This article demonstrates the biphasic synthesis of "block" copolymers and star-shaped polymers via monomer-selective cationic copolymerization in the biphasic system of organic/fluorinated solvents. The first monomer and second monomer were dissolved in hexane (upper phase) and dichloropentafluoropropane (lower phase), respectively. The first monomer was polymerized in the upper phase, while the comonomer still remained unreacted in the lower phase. At the point of almost complete consumption of the first monomer, two phases were stirred vigorously to become a single copolymer with low polydispersity. Monomer-selective reaction was achieved even from a combination of monomers with similar reactivities. Moreover, an efficient one-shot formation of star-shaped polymers was feasible through the copolymerization of monomer and divinyl compound in the biphasic system.