Zr(iv) complexes containing salan-type ligands: synthesis, structural characterization and role as catalysts towards the polymerization of ε-caprolactone, rac-lactide, ethylene, homopolymerization and copolymerization of epoxides with CO2

Abstract

Zr(IV) complexes containing salan-type diamine bis(phenolato) ligands were synthesized and characterized by various spectroscopic techniques and X-ray crystallography. We observed the formation of dinuclear Zr complexes where each Zr centre adopts a distorted octahedral geometry. The metal centers are bridged with two isopropoxide groups and each metal center has two terminal isopropoxide moieties attached to it. These compounds show notable activities towards the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA) in the presence and absence of benzyl alcohol as an external initiator, resulting in very high number average molecular weight (M_n) polymer with controlled molecular weight distributions (MWDs). There is a close proximity between observed molecular weight (M^obs_n) and theoretical molecular weight (M^theo_n). Analysis of MALDI-TOF and ¹H NMR spectra of low M_n oligomers reveals that the isopropoxide groups as well as the OBn groups initiate the ROP. Kinetic studies reveal that the polymerization follows first-order kinetics and is faster in the presence of benzyl alcohol (BnOH). All the complexes were active precatalysts towards the polymerization of ethylene. In addition, all the complexes were found to copolymerize cyclohexene oxide (CHO) and propylene oxide (PO) with CO₂, affording the formation of degradable polycarbonate with moderate M_n values and narrow MWDs. Their thermal properties were examined using DSC and TGA analysis. In the case of styrene oxide (SO), the coupled product of CO₂ and SO was observed.