Catalyst composition for polymerizing conjugated diene monomer

A catalyst composition, comprising (i) component (a), (ii) component (b) or (d), and (iii) component (c), wherein:

• the component (a) is a lanthanoid-aluminum crosslinked complex having a structure represented by the following formula (1) or (2):
• wherein Ln represents one selected from the group consisting of lanthanoid elements, Sc, and Y;
• and R1 to R4 may be the same or different, and represent one selected from the group consisting of a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an alkylsilyl group, an alkyloxy group, and a dialkylamide group;
• the component (b) is an alkylaluminum compound represented by a formula HnAlR103-n wherein R10 may be the same or different, and represents an alkyl group having 1 to 8 carbon atoms or an alkenyl group having 1 to 8 carbon atoms;
• and n represents an integer of 0 to 3;
• the component (c) is one or more compounds selected from the group consisting of metal halide compounds and halosilyl compounds;
• and
• the component (d) is one or more compounds selected from the group consisting of aluminum oxy compounds represented by the following formula (3) or (4), borane compounds and borate compounds:
• wherein R13 to R19 may be the same or different, and represent a hydrogen atom or an alkyl group having 1 to 8 carbon atoms;
• and x and y represent an integer of 1 to 10000.