2. Multigrain model

For highly porous catalyst monomer diffusion is less limited and monomer can penetrate into the pores of the catalyst more easily. Consequently polymer can grow throughout the particle and result in immediate fragmentation of the catalyst particles (Fig. 5).

 

Fig 5. Growth of polymer within the pores of porous catalysts

After initial breaking of catalyst particles into small fragments (microparticles), polymerization reaction occurs on surface of microparticles according to core–shell model. These microparticles together form porous macroparticles. This is the most accepted model for particle growth in olefin polymerization. Scheme of polyethylene morphology development during gas phase polymerization is shown in Fig. 6.

 

Fig. 6. Scheme of development of polymer with catalysts fragmentation

Several researchers have studied fragmentation of Ziegler–Natta catalyst for olefin polymerization. The fragmentation behavior of the emulsion-based Ziegler–Natta catalyst for propylene polymerizations was observed to be faster and more uniform than that of the MgCl₂ -supported and silica-supported catalysts of similar chemical composition [1-2].