Novel sodium benzoate modified silsesquioxane (nbPOSS) for isotactic polypropylene modification was synthesized and characterized. The influence of the novel silsesqioxane based nucleating agent on mechanical and thermal properties of commercial isotactic polypropylene was observed and compared with sodium benzoate. The mechanical investigations revealed changes in polymeric structure indicating that nucleating process had occurred. Synthetic route of 1-(3-benzoyloxypropyl)-3,5,7,9,11,13,15-hepta(isobutyl)pentacyclo-[9.5.1.13,9.15,15.17,13]octasiloxane and its characterization were presented. [ABSTRACT FROM PUBLISHER]