the present invention relates to diblock copolymers and more particularly to polycaprolactone - b - polyethylene oxide 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 polymer , the hydrophobic block comprising a polycaprolactone 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 . the hydrophobic reservoir obtained enables a larger amount of agent to be incorporated therein , with a slower release and without the damaging effects that such high doses would induce with known delivery systems .