Strongly asymmetric chlorinated polybutadiene-b-polystyrene, [P((CB)(x)-b-(PS)(y))] diblock copolymers with increasing x/(x + y) ratios (up to 5.2 mol %) have been synthesized by the selective chlorination of the polybutadiene (PB) block in solution. Chlorination has been performed in anhydrous dichloromethane added with an antioxidant [2,2＇-methylenebis-(6-tert-butyl-4-methyl-phenol)], at -50 degrees C, under a continuous Ar flow and in the dark. Under the optimized experimental conditions, the PB chlorination is not complete, but the PS block is left unmodified. Even in the presence of a large chlorine excess (Cl-2/butene unit molar ratio of 2.5), the experimental degree of chlorination of homo PB does not exceed 85%. The chlorinated copolymers have been characterized by H-1-NMR, IR spectroscopy, size-exclusion chromatography, and elemental analysis. The chlorinated copolymers have also been studied by DSC and SAXS after annealing at 150 degrees C. Although at this temperature the parent homopolymers are immiscible, no microphase separation has been observed for the block copolymers.