 The study examined the effect of shear stress on the crystallization of isotactic polypropylene, high PP, nucleated with either a sulcesquioxane-based or commercial nucleating agent. The sulcesquioxane-based nucleating agent was more efficient than the commercial one at reducing the amount of amorphous material in the polymer. Shear stress had a significant impact on the crystal size and morphology of the polymers, regardless of whether they were nucleated with the sulcesquioxane-based or commercial nucleating agent. The study also showed that the sulcesquioxane-based nucleating agent was more effective at preventing the formation of large crystals compared to the commercial nucleating agent. This article was authored by Mateusz Barksowski, Olga Misijukiewicz, Michael Dutkiewicz, and others.