27Al MAS solid state NMR study on coordinative nature of alkyl-Al cocatalysts on a novel SiO2-supported Ziegler–Natta catalyst for controlled multiplicity of molecular weight distribution

Molecular weight distribution (MWD) is most important factor to determine the mechanical property and processability of polyethylene (PE). The control of MWD is one of the main targets for development of new generation polymerization catalyst. In this work, a novel SiO2-supported Ziegler–Natta catalyst designed in a molecular level on the silica gel surface modified with linear long alkene chains was developed for ethylene polymerization. The surface modification of silica gel support was performed through the reaction between unsaturated alcohol, 9-decene-1-ol and hydroxyl groups on the SiO2 surface. This catalyst showed some unique catalytic properties for the control of MWD of PE, which varied significantly from broad and multimodal to narrow and unimodal by solely changing the types of alkyl-Al cocatalyst. The differences in molecular weight (MW) and MWD of the polymers were considered to be caused by different states of active sites in terms of different effects of alkyl-Al cocatalyst. Therefore, the coordinative nature of alkyl-Al should be expected to vary with its structure. The catalysts modified by various alkyl-Al cocatalysts were studied by 27Al magic angle spinning (MAS) solid state NMR method, and the relationship between the coordinative states of Al species and the MW and MWD of PE produced was clarified. Plausible models of active sites formed by various alkyl-Al cocatalysts were proposed as well.