Poly(methyl methacrylate)/montmorillonite (MMT) nanocomposites were prepared by in situ bulk polymerization. The results showed that the silicone coupling agent affected the structure and properties of hybrid materials. XRD analysis showed that the dispersion of clay in nanocomposites with silicone-modified organophilic MMT was more ordered than that in nanocomposites with unmodified organophilic MMT. The glass transition temperature (T-g) of the nanocomposites was 6-15degreesC higher and the A decomposition temperature (T-d) was 100-120degreesC thermal higher than those of pure PMMA. (C) 2003 Wiley Periodicals, Inc.