Xiubo Tang, Tielong Gao, Wenjuan Zhang, Junxian Hou and Wen-Hua Sun*
CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.

        The oligomerization of ethylene provides the major industrial process for the production of linear α-olefins, which are extensively used as comonomer of polyolefin, surfactants and lubricants. The linear α-olefins were originally manufactured by the Ziegler (Alfen) process,1 and current industrial catalysts include either a combination of alkylaluminum compound with early transition metal compounds or nickel (II) with bidentate monoanionic [P,O] ligands (the SHOP process).2 The late-metal catalysts have the advantages such as less oxophilicity, the tolerance of expanded functional groups and the controllability of products.3 Here, we report the synthesis of a series of nickel (II) complexes bearing ester-substituted pyridylimine ligands and their application in catalyzing ethylene oligomerization and polymerization upon activation with methylaluminoxane (MAO).

         The ligands 1a−f were prepared according to the previously reported procedures.4 Complexes 2a−c were obtained by stirring the ethanol solution of NiCl2⋅6H2O with the ligands 1a−c, while the complexes 2d−f were prepared by the reaction of (DME)NiBr2 (DME = 1,2-dimethoxyethane) with the corresponding ligands 1d−f (Scheme 1). These complexes were characterized by IR spectroscopic and elemental analysis. Furthermore, complexes 2a, 2b, 2d, 2e and 2f were characterized by X-ray diffraction and selected molecular structures of 2a and 2f are shown in Fig 1 and Fig 2. As is shown by the X-ray crystal structure, the carbonyl oxygen in the ester group in 2a coordinates with the nickel center, while the carbonyl oxygen in 2f is free from coordination.
        Ethylene oligomerization and polymerization using 3a−f/MAO were performed under 1 atm and 10 atm. These catalysts mainly produce oligomers with carbon number from C4 to C8 at 1 atm. Selectivities for linear α-olefin are relative low, which is the typical character of the nickel catalysis systems.5 The selected results at 10 atm are listed in Table 1. The bulkier diisopropyl-substituted complex 2c displays higher polymerization activity than and the dibromo-substituted complex 2f. Additionally, 2c produces polyethylene with higher Mn value than 2f, which reflects the steric effects of the substitutents. Analysis of the PE sample (entry 1 in Table 1) by 13C NMR spectrum reveals that the PE contains small amount of n-butyl branch. The branching degree is about 2 n-butyls per 1000 methylenes in the main chain. GPC analysis reveals the multimodality molecular distribution characters of the PE samples obtained at 10 atm.



Acknowledgement: We are grateful to the National Natural Science Foundation of China for financial supports under Grant No.20272062, National 863 Foundation (2002AA333060).

References
[1] D. Vogt, in Applied Homogeneous with Organometallic Compounds, Vol. 1 Eds: B. Cornils, W.A. Herrmann, VCH, New York. 1996. 245-258.
[2] W. Keim, A. Behr, B. Limbacker, C. Kruger, Angew. Chem., Int. Ed. Engl. 1983, 22, 503.
[3] S. D. Ittel, L. K. Johnson, M. Brookhart, Chem. Rev. 2000, 100, 1169.
[4] W-H. Sun, X. Tang, T. Gao, B. Wu, W. Zhang, H. Ma, submitted to Organometallics.
[5] F. Speiser, P. Braunstein and L. Saussine, Organometallics 2004, 23, 2613–2624.

论文来源：Asia Polymer Symposium(APOSYM/2004)October 10-16