Hydrogen Responses of Propylene Polymerization with MgCl2-Supported Ziegler–Natta Catalysts in the Presence of Different Silane External Donors

External donor (De) modification is an effective way to enhance the stereoselectivity of propylene polymerization with supported Ziegler–Natta catalysts. Aminosilane as a novel type of external donor has been found to have the excellent ability of enhancing the isoselectivity of propylene polymerization. In this work, dipiperidyldimethoxysilane (Donor-Py) was compared with cyclohexyl(methyl)dimethoxysilane (Donor-C) and dicyclopentyldimethoxysilane (Donor-D) for propylene polymerization in the presence or absence of hydrogen. By analyzing the effects of external donors on catalytic activity, polymer chain structures and their distributions, and the number and reactivity of three groups of active centers with different stereoselectivities, the performance of each De in enhancing stereoselectivity was compared. Propylene polymerization in the presence of RSi(OR’)3-type De (R = n-propyl or i-butyl, R’ = methyl or ethyl) and hydrogen was also studied. Donor-Py produced PP with higher molecular weight and was more sensitive to hydrogen than Donor-D. According to the fractionation results, Donor-Py produced PP with the lowest content of medium-isotactic PP and the highest content of highly isotactic PP, especially at high hydrogen concentrations. By raising hydrogen concentrations, the number of active centers was enhanced in systems with Donor-C and Donor-D while it was reduced when Donor-Py was added. When Donor-Py was used as De, the effects of H2 concentration on active center distributions and the reactivity of different active centers were evidently different from those of Donor-C and Donor-D. Donor-Py showed the best performance among the De used in this work in producing PP with both high isotacticity and good processability. The mechanism of De effects and hydrogen effects is discussed based on the results of polymerization kinetics and PP chain structures.