Transition metal compound, polymerization catalyst comprising the same and process for producing styrenic polymer using said polymerization catalyst

There are disclosed a transition metal compound of the general formula RMXa-1Lb (R is as a pi ligand, a fused polycyclic cyclopentadienyl group in which at least one of many-membered rings to which cyclopentadienyl groups are fusedly bonded is a saturated ring, M is a transition metal, X is a sigma ligand, L is a Lewis base, a is the valency of M, and b is 0, 1 or 2); a polymerization catalyst for styrene, etc. comprising the above transition metal, preferably further comprising an oxygen atom containing compound ionic compound or organoboron compound and optionally a Lewis base; and a process for producing a polymer of a compound containing an ethylenically unsaturated double bond or an acetylenic polymer, especially a syndiotactic polystyrene by using the above polymerization catalyst. The catalyst is particularly effectivive for producing highly syndiotactic polystyrene minimized in residual metals amounts at a low cost in enhanced efficiency.