A method for the syntheses of thermoresponsive amphiphilic diblock copolymers through sequential organocatalyzed polymerizations of vinyl-based and lactone-based monomers using group transfer polymerization (GTP) and ring-opening polymerization (ROP), respectively, is described. The organocatalysts were switched between the two polymerization stages and all syntheses could be conducted in a one-pot manner. The polymerization systems directly produced diblock copolymers composed of poly(N,N-diethylacrylamide) and biodegradable polyesters (poly(ε-caprolactone), poly(trimethylene carbonate), and poly(L-lactide) in various content ratios. Aqueous solutions of the obtained block copolymers below their cloud point temperatures (T_cp) were carefully characterized, revealing a close relationship among the structures of the block copolymers, T_cp, and micellar forming properties below the T_cp.