Coadsorption model for first-principle description of roles of donors in heterogeneous Ziegler–Natta propylene polymerization

Systematic periodic density functional calculations were conducted to clarify the mechanism for donors to exert steric and electronics influences on propylene polymerization using heterogeneous Ziegler–Natta catalysts. It was concluded that TiCl4 preferentially adsorbs as mononuclear species on the MgCl2 (1 1 0) surface, and the coadsorption of donors with it is energetically viable. The coadsorption of donors on the (1 1 0) surface reinforces the electron density of the Ti mononuclear species and sterically transfers the underlying C2 symmetry to convert the originally aspecific mononuclear species into isospecific one. The nearest coadsorption of ethylbenzoate (EB) not only sterically induces the isospecificity of the Ti mononuclear species but also electrostatically improves the regiospecificity in propylene insertion. In addition, EB prevents sterically demanding chain transfer to propylene, increasing the molecular weight of the produced polypropylene. Thus theoretically derived “coadsorption model” is highly consistent with a variety of experimentally known facts and believed to be useful for the ab initio prediction of new donor structures.

Systematic density functional calculations have been conducted to elucidate molecular roles of donors in heterogeneous Ziegler–Natta propylene polymerization. Coadsorption of donors with Ti mononuclear species on the MgCl2 (1 1 0) surface, which is energetically advantageous, can reproduce a variety of experimental facts such as the improvements in the stereo- and regiospecificities, the increase in polymer molecular weight.Figure optionsDownload high-quality image (144 K)Download as PowerPoint slideHighlights
► DFT calculations on roles of donors in Ziegler–Natta propylene polymerization.
► Coadsorption of donors with TiCl4 mononuclear species on the MgCl2 (1 1 0) surface.
► Coadsorbed donors amplify surface C2 symmetry to induce the stereospecificity.
► Coadsorbed donors enrich the Ti electron density to enhance the regiospecificity.
► A theoretically and experimentally consistent “coadsorption model” is proposed.