Al-alkyl cocatalyst

In ternary Nd Ziegler/Natta catalyst systems of the type Nd compound/cocatal-yst/halide donor Al alkyl cocatalysts yield high czs-l,4-BR whereas Mg alkyl cocatalysts yield fra s-l,4-poly(butadiene). To date, it is not clear why the use of these two classes of co catalysts leads to completely different polymer microstructures. 

Using authentic MgCl2 as the support led to even better catalysts for polyethylene, whereas the performance for polypropylene was ambivalent high productivity (>150 kg of polymer per gram of Ti) but poor stereoselectivity (less than 40% highly isotactic polymer) [11], However, the addition of proper Lewis bases to the catalyst formulation (Table 1), as components of the solid precatalyst ( internal donor ) or complexed with the Al-alkyl cocatalyst ( external donor ), improved both the productivity (up to 2-3 tons of polymer per gram of Ti) and the stereoselectivity (>95% highly isotactic polymer) [11, 38, 39]. 

During the last few decades, experimental reports about the combination of Al-alkyl cocatalyst with the Phillips catalyst have been very limited. Spitz et al. [104] reported the significant effect of TEA on Phillips catalyst for the activity. 

The Al-alkyl cocatalyst could also be introduced during the polymerization stage, with simultaneous interaction of catalyst with Al-alkyl cocatalyst and monomer within the polymerization reactor. Ethylene homopolymerization using Phillips catalyst PC600 calcined at 600°C followed by activation with TEA cocatalyst... 

Table 2 Results of ethylene polymerization/oligomerization runs using BC (3f) and [(PhaSiO) Cr (THF)]2 0/-OSiFTi3)2 (9f) with Al-alkyl cocatalysts ...

The transformation from ethylene polymerization to ethylene nonselective oligomerization over the two model catalysts (3f and 9f) in the presence of Al-alkyl cocatalyst MAO with the increase in Al/Cr molar ratio is shown in Scheme 14. Such interesting transformation phenomenon could not be found using the same catalysts combined with TiBA. Similar polymerization/oligomerization transformation behavior has also been reported recently on Cr-based ethylene trimerization... 

The activation of the Phillips chromium catalyst by Al-alkyl cocatalyst was also systematically studied by XPS and solid state nuclear magnetic resonance (NMR) (Liu et al., 2005 Xia et al., 2006). As indicated by XPS characterizations, there are four oxidation states including +2, +3, +5, and +6 of surface chromium species of triethylaluminum-modified PhiUips catalyst. The observations from above experiments suggested that the active precursor of the chromium site may have the flowing form Cr -2Cr composed of one Cr(II)Ox,surf species and two Cr(VI)Ox,surf species, in which Cr(II)Ox,surf species acted as the real active chromium... 

Fang Y, Liu B, Hasebe K, Terano M Ethylene and 1-hexene copolymerization with CO-prereduced Phillips CrOx/Si02 catalyst in the presence of Al-alkyl cocatalyst, J Polym Sci A Polym Chem 43(19) 4632—4641, 2005b. 

Li L, Wu Y, Dong Q, et al Effect of Al-alkyl cocatalyst on ethylene polymerization behavior using silyl chromate (SC)/Si02 catalyst, Asia PacJ Chem Eng 8(4) 539—546, 2013. 

Liu B, Fang Y, Nakatani H, Terano M Surface physico-chemical state of CO-prereduced Phillips Cr0x/Si02 catalyst and unique polymerization behavior in the presence of Al-alkyl cocatalyst, Macromol Sytnp 213(1) 37—46, 2004b. 

Xia W, Liu B, Fang Y, Hasebe K, Terano M Unique polymerization kinetics obtained from simultaneous interaction of Phillips Cr(VI)Ox/Si02 catalyst with Al-alkyl cocatalyst and ethylene monomer, J Mol Catal A Chem 256(1—2) 301—308, 2006. 

The fraction of r-rich sequences is higher in polypropene samples obtained with MgCl2-supported high-yield catalysts, particularly when suitable Lewis bases are added to the Al-alkyl cocatalyst [17-20]. High-yield catalyst systems have been disclosed which are able to produce polypropene samples containing up to one-third by weight of such sequences [19]. 

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