Thermosensitive random copolymers of hydrophilic and hydrophobic monomers are synthesized via living cationic copolymerization. The synthesis starts with the copolymerization of isobutyl vinyl ether (IBVE) and 2-(tert-butyldimethylsilyloxy)ethyl vinyl ether (BMSiVE) using the cationogen/ Et1.5AlCl1.5 initiating system in the presence of an added base to give a copolymer with a narrow molecular weight distribution. The relative reactivities determined by the Fineman-Ross method indicate that the product copolymer has a highly random sequence distribution. Subsequent desilylation gave a well-defined amphiphilic random copolymer of hydrophobic (IBVE) and hydrophilic units (2-hydroxyethyl vinyl ether: HOVE). On heating, an aqueous solution of the product random copolymer undergoes thermally induced phase separation at a critical temperature. This phase separation is quite sensitive and reversible on heating and cooling. The randomness of the sequence distribution is indispensable to realizing such highly sensitive phase separation. For example, diblock copolymers are soluble and form micelles over a wide range of temperature, whereas copolymers with both block and random segments exhibit only slightly sensitive phase separation behavior with hysteresis. The critical temperature of the random copolymer presented here can be controlled by the composition of IBVE and HOVE. In addition to the composition, the structure of the hydrophobic repeating unit is a major factor determining the critical temperature.