Abstract: A novel catalytic system to prepare branched polyethylene with single catalyst activated by two different co-catalysts is reported in this paper via the tandem catalysis, with ethylene as single monomer. Such catalytic system is composed of β-diketonate zirconium complexes, i.e.(acac)₂ZrCl₂(Ⅰ) and (dbm)₂ZrCl₂(Ⅱ) as catalyst precursors, diethylaluminium chloride(AlEt₂Cl) as a co-catalyst for ethylene oligomerization and methylaluminoxane(MAO) as another one for the in situ copolymerization with ethylene. The oligomers obtained were mainly α-olefins, and the contents of them in weight were 76.84% for Ⅰ and 65.99% for Ⅱ, while the part being able to be copolymerized was 62.12% and 55.32% for Ⅰ and Ⅱ, respectively. The branched polyethylene via in situ copolymerization of ethylene was prepared by the tandem catalytic system Ⅰ/AlEt₂Cl/MAO or Ⅱ/AlEt₂Cl/MAO. ¹³C NMR spectrum reveals that the resultant copolymer was branched polyethylene with total branches of 2.1/1000C, including ethyl group of 1.2/1000C, butyl group of 0.8/1000C and longer branches of 0.1/1000C. With increasing the molar ratio of AlEt₂Cl to MAO, the melting temperature of the polymer obtained decreased from 133.7 to 116.7 ℃ for Ⅰ/AlEt₂Cl/MAO and from 131.7 to 118.8 ℃ for Ⅱ/AlEt₂Cl/MAO as well as the change of crystallinity. The catalytic activities for catalyst Ⅰ/AlEt₂Cl/MAO and Ⅱ/AlEt₂Cl/MAO were from 2.48×10⁴ to 0.10×10⁴ g PE/(mol Zr·h) and from 1.00×10⁴ to 0.32×10⁴ g PE/(mol Zr·h), respectively. The results indicate that the branched polyethylene can be obtained by such a catalytic system.

Key words: In situ copolymerization of ethylene, Branched polyethylene, β-Diketonate zirconium complex, Diethylaluminium chloride, Methylaluminoxane