The soluble poly(methyl methacrylate-co-octavinyl-polyhedraI oligomeric silsesquioxane) (PMMA-POSS) hybrid nanocomposites with improved T, and high thermal stability were synthesized by common free radical polymerization and char acterized. using FTIR, high-resolution H-1 NMR, Si-29 NMR, GPC, DSC, and TGA. The POSS contents in the nanocomposites were determined based on FTIR spectrum, revealing that it can be effectively adjusted by varying the feed ratio of POSS in the hybrid composites. On the basis of the 1H NMR analysis, the number of the reacted vinyl groups on each POSS molecules was determined to be about 6-8. The DSC and TGA measurements indicated that the hybrid nanocomposites had higher T-g and better thermal properties than the pure PMMA homopolymer. The T-g increase mechanism was investigated using FTIR, displaying that the dipole-dipole interaction between PMMA and POSS also plays very important role to the T, improvement besides the molecular motion hindrance from the hybrid structure. The thermal stability enhances with increase of POSS content, which is mainly attributed to the incorporation of nanoscale inorganic POSS uniformly dispersed at molecular level. (c) 2007 Wiley Periodicals, Inc.