Kaminsky's catalyst (metallocene/methylaluminoxane) is widely known as a homogeneous catalyst for olefin polymerization. This catalyst system is characterized by its markedly high activity per unit transition metal thereof. Among metallocenes which are widely known and have been classically used is bis(cyclopentadienyl)zirconium dichloride (zirconocene dichloride). Kaminsky et al proposed to copolymerize ethylene and an .alpha.-olefin using a catalyst system composed of zirconocene dichloride and methylaluminoxane as disclosed in JP-A-58-19303 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the above catalyst system is disadvantageous in that it is difficult to obtain a copolymer having a sufficiently high molecular weight in an industrially advantageous temperature range (50.degree. to 70.degree. C.) and that a large quantity of a comonomer must be charged ina polymerization system in order to obtain a linear low-density polyethylene having a desired density.
Ewen et al succeeded in obtaining a high-molecular-weight copolymer in a preferred temperature range (50.degree. to 70.degree. C.) by using a metallocene having a substituent on the cyclopentadienyl ligand (see JP-A-60-35007). However, the technique involves a problem from an industrial view point because very expensive methylaluminoxane must be used in a large quantity as a co-catalyst.
Further, Welborn et al proposed to increase the polymerization activity per unit aluminoxane by supporting a metallocene and aluminoxane on a porous inorganic metal oxide (see JP-A-61-296008). The polymerization activity reached, however, is not sufficiently high. Besides, the polymerization activity of the catalyst system is decreasing with time immediately after contact between a metallocene and aluminoxane.
Furthermore, Ishihara et al proposed to use a co-catalyst composed of methylaluminoxane and other organoaluminum compound in an attempt to increase the polymerization activity per aluminoxane (see JP-A-60-130604 and JP-A-60-260602). However, the effect obtained is not so high as expected.
It has recently been revealed that use of a Kaminsky's catalyst in polymerization of .alpha.-olefins, chiefly propylene, enables stereospecific polymerization. For example, it is reported that atactic polypropylene (Macromol. Chem. Rapid Commun., Vol. 4, pp. 417-421 (1983), JP-A-58-19309), isotactic polypropylene (Angew. Chem. Int. Ed. Engl., Vol. 24, pp. 507-508 (1983), J. Am. Chem. Soc., Vol. 106, p. 6355 (1984), J. Am. Chem. Soc., Vol. 109, p. 6544 (1987), Chem. Lett., pp. 1853-1856 (1989), JP-A-2-76887), and syndiotactic polypropylene (J. Am. Chem. Soc., Vol. 110, p. 6255 (1988)) can be prepared. The ligand structure and stereostructure of metallocenes are key points for manifestation of stereospecificity in these polymerization systems.
However, metallocenes which make it possible to produce industrially important isotactic polypropylene are very limited in kind and performance. For example, ethylenebis(.eta.5-indenyl)zirconium dichloride or ethylenebis(.eta.5-tetrahydroindenyl)zirconium dichloride disclosed in JP-A-63-295607 enables production of isotactic polypropylene in the presence of methylaluminoxane, but the stereospecificity of the isotactic polypropylene obtained is relatively low as about 95% in terms of mm %. Moreover, the resulting polymer has a melting point as low as 135.degree.-146.degree. C. because of the occurrence of hydrogen shift polymerization in the polymer chain, called 1,3-insertion (see Macromol. Chem., Rapid Comun., Vol. 8, p. 305 (1987)).
JP-A-2-75609 and JP-A-2-75610 describe that an isoblock polymer is obtained by polymerization of propylene in the presence of indenyl(cyclopentadienyl)(dimethylsilyl)hafnium and methylaluminoxane. The polymer obtained here is a soft polymer having a low melting point and low stereospecificity (mm %), differing from so-called isotactic polypropylene of industrial importance.
Ishihara et al. report production of isotactic polypropylene by the use of (CHI)HfCl.sub.2 (Polymer Preprints, Japan, Vol. 40, p. 265 (1991)), but the polymer obtained has a very low mm %.
It is reported in JP-A-1-301704, JP-A-1-319489, JP-A-2-76887, and Chem. Lett., pp. 1853-1856 (1989) that dimethylsilylbis(2,3,5-trimethylcyclopentadienyl)zirconium dichloride enables production of isotactic polypropylene in the co-presence of methylaluminoxane, achieving stereospecificity as high as 99 mm % or more. It turned out, however, that the polymer chain contains chemical inversion, which is a polymer chain defect resulting from inversion of the direction of monomer insertion and which reduces rigidity of the polymer (see KOBUNSHI KAKO, Vol. 41, p. 28 (1992)). In addition, the greatest problem associated with this catalyst system resides in that the above zirconium complex includes from the nature of its structure two stereoisomers, a racemic modification and a meso form, and that it is only the racemic modification that enables production of highly isotactic polypropylene. The-other isomer, a meso form, catalyzes formation of atactic polypropylene with non-negligible activity. Therefore, the preparation of the complex involves a complicated purification step for complete removal of the meso form and thus incurs increased cost.
Collins et al report that isotactic polypropylene is prepared in the presence of a combination of bis(t-butylcyclopentadienyl)zirconium dichloride and methylaluminoxane (see Organometallics, Vol. 10, p. 2061 (1991)). In this case, too, it is necessary to separate a racemic mixture from an undesired meso form in the preparation of the zirconium complex.
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