Synthesis of Salicylaldiminato Zirconium Complexes and Its Catalysis Application for Ethylene Polymerization

Salicylaldehyde imine ligands ( 1, 2 ) and corresponding salicylaldiminato zirconium complexes ( 3, 4 ) were prepared via the condensation reaction by using 3-tert-butyl-5-methoxy salicylaldehyde and ortho-substituted anilines as the materials. Both ligands and complexes were characterized by mass spectrum, ¹H-NMR and ¹³C-NMR spectroscopy. In the presence of methylaluminoxane (MAO) as a co-catalyst, both zirconium complexes exhibited good catalytic activity for ethylene polymerization in toluene, producing ultra high molecular weight polyethylene (UHMWPE). Complex 3 was found to exhibit the activity of 50.2 kg/(mmol·h) at 50℃ and 0.9 MPa pressure in toluene, resulting in UHMWPE with a viscosity average molecular weight value of 1.5×10⁶. In case of complex 4 , the activity increased from 45.5 kg/(mmol·h) at 17℃ to 55.6 kg/(mmol·h) at 50℃. An obvious decrease of activity could be observed when the temperature was above 50℃. This decrease was probably due to the decomposition of active species at high temperature. High n_Al:n_Zr ratio led to an enhancement in activity and a decrease in molecular weight, which could be ascribed to the more active species and higher chain transfer rate in the catalytic system. In addition, complex 4 showed higher ethylene polymerization activity of 60.2 kg/(mmol·h) at 30℃ and 0.9 MPa when n-hexane was employed as the solvent, affording the polyethylene with a viscosity average molecular weight value up to 6.6×10⁶. The rheometer analysis of polymer generated from complex 4 revealed a narrow molecular weight distribution 2.4, indicating a well-controlled manner during the polymerization process. Characteristic absorption appearing at 1 471 cm^-1 and 719 cm^-1 from infrared spectrum proved that the microstructure of UHMWPE sample was linear. The high melting points of obtained polymers measured by Differential Scanning Calorimetry (DSC), ranging from 138℃ to 140℃, were typical for linear polyethylene.