A series of novel rod-coil diblock copolymers on the basis of mesogen-jacketed liquid-crystalline polymer were successfully prepared by atom transfer radical polymerization from the flexible polydimethylsiloxane (PDMS) macroinitiator. The hybrid diblock copolymers, poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene}-block-polydimethylsiloxane, had number-average molecular weights (M.'s) ranging from 9500 to 30,900 and relatively narrow polydispersities (less than or equal to1.34). The polymerization proceeded with first-order kinetics. Data from differential scanning calorimetry validated the microphase separation of the diblock copolymers. All block copolymers exhibited thermotropic liquid-crystalline behavior except for the one with M-n being 9500. Four liquid-crystalline diblock copolymers with PDMS weight fractions of more than 18% had two distinctive glass-transition temperatures. (C) 2003 Wiley Periodicals, Inc.