Activation and Stereospecific Control in Propylene Polymerization with MgCl2 Supported Ziegler-Natta Catalysts

Polymerization Stereochemistry with Ziegler−Natta Catalysts Containing Dialkylpropane Diethers: A Tool for Understanding Internal/External Donor Relationships

“…If this is the case, the position in which first the internal and then the external donor is absorbed should be, at least for some active centers, close enough to the active titanium to affect the stereochemistry of the monomer insertion. Other considerations also seem to disfavor the hypothesis that the whole complex takes part in the formation of the isospecific active centers (equilibrium 4) as has been proposed. − Polypropene samples have been prepared in the presence of 13 C selectively enriched aluminum alkyls bearing, besides the 13 C-enriched ethyl group, alkyl groups of different sizes: Al( 13 CH 2 CH 3 )(iC 4 H 9 ) 2 and Al( 13 CH 2 CH 3 )(CH 3 ) 2 …”

“…Other considerations also seem to disfavor the hypothesis that the whole complex takes part in the formation of the isospecific active centers (equilibrium 4) as has been proposed. [23][24][25] Polypropene samples have been prepared in the presence of 13 C selectively enriched aluminum alkyls bearing, besides the 13 C-enriched ethyl group, alkyl groups of different sizes: Al( 13 CH 2 -CH 3 )(iC 4 H 9 ) 2 and Al( 13 CH 2 CH 3 )(CH 3 ) 2 . 26 The analysis of first-step stereoregularity has shown that the mechanism of steric control on the first monomer insertion is completely independent of the steric bulkiness of the aluminum alkyl cocatalyst.…”

Impact of Process Poisons on the Performance of Post‐Phthalate Supported Ziegler–Natta Catalysts in Gas Phase Propylene Polymerization

“…Similar behavior has been reported when aromatic ester compounds are used as electron donors and some studies have shown it to be the strongest in poisoning the active sites, and decreases the atactic portion of the polymer quite well. [21][22][23] Conventional silanes which are used for controlling the polymer tacticity are known to cause indiscriminate poisoning of all types of active sites beyond a specific threshold if the molecule is not bulky enough to favor selective poisoning of aspecific sites. [24] Moreover, both ethyl acetate and dimethyl sulfoxide have been used as complexing agents with co-catalysts in polymerizations of polar monomers such as vinyl chloride, and methyl methacrylate with Ziegler-Natta type catalysts and their ability to control the polymers' stereoregularity whereby similar polymers as those produced using radical initiators are obtained have been observed.…”