Influence of porosity on the fragmentation of Ziegler-Natta catalyst in the early stages of propylene polymerization

Catalyst fragmentation and polymer growth in the early stages of propylene polymerization has been investigated using different Ziegler-Natta catalysts. Polymerization was carried out in slurry under mild conditions and stopped at low yield. Scanning electron microscopy (SEM) has been used to characterize the surface and cross sectional morphology of polymer particles at different stages of particle growth. Different fragmentation behavior is observed, and it is found that the way that catalyst fragments in the early stages of polymerization is greatly influenced by the porosity of the catalyst. When a less porous catalyst was used, the resulting mass diffusion limitation causes layer-by-layer fragmentation, starting at the outer surface of the catalyst particle to the center. In contrast, for highly porous catalyst, monomer can easily penetrate into the pores of the catalyst /polymer particle. The polymer growing throughout the particle results in a coarse and instantaneous fragmentation.