A series of alkyne-functionalized poly(4-(phenylethynyl)styrene)-block-poly(ethylene oxide)-block-poly(4-(phenylethynyl)styrene) (PPES-b-PEO-b-PPES) ABA triblock copolymers was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization. PESn[Co-2(CO)(6)](x)-EO800-PESn[Co-2(CO)(6)](x) ABA triblock copolymer/cobalt adducts (10-67 wt % PEO) were subsequently prepared by reaction of the alkyne-functionalized PPES block with Co-2(CO)(8) and their phase behavior was studied by TEM. Heating triblock copolymer/cobalt carbonyl adducts at 120 degrees C led to cross-linking of the PPES/Co domains and the formation of magnetic cobalt nanoparticles within the PPES/Co domains. Magnetic hydrogels could be prepared by swelling the PEO domains of the cross-linked materials with water. Swelling tests, rheological studies tests demonstrated that the water capacity and modulus of the hydrogels were dependent upon the composition of the block copolymer precursors.