Inorganic-organic hybrid materials were prepared by free radical polymerization of styrene in the presence of varying amounts of the cluster Zr6O4(OH)(4) (methacrylate)(12). Stepwise polymerization allowed the preparation of bubble- and crack-free, transparent bulk samples on a 30 g scale with dimensions required for mechanical testing. Small-angle X-ray scattering investigations and transmission electron micrographs revealed that the clusters formed randomly distributed aggregates of random size. Solvent uptake in swelling experiments was related to the cluster proportion. Storage moduli in the glassy state were slightly increased when compared with neat polystyrene, but pronounced plateau moduli were observed above the glass transition temperature, which correlated to the cluster proportion. Plateau moduli were used to calculate network parameters such as network density. Onset temperatures of thermal decomposition and the glass transition temperatures of the cluster-crosslinked polymers were higher than that of neat polystyrene. Thermal expansion coefficients were unaffected in the glassy state, but were gradually reduced above the glass transition temperature with increasing cluster proportion. Both the tensile moduli at room temperature and the yield points increased when polystyrene was doped with the cluster. The strain hardening moduli, as determined in compression tests at large deformations, increased linearly with the cluster proportion. (c) 2007 Wiley Periodicals, Inc.