One aspect of the present invention provides an end-modified conjugated diene polymer represented by Formula 1 below: ##STR00001## wherein p is a conjugated diene polymer chain; R.sup.1 to R.sup.7 are each independently C.sub.1-C.sub.20 saturated or unsaturated hydrocarbon chains; X is carbon (C), silicon (Si), or nitrogen (N); a is 1 or 2; and n is an integer of 1 to 200, and a method for preparing the same.

A bis-imine pyridine complex of lanthanides having general formula (I):

##STR00001## Said bis-imine pyridine complex of lanthanides having general formula (I) can be advantageously used in a catalytic system for the (co)polymerization of conjugated dienes.

The invention provides a rubber composition prepared by mixing, per 100 parts by mass of the rubber component comprising at least 50% by mass of a diene-based rubber, from 20 to 150 parts by mass of a filler, and from 0.05 to 30 parts by mass of a compound A which has a specific amidine structure and has a functional group reactive with the diene-based rubber. The rubber composition improves both a high elastic modulus and a low tan . The invention also provides a pneumatic tire using the rubber composition.