Abstract
In heterogeneous Ziegler-Natta catalysts for olefin polymerization, isolation of a single type of active sites is a kind of ambition, which would solve long-standing questions on the relationship between active site and polymer structures. In this paper, polypropylene produced by TiCl3/MgCl2 model catalysts with minimum Ti heterogeneity was analyzed by combined solvent fractionation and the two-site statistical model. We found that the active sites of the model catalysts were classified into only three types, whose proportions were dependent on the Ti dispersion state. The addition of external donors not only newly formed highly isospecific sites, but also altered the stereochemical nature of the other active sites.
Highlights
Heterogeneous Ziegler-Natta catalysts are dominantly responsible for the industrial production of polypropylene (PP), due to their high activity, high isospecificity, superior morphology and broad molecular weight distribution (MWD) of produced PP
The increase of the Ti content in the model catalysts led to increase in the mmmm value, and accompanied a slight increase in the rrrr value, suggesting that both of the ES and CE natures were emphasized at higher Ti contents
TiCl3/MgCl2 model catalysts with controlled dispersion of Ti species were applied for the purpose of enabling active-site analyses at the single-site level with the aid of solvent fractionation and two-site statistical analysis
Summary
Heterogeneous Ziegler-Natta catalysts are dominantly responsible for the industrial production of polypropylene (PP), due to their high activity, high isospecificity, superior morphology and broad molecular weight distribution (MWD) of produced PP. 0.1 wt-%, the mmmm value monotonously increased from 37 to 50 mol%, which corresponds to the formation of TiCl3 multinuclear species [8,9] The other properties, such as the polymerization activity and MWD, follow the same dependences on the Ti content. We have combined the model catalyst approach with the two types of the fractionation approaches, aiming at precise active site separation for Ziegler-Natta propylene polymerization catalysts to clarify the following issues: the origin of polydispersity of molecular weight—even though all Ti species were isolated below. PP produced by the TiCl3/MgCl2 model catalysts was first fractionated by boiling hexane and subjected to statistical analysis based on the two-site model. Mononuclear Ti species enhanced the formation of the most isospecific site in the presence of EB
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