Copolymerization of Ethylene and Methyl Methacrylate by Bis-(salicylaldiminato) Nickel(Ⅱ) Complexes

Three bis-(salicylaldiminato) nickel (Ⅱ) complexes with different steric hindrances on the adjacent position of aniline group,O-(3,5-I₂)C₆H₃-o-C(H)＝N-2,6-C₆H₃(Me)₂₂Ni (C1),O-(3,5-I₂)C₆H₃-o-C(H)＝N-2,6-C₆H₃(Et)₂₂Ni (C2) andO-(3,5-I₂)C₆H₃-o-C(H)＝N-2,6-C₆H₃(i-Pr)₂₂Ni (C3), were successfully synthesized based on the reaction of Ni(CH₃COO)₂·4H₂O with ligands L1-L3, HO-(3,5-I₂)C₆H₂-o-C(H)＝N-2,6-C₆H₃(Me)₂(L1), HO-(3,5-I₂)C₆H₂-o-C(H)＝N-2,6-C₆H₃(Et)₂(L2) and HO-(3,5-I₂)C₆H₂-o-C(H)＝N-2,6-C₆H₃(i-Pr)₂(L3). The ligands were characterized by ¹H-NMR and FT-IR, and the formed complexes were characterized by ¹H-NMR, FT-IR, element analysis (EA) and mass spectrometry (MS). The complexes were applied for copolymerization of ethylene and methyl methacrylate (MMA) in the presence of methylaluminoxane (MAO). The effects of different polymerization factors, such as Al、Ni molar ratio (n_Al/n_Ni), catalyst dosage, polymerization reaction temperature and ethylene pressure, the concentration of MMA and polymerization reaction time were investigated systematically. The optimum polymerization conditions were as follows:n_Al/n_Ni 150, 25℃, ethylene pressure of 2 MPa and 2 h polymerization. With the increase of n_Al/n_Ni, the catalyst activity first increased and then decreased. The more catalyst dosage uses, the higher molecule mass was obtained. The activity increased with the increasing of polymerization pressure. The increases of polymerization time led to an increase followed by a decrease of catalyst activity. The complexes C1-C3 were evaluated for the copolymerization under the optimum polymerization conditions. The complex C1 with small substituents on the adjacent position of aniline group displayed higher catalytic activity of 12.2 kg/(mol·h), but lower molecular mass of 5.97×10⁵. While C2 and C3 with bulky substituents on the adjacent position of aniline group showed low catalytic activity, 6.8 kg/(mol·h) and C3 5.4 kg/(mol·h) respectively, and high molecular mass with 8.06×10⁵ for C2 and 9.85×10⁵ for C3. The microstructures of the obtained polymers were characterized by ¹H-NMR, ¹³C-NMR and FT-IR spectra, demonstrating that the polymers were the copolymers of ethylene and MMA.