Cr[CH(SiMe3)2]3/SiO2 catalysts for ethene polymerization: The correlation at a molecular level between the chromium loading and the microstructure of the produced polymer

Several spectroscopic techniques have been applied in a concerted way to investigate the local structure and the function of the chromium sites in a series of Cr[CH(SiMe3)2]3/SiO2 catalysts for ethene polymerization that differ in chromium loading and to correlate them with the microstructure of the produced polyethylene (PE). According to previous studies, the density of the produced PE is proportional to the Cr loading: the higher the Cr loading, the lower the PE density, because of an increase in the content of short-chain branching. Here, the nature of the branches and their enchainment in the polymer have been further investigated to determine the different properties of the produced polymers, and the differences detected have been traced back by spectroscopic analysis to the presence of two different types of Cr sites working in tandem: monografted [tbnd]SiO[sbnd]Cr[CH(SiMe3)2]2 species that are responsible for ethene oligomerization, and bis-grafted ([tbnd]SiO[sbnd])2CrCH(SiMe3)2 species that account for ethene homo- and co-polymerization. The relative abundance of the two species depends on the chromium loading. Moreover, we have also found that carbon monoxide selectively poisons the sites responsible for ethene oligomerization, indicating a possible strategy for the fine regulation of the properties of the produced PE.