Basicity of stereoregulating electron-donor compounds in ziegler-natta catalysts: A study by infrared spectroscopy and chemical exchange reactions

Abstract

The Lewis basicity of typical electron donor compounds used to modify heterogeneous Ziegler-Natta catalysts, such as diakyl ethers, silyl ethers, esters, and ketones, was characterized by determining their effects on the IR frequency of surface silanol groups of silica gel, which correlate with the heat of this specific interaction and with their "donicity", i.e., with the heat of formation of their complexes with SbCl5. In this manner, ethers and silyl ethers are found to be more basic than ketones and esters. These differences in basicity are likely to affect the stereoregulating properties of such electron donor compounds in Ziegler-Natta polymerization catalysis. By consecutively adsorbing electron donors with various strengths on a MgCl2 support, it was shown that weakly basic donors, such as ethyl benzoate or dibutyl phthalate, are partially displaced by donors with higher basicity, such as valerophenone and propyltrimethoxysilane. This indicates that the displacement of inner donors of the ester type from a titanium-magnesium catalyst upon addition of typical external donors, such as silyl ethers, is controlled by the relative basicities of these donor molecules.