the highest apparent rate constant, k app of 0.4043 h −1 , being obtained for the complex of the ligand with an electron-withdrawing group (chloro). The complex was used to investigate activity and stereo-selectivity towards the ring-opening polymerization of L -lactide and rac -lactide. Tetrad analysis of the polylactides obtained from homonuclear 1 H and 13 C NMR afforded isotactic PLA with L -LA and heterotactic (Pr = 0.68) stereo-block PLA with rac -LA.