The invention concerns a block copolymer of polyethylene oxide and polycaprolactone, the polyethylene oxide having a number average molecular weight from about 2.0 to about 3.8 kD, the block copolymer having a fraction of polyethylene oxide of from about 11.8 to 18.8 percent by weight. The invention also concerns polymersomes made from such copolymers and to methods of making the polymersomes.