The present invention relates to a macromolecule having a controlled terminal group stoichiometry, the macromolecule including a surface layer, at least one subsurface layer and at least two terminal groups including: a first terminal group which is a residue of a pharmaceutically active agent, a derivative thereof or precursor therefor; and a second terminal group selected to modify the pharmacokinetics of the pharmaceutically active agent and/or macromolecule, wherein terminal group stoichiometry refers to the number and type of terminal groups.