Inorganic-organic hybrid polymers were prepared by co-polymerization of styrene or methyl methacrylate in the presence of small proportions of the cluster Zr4O2(methacrylate)(12). The thermal properties of the hybrid polymers were improved by the cluster-crosslinking. Variation of the reaction conditions had little influence on the thermal properties, although the gelation times were clearly influenced. The cluster-crosslinks were completely and selectively degraded by means of acetylacetone. This allowed the unique possibility of determining the number average molecular weights and polydispersities of the organic polymers which were initially interconnected by the clusters. (c) 2007 Elsevier Ltd. All rights reserved.