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G. Trimmel, S. Gross, G. Kickelbick, U. Schubert:
"Swelling behavior and thermal stability of poly(methylmethacrylate) crosslinked by the oxozirconium cluster Zr₄O₂(methacrylate)₁₂";
Applied Organometallic Chemistry, 15 (2001), 401 - 406.

A new isomer of the oxozirconium cluster Zr₄O₂(methacrylate)₁₂ was obtained by reaction of zirconium butoxide with an excess of methacrylic acid. Contrary to the known isomer, the structure is centrosymmetric with a planar, rhombic Zr₄ core. Copolymerization with a 56-207-fold excess of methylmethacrylate results in cluster-crosslinked poly(methylmethacrylate) that swells in ethyl acetate but does not dissolve. The solvent uptake during swelling decreases and the thermal stability improves with an increasing portion of the cluster.