Polyolefins such as polyethylene and polypropylene are clean materials consisting of carbon and hydrogen not detriment to the environment, and are excellent in processing moldability and physical properties. Because of these characteristics, these materials have been used in a wide variety of fields in automobiles, electric instrument parts, food packages, drink/cosmetic/medical containers, civil engineering materials, and agricultural materials etc. In recent years, however, there are various demands for physical properties of polyolefin, and polyolefin having various characteristics are desired. Further improvement in productivity is also desired. Particularly when blow molding or sheet molding is conducted, melt tension (hereinafter abbreviated sometimes as MT) and swell ratio are desired to be high.
As ethylene-based polymers excellent in these physical properties, polyethylene produced by a high-pressure process and polyethylene obtained by a Cr-based Philips type catalyst are known, but there is a large amount of long chains and branched chains therein, and thus rigidity and impact strength are lowered. Further, regulation of the amount of long chains and branched chains and introduction of α-olefin comonomers are difficult, so there is limit to physical properties which can be attained. Further, such high-pressure polyethylene has the problem of high fluidization activating energy as a measure of temperature dependence for generating resin fluidization at the time of molding.
Conventionally, a Zeigler type titanium-based catalyst comprising a titanium-based compound and an organoaluminum compound is well known as a polymerization catalyst for producing an ethylene polymer and an ethylene-based polymer such as an ethylene/α-olefin copolymer. In recent years, a metallocene-based catalyst comprising a metallocene compound having a cyclopentadienyl group and an organoaluminum oxy compound (aluminoxane) comes to be known as a catalyst by which an olefin polymer can be produced with a high polymerization activity, and recently, a novel catalyst system comprising a transition metal compound having a ligand of diimine structure has also been proposed (see WO96/23010A2). Recently, a metallocene compound having a cyclopentadienyl group preferable as a catalyst for producing an ethylene-based polymer with narrow compositional distribution has also been proposed as a novel olefin polymerization catalyst by the present applicant in WO2004/029062A1. Further, the present applicant has also proposed, in Japanese Patent Application Laid-Open No. 11-3151.09 and EP0874005A1, a transition metal compound containing a salicyl aldimine ligand. The transition metal compound containing a salicyl aldimine ligand is characterized in that it is easily synthesized, has a high ethylene polymerization performance, and can regulate polymerization performance such as molecular weight, copolymerizability etc. by changing the structure of the ligand.
Ethylene-based polymers obtained by using these polymerization catalysts are excellent in rigidity and impact strength as compared with the above-mentioned high-pressure process polyethylene or Cr-based catalyst product, but cannot be said to be satisfactory in respect of MT and swell ratio, so there is room for improvement.
Some proposes have been made to solve these problems for the above-mentioned catalysts. For example, Japanese Patent Application Laid-Open No. 7-278221 describes that an ethylene-based polymer excellent in MT and swell ratio is obtained by combination of a specific Ti compound, a liquid Mg compound and a compound having an ether linkage. In this case, however, the swell ratio is 1.35 or less, which cannot be said to be sufficiently high. The present inventors have disclosed a process for producing a long-chain and branched chain-containing polyolefin by using a combination of specific transition metal compounds in Japanese Patent Application Laid-Open No. 2002-105132. In this case, however, the activation energy of fluidization is increased by introduction of long chains and branched chains, but the effect on improvement of MT is low.
In view of the process for production of polyolefin including the polyethylene-based polymer, high-density polyethylene has been produced generally by slurry polymerization, generally in a low-pressure process using a Ziegler type catalyst. When high-density polyethylene having an arbitrary molecular-weight distribution, among the above high-density polyethylenes, is produced for the purpose of regulating moldability and physical properties, the polymerization is conducted in multistage, and usually the molecular weight and density of a polymer formed in each stage is regulated in the polymerization. Specifically, the polymerization is constituted of a multistage slurry polymerization process consisting of a low-molecular-weight polyethylene polymerization step and a high-molecular-weight polyethylene polymerization step, but in such multistage process, there remain problems to be solved with respect to the process and cost because of troublesome operation due to the multistage process and necessity for use of a large amount of hydrogen in the process of formation of low-molecular-weight polyethylene.
The present inventors made extensive studies for improvement of melt tension and rationalization of the process for polyolefin by the multistage polymerization method, and as a result, they found polyolefin exhibiting excellent melt tension and low activation energy of fluidization and overcoming the problems of the conventional resin by regulating the length and amount of long chains and branched chains by using a specific olefin polymerization catalyst.