Roles of malonate donor on activity and stereoselectivity of Ziegler–Natta catalyzed propylene polymerization

• The chelate mode is the preferred donor-catalyst adsorption mode.
• The Ziegler–Natta catalyst provides the regioselectivity for propylene insertion.
• The electron donor provides stereoselectivity for propylene insertion.
• The electron transfer from the donor to Ti stabilized the transition state.

The role of malonate compounds as electron donors in the propylene polymerization using Ziegler–Natta (ZN) catalyst were investigated. Adsorption modes of a malonate compound on the MgCl2(110) surface were also studied. Calculations revealed that the chelate mode was preferred over other modes (mono, bridge, and zip). For the malonate donor, the energy of the transition state of the primary (1,2) insertion of the propylene to Ti–iBu bond is lower than that of the secondary (2,1) insertion and that of the primary (1,2)-si is lower than that of the primary (1,2)-re. Without the malonate donor, the transition state of the primary (1,2)-si and -re has the similar energy. This suggests that in the presence of the malonate donor the ZN catalyst shows both stereo (favoring si face) and regioselectivity (favoring primary (1,2)) for polypropylene polymerization in agreement with experiments. The steric repulsion between the methyl group of propylene and the alkyl moiety of the catalyst controls the regioselectivity while the steric interaction between the propylene and the electron donor dictates the stereoselectivity. The electron donor also stabilized the transition state structure by transferring electrons to Ti resulting in the reduction of the activation energy and hence the catalyst becomes more active.

Adsorption modes of malonate donor on MgCl2(110) surface together with the kinetics of the ZN catalyzed propylene polymerization reaction in the presence and absence of the malonate donor through the primary (1,2) and the secondary (2,1) insertion steps for both re and si faces were investigated using DFT calculations.Figure optionsDownload as PowerPoint slide