Diblock copolymers of vinyl ethers with two thermosensitive segments possessing different phase separation temperatures (T-PS) have been synthesized by sequential living cationic copolymerization. Examples include 2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) and 2-methoxyethyl vinyl ether (MOVE), which were sequentially polymerized using a cationogen/Et1.5AlCl1.5 initiating system in the presence of tetrahydrofuran to give diblock copolymers with a very narrow molecular weight distribution. When an aqueous solution of a diblock copolymer (EOEOVE/MOVE = 200/400) was heated, four different viscoelastic stages were observed: clear liquid (sol, less than or equal to40 degreesC, transparent gel (42-55 degreesC), hot clear liquid (sol, 57-63 degreesC), and opaque mixture by phase separation (>63 degreesC). Micelle formation during physical gelation was confirmed by the change in particle diameter. The temperatures of the first and third transitions corresponded to each T-PS for two segments, whereas that of the second transition was dependent on factors such as structure (sequence, composition, and molecular weight distribution) and physical properties (concentration, additives). Freeze-fracture transmission electron microscopy revealed the physical gels to consist of a regular arrangement of spherical micelles with controlled size.