Poly(styrene-b-4-vinylphenoxyphthalonitrile)−Cobalt Complexes and Their Conversion to Oxidatively Stable Cobalt Nanoparticles

Oxidatively stable, magnetic cobalt nanoparticles have been prepared by heating cobalt nanoparticles encapsulated in poly(styrene-b-4-vinylphenoxyphthalonitrile) block copolymers at elevated temperatures. The block copolymers were synthesized through the sequential anionic polymerization of styrene and tert-butyldimethylsilyloxystyrene. The silyl ether protecting groups on the second block were hydrolyzed under acidic conditions to afford poly(styrene-b-4-vinylphenol), and then the pendent phenols of the diblock copolymer were chemically modified with 4-nitrophthalonitrile to afford poly(styrene-b-4-vinylphenoxyphthalonitrile). Stable suspensions of ~8−10 nm diameter cobalt metal nanoparticles were formed by thermolysis of dicobalt octacarbonyl in solutions of toluene containing poly(styrene-b-4-vinylphenoxyphthalonitrile). The cobalt−polymer nanoparticle complexes were pyrolyzed under argon to afford highly magnetic cobalt nanoparticles encased in graphitic-like coatings. Magnetic susceptibility measurements indicate that the cobalt−graphitic particles are oxidatively stable and retain their high saturation magnetizations (~95−100 emu g^-1) for at least 1 year under ambient conditions.