Catalyst for producing aromatic vinyl compound polymer and process for producing aromatic vinyl compound polymer by using same

There are disclosed a catalyst for producing an aromatic vinyl compound polymer which catalyst comprises a contact product between an (A) trivalent titanium compound and a (B) compound represented by the general formula (II) or (III> R kM L m (II> R kM X lL m (III), a (C) an ionic compound comprising a noncoordinate anion and a cation, an aluminoxane or an organoboron compound; and optionally a (D) Lewis acid, and a process for producing an aromatic vinyl compound polymer having a high degree of syndiotactic configuration which process comprises polymerizing (a) an aromatic vinyl compound or (b) an aromatic vinyl compound together with an olefin or a diolefin in the presence of the above catalyst. By virtue of using the trivalent titanium compound, it is made possible to readily prepare the above catalyst in high yield, to efficiently produce the above (co)polymer in high catalytic activity by using the catalyst and further to enhance the syndiotacticity of the objective (co)polymer, while minimizing the by-production of unfavorable atactic (co)polymer.