Abstract: Slurry-type Ziegler-Natta (Z-N) catalysts are predominantly employed in industrial olefin polymerization process. However, the influence mechanisms of the dispersion mediums on the catalytic behaviors of the slurry Z-N catalysts remain insufficiently understood. This study investigated the propylene polymerization behaviors using particulate Z-N catalyst (Cat-P) and slurry Z-N catalysts (Cat-M1 and Cat-M2) derived from Cat-P with different dispersion media. The results demonstrated that Cat-P exhibited faster activation, higher catalytic activity and yielded polypropylene (PP) with higher isotacticity and the lowest fine powder content. In contrast, the slurry catalyst Cat-M1 showed the lowest catalytic activity, produced PP with a high fine powder content (15.7 wt.%) and reduced isotacticity. The slurry catalyst Cat-M2, prepared by modifying the viscosity and surface tension of dispersion medium M1, showed significant improvements in catalytic activity, PP isotacticity, and fine powder content compared to Cat-M1. The study elucidated that the dispersion medium with high viscosity and surface tension impeded the diffusion of co-catalyst, external donor, and propylene monomer into the catalyst particles. Diffusion control slowed the formation of highly stereospecific active sites and, while also leading to insufficient monomer supply locally, which in turn facilitated the deactivation of some active sites via further reduction by alkylaluminum. Consequently, Cat-M1 exhibited lower initial activity, decreased isotacticity, and increased fine powder generation during the early polymerization stage. The particulate Cat-P and modified Cat-M2 catalysts allowed for rapid initiation and polymerization at the Ti active sites. The rapidly growing polymer chains effectively exerted a “guard effect” to protect the active sites from deactivation. This work clarified the diffusion-controlled effects during the activation and chain propagation of heterogeneous Z-N catalysts, providing a theoretical basis for the stable control of industrial olefin polymerization.

Key words: Slurry catalyst; Ziegler-Natta catalyst, Propylene; Diffusion control, Guard effect