Methoxy poly(ethylene glycol)-poly(α-benzyl carboxylate-ε-caprolactone) (MPEG-PBCL) is a linear amphiphilic block copolymers. One hydrogen at α-position on PCL block is substituted by benzyl carboxylate. It can be used in preparation of micelles or nanoparticles for drug encapsulation and drug delivery. The benzyl group can be removed by hydrogenation to obtain PEO-b-PCL which carries functional carboxylic acid side groups on PCL , i.e. PEO-b-poly(α-carboxyl-ε-caprolactone) (PEO-b-PCCL). Drug moieties can be covalently conjugated to the hydrophobic block. Novel PEO-b-poly(ester) block copolymers with aromatic and reactive side groups on the polyester block have tremendous potential in the design of optimized carriers for the delivery of various therapeutic agents, as they can assemble to biodegradable nanoscopic micelles with chemically tailorable core structures.