The IR spectra of neodymium complexes supported on styrene-acrylic acid copolymer were investigated. The complexes possess a bidentate carboxylate structure, and the Nd‒O bond in the complexes is highly covalent. The effect of the nature and constitution of the supported neodymium complexes on their activity in butadiene polymerization was studied. The copolymer was prepared in THF or dioxane was better for the synthesis of highly active supported complexes. Polymer-supported neodymium complexes containing about 12 wt% of -COOH gave optimum catalytic activity at a Nd/‒COOH mole ratio of 0.20.