Porous versus novel compact Ziegler-Natta catalyst particles and their fragmentation during the early stages of bulk propylene polymerization

The effect of the porosity of Ziegler-Natta catalyst particles on early fragmentation, nascent polymer morphology, and activity were studied. The bulk polymerization of propylene was carried out with three different heterogeneous Ziegler-Natta catalysts under industrial conditions at low temperatures, that is, with a novel self-supported catalyst (A), a SiO2-supported catalyst (B), and a MCl2-supported catalyst (C), with triethyl aluminum as a cocatalyst and dicyclopentyl dimethoxy silane as an external donor. The compact catalyst A exhibited no measurable porosity and a very low surface area (< 5 m(2)/g) by Brunauer-Emmet-Teller analysis, whereas catalysts B and C showed surface areas of 63 and 250 m(2)/g, respectively. The surface and cross-sectional morphologies of the resulting polymer particles at different stages of particle growth were analyzed by scanning electron microscopy and transmission electron microscopy. The compact catalyst A showed homogeneous and instantaneous fragmentation already in the very early stages of polymerization, which is typically observed for porous MgCl2-supported Ziegler-Natta catalysts. Moreover, the compact catalyst particles gave rise to almost perfectly spherical polymer particles with a smooth surface. In contrast, the silica-supported catalyst B gave rise to particles having a cauliflower morphology, and the second reference catalyst C produced fairly spherical polymer particles with a rough surface. All of the three catalysts exhibited similar activities of 450 g of polypropylene/g of catalyst after 30 min of polymerization, and most interestingly, the comparative kinetic data presented indicated that the reaction rates were not influenced by the porosity of the catalyst. (C) 2008 Wiley Periodicals, Inc.