Polymer cymantrenecarboxylate complexes of rare-earth metals, [Ln(η2-O2CCym)2(µ-O2CCym)4Ln(ROH)4] n · mSolv (Ln = Nd (I), Gd (II), Dy (III), Ho (IV), Er (V, Va); R = H and Me; Cym = (η5-C5H4)Mn(CO)3; Solv is a solvent molecule), are synthesized by the exchange reactions between LnCl3 and CymCO2K in aqueous–organic media. Two types of coordination sites alternate in the polymer chain: Ln3+ ions coordinating water or methanol molecules and oxygen atoms of the carboxylate anions and Ln3+ ions coordinating oxygen atoms of the bridging and chelate carboxylate anions. In both cases, the coordination number of Ln is 8. Polymer [Dy4(O2CCym)12(HOCH2CH2OH)3.76(H2O)0.48] n · 3nTHF (VI) containing four crystallographically independent Dy3+ ions is obtained when ethylene glycol is used instead of methanol. The thermal behavior of complexes I–V in argon and magnetism of complexes II, III, and IV are studied. Complex III exhibits the properties of a single-molecule magnet with the chain structure (single-chain magnet) (CIF files CCDC 1410905 (I), 1410917 (II), 1410906 (III), 1410911 (IV), 1410918 (V), and 1410934 (VI)).