Block copolysiloxanes have been prepared and utilized to form complexes of cobalt nanoparticles encased in the copolymers. The coated nanoparticles could be dispersed in polydimethylsiloxane (PDMS) to afford PDMS ferrofluids. Poly(dimethylsiloxane-b-methylvinylsiloxane) (PDMS-b-PMVS) diblock copolymers were synthesized via anionic living polymerization with controlled molecular weights and narrow molecular weight distributions. The PMVS blocks were functionalized with trimethoxysilethyl or triethoxysilethyl pendent groups to yield poly(dimethylsiloxane-b-(methylvinyl-co-methyl(2-trimethoxysilethyl)silo xane) (PDMS-b-(PMVS-co-PMTMS)) or poly(dimethylsiloxane-b-(methylvinyl-co-methyl(2-triethoxysilethyl)silox ane) (PDMS-b(PMVS-co-PMTES)) copolymers, respectively. Stable suspensions of mostly superparamagnetic cobalt nanoparticles were prepared in toluene in the presence of PDMS-b-(PMVS-co-PMTMS) and PDMS-b-(PMVS-co-PMTES) copolymers via thermolysis Of Co-2(CO)(8). TEM micrographs showed non-aggregated cobalt nanoparticles with mean particle diameters ranging from approximate to 10- 15 nm. Specific saturation magnetizations of the cobalt-copolymer complexes ranged from similar to40-110 emu g(-1) of cobalt. (C) 2004 Published by Elsevier Ltd.