1 Mole of poly(propylene glycol) (PP6) with molecular weight of 2000 was reacted with 2 moles of hexamethylene diisocyanate (HDI) at 12$0^{circ}C$ for 15 hours under nitrogen atmosphere to give HDI-PPG-HDI intermediate. Then s-caprolactam (CL) was added to the system to block the end NCO groups. The resultant prepolymer has the form of H-carbamoyl caprolactam and can be used as an activator with NaH catalyst for the anionic ring opening polymerization of CL. Using this method. PCL-b-PPG-b-PCL triblock copolymers with various content of PPG up to 40 wt% were obtained. Homo poly(s-caprolactam) (PCL) was also polymerized using newly synthesized CL-blocked HDI activator (hexamethylene dicarbamoyl dicaprolactam). The yields of the polymerizations were determined by gravimetry after successive soxhlet extraction with water and acetone. Homo PCL has the yield of about 94% The yields of triblock copolymers were (lecreased with increasing PPG content antral the copolymer with 40 wt% PPG feed content has the yield of about 80%. According to the results of copolymer composition. determined by 1 H-NMR spectroscopy. it has been revealed that the PPG content in the block copolymer is greater than the prepolymer composition in the feed when the prepolymer composition is less than 30 wt%. On the contrary when the prepolymer composition in the feed is over 30 wt%. the PPG content is lower than the feed composition.