A polymerization catalyst composition contains a rare earth element carboxylate, a phosphate ester represented by the following general formula (1):

##STR00001##

(wherein Rs are each independently an alkyl group having 5 to 20 carbon atoms), and water.

A polymerization catalyst composition contains a rare earth element carboxylate, a phosphate ester represented by the following general formula (1):

##STR00001##

(wherein Rs are each independently an alkyl group having 5 to 20 carbon atoms), and water.

An object of the present disclosure is to provide a terminal-modified conjugated dime polymer capable of improving fracture resistance of a rubber article. Specifically, a terminal-modified conjugated diene polymer is characterized in that a content of catalyst residue therein derived from a catalyst used in polymerization is less than 60 ppm.

An object of the present disclosure is to provide a terminal-modified conjugated dime polymer capable of improving fracture resistance of a rubber article. Specifically, a terminal-modified conjugated diene polymer is characterized in that a content of catalyst residue therein derived from a catalyst used in polymerization is less than 60 ppm.

An object of the present disclosure is to provide a terminal-modified conjugated diene polymer having a high cis-1,4 content and a low molecular weight distribution value and being excellent in durability (such as wear resistance, fracture resistance, cracking resistance, and the like). In order to achieve the object, a terminal-modified conjugated diene polymer of the present disclosure characteristically has: molecular weight distribution determined by gel permeation chromatography (GPC) of less than 2; and content of cis-1,4 bond of 95% or more.

An object of the present disclosure is to provide a terminal-modified conjugated diene polymer capable of improving fracture resistance of a rubber article. Specifically, a terminal-modified conjugated diene polymer is characterized in that a content of catalyst residue therein derived from a catalyst used in polymerization is less than 60 ppm.