Poly(lactide-b-siloxane-b-lactide) triblock copolymers with pendent carboxylate groups have been synthesized to serve as dispersants for magnetite nanoparticles. Magnetic nanoparticles are of interest for potential biomedical applications including magnetic field-directed drug delivery and magnetic cell separations. For in-vivo applications, it is important that the magnetic particle be coated with biocompatible organic materials to afford dispersion characteristics or to further modify the surfaces of the complexes with biospecific moieties. Synthesis of the triblock copolymer dispersants comprises three reactions. First, difunctional, controlled molecular weight, telechelic, polymethylvinylsiloxane (PMVS) oligomers with 3-aminopropyl endgroups were prepared in ring-opening redistribution reactions. Secondly, these oligomers were utilized as macroinitiators for ring-opening L-lactide or D,L-lactide to provide triblock materials with PMVS central blocks and polylactide end blocks. Both the PMVS oligomers and the poly(lactide-b-siloxane-b-lactide) copolymers had the expected molecular weights and compositions. Thirdly, the vinyl groups on the polysiloxane center were functionalized with carboxylic acids by adding mercaptoacetic acid across the pendent double bonds. At neutral pH, the Carboxylate-functional polysiloxane central block binds to the surfaces of magnetite nanoparticles, while the polylactides serve as tail blocks to provide dispersibility in polylactide solvents through interparticle steric repulsive forces. Analyses confirmed that the copolymers indeed adsorbed onto the Surfaces of the magnetite nanoparticles, but fractionations of these materials suggested that the composition distribution of the components was somewhat inhomogeneous. (C) 2008 Published by Elsevier Ltd.