Precise Active Site Analysis for TiCl3/MgCl2 Ziegler-Natta Model Catalyst Based on Fractionation and Statistical Methods

Abstract

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 TiCl 3 /MgCl 2 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.

Effects of titanium dispersion state on distribution and reactivity of active centers in propylene polymerization with MgCl2‐supported Ziegler‐Natta catalysts: A kinetic study based on active center counting

“…Cat-1 with ultralow Ti content is used as a model catalyst that mainly contains isolated mononuclear Ti species. [18][19][20][21][22][23][24][26][27][28][29][30] Cat-2 with higher Ti content and more clustered Ti species is similar with those supported ZÀ N catalysts used in polyolefin industry. Therefore, the catalysts used here can be taken as models for the two extremes of Ti dispersion states.…”

“…By comparing olefin polymerization behaviors of different model ZÀ N catalysts with controlled Ti content, close correlations between the amount of clustered Ti species and stereoselectivity of TiCl 4 /MgCl 2 or TiCl 3 / MgCl 2 catalysts have been reported. [19][20][21][22][23][24] The model catalysts with ultralow Ti load were found to produce polypropylene with very low isotacticity, implying that clustered Ti species are the main active sites of isospecific polymerization. In our previous work, strong effects of Ti dispersion state on the responses of TiCl 4 / MgCl 2 type model catalysts to electron donor addition were reported, and mechanistic explanation was proposed based on the results of counting active centers in propylene polymerization.…”

Genesis of MgCl2‐based Ziegler‐Natta Catalysts as Probed with Operando Spectroscopy

“…A slight decrease of the n(OH) absorption band at around 3500 cm À1 indicates that TiCl 4 reacts with residual alcohol-derived species and/or with traces of adventitious H 2 O on MgCl 2 surface. [113][114][115] We are tempted to exclude Ti over-reduction on account of the low amount of TEA used during the experiment (Al : Ti = 2 : 1), [116] but the co-presence of both isolated TiCl x species and Ti clusters, each one expressing a specific activity, [117] is plausible and cannot be discarded at all. [104] During the activation with TEA at 25 8C, an extensive absorption slowly appears in the DR UV-Vis spectra in the d-d region (from spectrum 2 to 3 in Figure 5a), showing the reduction of the Ti 4 + centers to Ti 3 + .…”

Effect of AlEt3and an External Donor on the Distribution of Active Sites According to Their Stereospecificity in Propylene Polymerization over TiCl4/MgCl2Catalysts with Different Titanium Content

“…According to ref ,. the TiCl 3 ·3Py/MgCl 2 model catalysts with a low titanium content (≤0.1 wt%) produce PP with a higher activity, a narrower MWD, and a lower stereoregularity compared to similar TMCs having the high titanium content.…”