Abstract: The present invention relates to diblock copolymers and more particularly to polycaprolactone-b-polyethylene oxide (PCL-b-PEO) diblock copolymers used in micellar systems, for delivering a biologically active agent to a site. There is provided a diblock copolymer compound comprising a hydrophilic block and a hydrophobic block, the hydrophilic block comprising a polyethylene oxide (PEO) polymer, the hydrophobic block comprising a polycaprolactone (PCL) polymer, the PCL polymer comprising a number of caprolactone monomers selected from 5 to 150, the PEO polymer comprising a number of ethylene oxide monomers selected from 30 to 100. The diblock copolymer compound of the present invention may be used to form a micellar delivery system for delivering biologically active agents such as lipophilic drugs to sites such as the central nervous system.