The present invention relates to polycarbonate-based diblock polymers, nanoparticles comprising the diblock polymers and a therapeutic agent, in particular the therapeutic agent is attached to a magnetic moiety, and methods to prepare the diblock polymers and nanoparticles. Said diblock polymer comprises : (i) a first block polymer comprising of a polyether repeating unit; and (ii) a second block polymer comprising of a repeating unit of a general Formula I wherein (a) R1, R2, R3, R4, and R5 is each independently selected from the group comprising : hydrogen, halogen, alkyl group comprising 1 to 20 carbon atoms, and aryl group comprising 6 to 20 carbon atoms, (b) p and q is each independently an integer from 0 to 6, wherein p and q cannot both be 0, (c) A is a linker, and (d) B is an aromatic moiety with two adjacent hydroxyl groups, for example catechol moiety. In a preferred embodiment, the nanoparticles encapsulated with iron-salen complexed paclitaxel are formed for use in breast cancer treatment.