PURPOSE: To prepare a raw rubber suitable for the preparation of various rubbers, esp. tire tread rubber, by feeding a monomer such as butadiene and a Lewis base/ organic lithium compound continuously to a plurality of seriesly connected polymerization zones under specific conditions.

CONSTITUTION: To prepare the titled polymer containing 0W40wt% of bonded styrene, 20W80mol% of the 1,2-bonded butadiene, Mn of pref. 100,000W300,000, Mw/Mn of 1.5W3.5, and the microscopic structure distribution of the butadiene segment as determined by GPC analysis characterized in that the 1,2-bond varies continuously along the molecular weight, by feeding butadiene or butadiene and styrene, an inert solvent such as hexane, and a catalyst comprising a combination of a Lewis base such as tetrahydrofuran and an organic Li compound such as butyl lithium, continuously to seriesly connected two or more polymerization zones keeping the feed amount of the catalyst, the ratio of the Lewis base/organic Li, and/or the polymerization temperature of each polymerization zone different between the zones.

EFFECT: Processability and the wet-skid resistance can be improved.