Role of a Multidentate Carbonate Donor in MgCl2-Supported Ziegler-Natta Olefin Polymerization Catalysis: An Experimental and Computational Approach

A new multidentate carbonate (3,3,3',3'-tetramethyl-2,2',3,3'-tetrahydro-1,1'-spirobiindane-5,S',6,6'-tetracarbonate) is studied as a potential internal electron donor for Ziegler Natta (Z-N) catalysts. The non-phthalate nature of the multidenate carbonate donor has an advantage in comparison to hazardous phthalate donors. The multidentate carbonate donor coordination to the MgCl2 surface is investigated through mono-, bi-, tri-, and tetradentate modes using density functional theory. These calculations indicate that the tridentate donor coordination is thermodynamically more favorable with respect to the mono-, bi-, and tetradentate ones. Ethylene polymerization with the multidentate carbonate as the internal electron donor is studied, and medium-to high-molecular-weight polyethylene is observed, which is confirmed by intrinsic viscosity (IV) and melt flow index analysis. The experimental findings are validated by quantum chemical calculations, which suggest that the multi(bi)dentate (multicarbonate donor with bidentate coordination) active site leads to high-molecular-weight polymers, whereas the multi(ti)dentate and multi(tetra)dentate donor active sites only form oligomers owing to less space on the titanium center for the incoming monomer. For a comparative study, phthalate, diether, and succinate donors are also employed for ethylene polymerization. Furthermore, the regio-and stereoselective behavior of a propylene monomer on Ti alkyl bonds (alkyl = -Et and -isobutyl) is investigated for all donor cases, and it is found that the multidentate carbonate is equally effective as other donors. Therefore, the current multidentate carbonate donor catalyst study sheds light on the nature of different adsorption modes of donors on the MgCl2 support and how this new multidentate carbonate donor influences the Z-N olefin polymerization process.