Stimuli-responsive ABC triblock copolymers with three segments with different phase-separation temperatures were synthesized via sequential living cationic copolymerization. The triblock copolymers exhibited sensitive thermally induced physical gelation (open association) through the formation of micelles. For example, an aqueous solution of EOVE200-b-MOVE200-b-EOEOVE200 [where EOVE is 2-ethoxyethyl Vinyl ether, MOVE is 2-methoxethyl vinyl ether and EOEOVE is 2-(2-ethoxy)ethoxyethyl vinyl ether; the order of the phase-separation temperatures was poly(EOVE) (20 degreesC) < poly(EOEOVE) (41 degreesC) < poly(MOVE) (70 degreesC)] underwent multiple reversible transitions from Sol (<20 degreesC) to micellization (20-41 degreesC) to physical gelation (physical crosslinking, 41-64 degreesC) and, finally, to precipitation (>64 degreesC). At 41-64 degreesC, the physical gel became stiffer than similar diblock or ABA triblock copolymers of the same molecular weight. Furthermore, the ABC triblock copolymers exhibited Weissenberg effects in semidilute aqueous solutions. In sharp contrast, another ABC triblock copolymer with a different arrangement, EOVE200-b-EOEOVE200-b-MOVE200, scarcely exhibited any increase in viscosity above 41 degreesC. The temperatures of micelle formation and physical gelation corresponded to the phase-separation temperatures of the segment types in the ABC triblock copolymer. No second-stage association was observed for AB and ABA block copolymers with the same thermosensitive segments found in their ABC counterparts. (C) 2004 Wiley Periodicals, Inc.