Why does the use of different external electron donors—even when the same Ziegler–Natta catalyst is employed for polypropylene (PP) production—result in PP with varying structures and properties?

The answer lies in the selective regulation of the catalyst's active sites.

Diphenyldimethoxysilane (DDS, CAS 6843-66-9) is a typical alkoxysilane compound that serves as an external electron donor in Ziegler–Natta propylene polymerization systems. Published research indicates that alkoxysilanes with different structures exert varying effects on the polypropylene chain structure; consequently, the selection of the external electron donor is a critical step in designing the PP catalyst system.

"Polypropylene isotacticity" can be simply understood as the degree of regularity in the arrangement of methyl groups along the polymer backbone. A higher isotactic content generally facilitates the formation of a more ordered crystalline structure in PP; therefore, industrial production requires the catalyst system to control this stereoselectivity.

The DDS molecule contains two phenyl groups and two methoxy groups. Its structural characteristics enable it to act as an electron donor, interacting with the Ziegler–Natta catalyst system and thereby influencing the reaction selectivity of the active sites. Studies have compared diphenyldimethoxysilane with other external electron donors, examining its impact on PP molecular weight, molecular weight distribution, and stereoregularity.

It is worth noting that simply adding more DDS does not necessarily yield better results. The dosage of the external electron donor must be optimized based on factors such as catalyst composition, Al/Ti ratio, Si/Ti ratio, hydrogen concentration, and polymerization temperature. In some industrial systems, DDS is combined with other external electron donors to achieve specific PP structures and processing characteristics. Research has documented cases where a combination of DDS and another external electron donor was used to produce iPP/EPR reactor alloys.

Therefore, when selecting DDS, the focus should be on purity, impurity control, batch-to-batch consistency, and compatibility with the existing catalyst system, rather than merely on the product name.