Ethylene polymerization by binuclear chromium complex with tetrakis(pyrazolyl-methyl)benzene ligand

摘要

The reaction of one equivalent of 1,2,4,5-tetrakis(pyrazolyl-methyl)benzene ligand (L) with two equivalents of Cr(THF)(3)Cl-3 in tetrahydrofuran has produced a binuclear neutral complex of general formula [Cr2L(THF)(2)Cl-6] (1). It was characterized by mass spectrometry, Fourier-transformed infrared, X-ray photoelectron spectroscopy, and magnetic susceptibility. Theoretical calculations supported the formation of binuclear complex (1) with each hexacoordinate metal center. This complex was active in ethylene polymerization employing methylaluminoxane as cocatalyst, with moderate activities (5.7-20.8 kg polymer/(mol precat.) (bar) (h)). The reaction parameters such as temperature, monomer pressure, and Al/Cr ratio were studied. An increase of reaction temperature produced a decreasing of catalytic activity. However, an increase in the monomer pressure did not produce a significant increase in catalytic activity. The Al/Cr ratios used to activate this complex are smaller than those reported for the tris(pyrazolyl)borate chromium complex. This system produced high-density polyethylenes with wide molecular weight, M-w, distributions between 42,280 and 74,346 and polydispersity index from 2.3 to 6.6.