(a) Field of the Invention
The present invention relates to a catalyst for preparing vinyl aromatic polymer and polymerization process of styrene using the same, and particularly to a transition metal half metallocene catalyst with a novel structure for preparing syndiotactic styrene polymer having high activity, superior stereoregularity, high melting point and various molecular weight distributions and a process for preparing polymer using the same.
(b) Description of the Related Art
Styrene polymer is largely classified into three kinds of polymers of atactic, isotactic and syndiotactic polystyrene according to arrangement of benzene ring attached to a polymer main chain.
Amorphous atactic polystyrene is a thermoplastic polymer obtained by general radical or ion polymerization. It is prepared by various processing methods such as injection molding, extrusion molding or vacuum forming, etc. and is used for packaging material, housing material for daily commodities, toy, electrical and electronic products, etc. Isotactic polystyrene having stereoregularity is mainly obtained using heterogeneous Ziegler-Natta catalyst but it is not widely used due to low productivity and low crystallization speed.
Meanwhile, syndiotactic polystyrene is used for engineering plastics more than general-purpose resin because it has properties of crystalline polymer due to high stereoregularity, i.e., heat resistance and chemical resistance, and good mechanical properties such as high crystallization speed while maintaining processing forming property and electrical property of general amorphous polystyrene. Therefore, syndiotactic polystyrene is suitable for material for electronic parts or automobile engine parts, and is used for parts of cellular phone or microwave oven using high frequency property.
Such syndiotactic polystyrene can be generally prepared using a 4 group transition metal compound metallocene catalyst with π-ligand and σ-ligand. As the π-ligand, cyclopentadienyl, indenyl, fluorenyl group or derivatives thereof can be used, and as the σ-ligand, alkyl, aryl, alkyl, alkoxy, aryloxy, thioalkoxy, thioaryloxy, amino, amido, carboxyl, alkyl sillyl group, halogen, etc. can be used.
EP 210,615 has disclosed a method for synthesizing syndiotactic polystyrene with high yield by combining metallocene catalyst with the above structure as a main catalyst with alkylaluminoxane as a cocatalyst. More particularly, main catalyst such as cyclopentadienyltitanium trichloride or pentamethylcyclopentadienyltitanium trichloride is activated with methylaluminoxane cocatalyst and used for polymerization of syndiotactic polystyrene with superior stereoregularity.
Japanese Laid-Open Patent Publication No. Hei 4-314,790 has described that when chloro group, coligand of a metallocene catalyst with the above-mentioned structure, is substituted with alkoxy group, specifically when pentamethylcyclopentadienyltitanium trimethoxide main catalyst and methylaluminoxane cocatalyst are used, syndiotactic polystyrene can be obtained with much higher yield.
Canadian Laid-Open Patent Publication No. CA 2,026,552 has disclosed a method for preparing syndiotactic polystyrene having broad molecular weight distribution by using two or more kinds of the metallocene catalysts together in polymerization. It is to diversify narrow molecular weight distribution obtained when only one kind of a catalyst exists. However, in such a case, since introduced catalysts independently participate in polymerization, produced polystyrenes are also independent each other and thus they are difficult to be uniformly mixed in molecular unit.
U.S. Pat. No. 6,010,972 has disclosed preparation of di-nuclear half metallocene catalyst in which two cycloalkandienyl groups are connected to both nuclei through alkylene or sillylene bridge and styrene polymerization using the same. The result of styrene polymerization showed high polymerization activity, superior stereoregularity and narrow molecular weight distribution similarly to mononuclear metallocene, indicating that even if two or more metal centers exist in one molecule, they do not hinder a polymerization. However, since both cycloalkandienyl groups are the same due to the used preparation process of main ligand, it is not effective for providing various polystyrenes, one of advantages of multinuclear metallocene catalyst, as can be seen from narrow molecular weight distribution.
EP 964,004 has disclosed preparation of multinuclear metallocene catalyst in which two or more half metallocenes are connected through coligand bridge having dialkoxy group or diaryloxy group and styrene polymerization using the same. The results of styrene polymerization also showed high polymerization activity, superior stereoregularity and narrow molecular weight distribution. However, since the used coligand has symmetric structure and acts as a leaving group when polymerization, produced polystyrenes show little difference from those produced using mononuclear metallocene catalyst.