Keywords: Zirconium / Zirconium oxo cluster / Hafnium oxo cluster / Metal carboxylate / Ligand exchange 12 ] 2 is also reported for comparison. The Zr12 clusters are structurally related to the Zr6 clusters since they are composed of two Zr6 sub-units which are bridged by four carboxylate ligands. In each compound, carboxylic acid molecules interact with the cluster by hydrogen bonding to part of the µ 3 -OH ligands. The clusters are highly dynamic in solution, due to site exchange of the

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Upon reaction of titanium propoxide and yttrium 2-methoxyethoxide with a sevenfold excess of methacrylic acid, three different mixed-metal clusters were obtained, depending on the titanium to yttrium alkoxide ratio.

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Pre-formed transition metal oxide clusters with methacrylate ligands were polymerized in the presence of organic co-monomers to form inorganic-organic hybrid polymers. The number of polymerizable ligands and thus the crosslinking density of the hybrid polymers can be varied for a given cluster type by partially exchanging the methacrylate ligands for non-reactive ones. The properties of the hybrid polymers can also be varied by adjusting the conditions of the polymerization reaction, especially by a pre-polymerization procedure. It is shown that the thermal stability of polystyrene crosslinked by Zr 6 O 4 (OH) 4 (OMc) 12 is thus greatly improved.