Synthesis and structural characterization of titanium and zirconium complexes containing half-salen ligands as catalysts for polymerization reactions

Abstract

Complexes 1–7 were synthesized from tridentate half-salen ligands in good yields and purity. They were characterized by various spectroscopic techniques, elemental analyses, and X-ray crystallography. With titanium, homoleptic and heteroleptic complexes could be synthesized independently whereas with zirconium only homoleptic complexes could be synthesized. All the complexes were efficient and good catalysts for the ring-opening polymerization (ROP) of lactides (LA) and ε-caprolactone (ε-CL). The molecular weights (M_n) of the resulting poly(lactic acid) (PLA) and poly(caprolactone) (PCL) are close to the theoretical values, and the molecular weight distributions (MWDs) are narrowly distributed. Kinetic study showed that the ROP had a first-order dependency on monomer concentration. Next, preliminary studies on copolymerization of cyclohexene oxide (CHO) with CO₂ were performed in the presence of tetrabutylammonium bromide (TBAB) as a cocatalyst. The results from DSC and TGA were used to describe the thermal properties of the copolymer. In addition, all the complexes can effectively catalyze the cycloaddition reactions of CO₂ with propylene oxide (PO) and styrene oxide (SO) using TBAB as a cocatalyst. The formation of the homopolymerization product from CHO is witnessed in the absence of any cocatalyst. All the complexes, when activated by methylaluminoxane (MAO), showed potent catalytic efficiency towards the polymerization of ethylene. The impact of polymerization parameters such as time, temperature and solvents on the polymerization of ethylene is discussed elaborately.