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.
Three ansa-metallocenes(Me_2C)(Me_2Si)Cp_2TiCl_2(1),[(CH_2)_5C](Me_2Si)Cp_2TiCl_2 (2)and (Me_2C)(Me_2Si)Cp_2ZrCl_2 (3)with larger dihedral angles and longer distance from metal to the center of Cp planes were synthesized and used as catalysts for ethylene polymerization in the presence of methylaluminoxane (MAO).In the case of ethylene polymerization,compared the feature structures of unbridged metallocenes, singly bridged metallocenes and doubly bridged metallocenes 1,2,3,there exhibit the relationship bet...
Thermal segregations of LLDPE were treated with successive self-nucleation/annealing (SSA) on dif-ferential scanning calorimetry (DSC). Information on molecular heterogeneity of LLDPE was obtained. After SSA was treated, the multiple endothermic peaks were observed in the DSC thermograms during heating experiment. It is obtained that the thickness of different lamellas formed by segments of vari-ous lengths was 4―10 nm. X-ray diffraction (XRD) results showed that the crystallites dimensions of various reflections were about several dozens of nanometers. The ethylene/α-olefin copolymers and the copolymer via in-situ copolymerization were similar to each other for molecular heterogeneity and XRD characteristics, which revealed that it was possible to use the ethylene/α-olefin copolymers to simulate the copolymer via in-situ copolymerization of ethylene to simplify the complexity of the structure of the ethylene in-situ copolymer.
