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Magneto-responsive organogels prepared through surface-initiated atom transfer radical polymerization on iron nanoparticles.

Iron nanoparticles (Fe(np)) with average diameter 50 nm have been synthesized by addition of NaBH4 to FeCl3 solution then radical initiator, (11-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane (1) has been immobilized on to Fe(np). Surface-initiated atom transfer radical polymerization (SI-ATRP) of different monomers such as styrene, methyl methacrylate, methacrylonitrile and N-vinylimidazol have been carried out from initiator-grafted Fe(np) and the amount of grafted polymers was determined by thermogravimetric analysis (TGA). Immobilization of PMMA has been monitored by TGA of samples taken from the reaction mixture after 4, 9 and 21 hours. Activators regenerated by electron transfer (ARGET) ATRP of methyl methacrylate and styrene were conducted from initiator-grafted Fe(np) in the presence of phenol and Na-phenoxide respectively and the yields of immobilized polymers were found lower than those we observed in the case of conventional ATRP. The concentration of Fe(np) (c(m)) in the reaction mixture was found to influence significantly the appearance of products; at higher c(m) values SI-ATRP of styrene resulted in the formation of magneto-responsive gels. Transmission electron microscope investigation of bare Fe(np) and magnetic gel revealed that the particles are well-dispersed in the gel while non-grafted Fe(np) form aggregates. XRD measurements were used to determine the composition of magnetic nanoparticles. It was found that the crystalline phase of Fe(np) is mostly reduced iron and the fraction of iron-oxide (hydroxide) species is below the detection limit of XRD. Immobilized polymers have been degrafted by digestion of Fe(np) in HCl and trifluoroacetic acid and the residual polymeric material were identified by FTIR and 1H-NMR spectroscopy. A plausible mechanism for the formation of magnetic gel was given in which polymer network is built up through interparticle termination reactions and Fe(np) act as functional cross-linkers.