Polyurethane iniferter prepared from isocyanate end capped prepolymer and 1,1,2,2-tetraphenyl-1,2-ethanediol, has been used to polymerize vinylbenzyl chloride to obtain polyurethane-polyvinylbenzyl chloride multiblock copolymers. Formation of the block copolymers proceeds with increase in both molecular weight and conversion with increasing polymerization time showing that the polymerization proceeds via a "living" radical mechanism. The block copolymers so obtained were converted into their cationomers by the treatment of triethylamine. The block copolymers and their cationomers have been characterized by FTIR, FTNMR, TGA, and DSC studies. The effect of thermal energy on the molecular weight of the macroiniferter in the absence of monomer has been studied in order to understand the mechanism of formation of the block copolymers.