The polymerization conditions for polystyrene and poly(methyl methacrylate) crosslinked by 0.5 mol % of the cluster Zr6O4(OH)(4)(methacrylate)(12) were optimized by applying a step polymerization procedure. The onset of thermal decomposition was thus increased up to about 50 degrees for polystyrene and about 110 degrees for poly (methyl methacrylate). The increase in thermal stability correlated with a higher char yield. The glass transition temperatures were also increased by about 15 degrees. (c) 2005 Wiley Periodicals, Inc.