Ethylene Polymerization with Supported Vanadium-Magnesium Catalyst: Hydrogen Effect
Macromolecular Chemistry and Physics
, E-ISSN: 1521-3935
Mikenas Tatiana B.
Zakharov Vladimir A.
Echevskaya Lyudmila G.
Matsko Michail A.
Boreskov Institute of Catalysis, Prospect Akademika Lavrentieva 5, Novosibirsk 630090, Russia
Ethylene polymerization on supported vanadium-magnesium catalysts with the composition VCl4/MgCl2 and VOCl3/MgCl2 was studied in the presence of hydrogen as the chain transfer agent. Vanadium-magnesium
catalysts (VMC) are known to exhibit a high reactivity towards hydrogen. This reactivity is manifested as a
sharp decrease in polyethylene (PE) molecular weight accompanied by a dramatic (2–6-fold) loss of catalyst
activity, when hydrogen is introduced. Catalyst deactivation in the presence of hydrogen is reversible. Catalyst
activity is restored, when hydrogen is removed from the reaction medium. With this regard we have studied the
effect of the concentration of hydrogen and of Al(i-Bu)3 and Al(i-Bu)2H, used as co-catalysts, on the activity of
VMC. We have also identified the reaction products formed after polymerization reaction in the presence of
hydrogen. Based on experimental data we suggest reaction the scheme leading to catalyst deactivation in the presence of hydrogen and its reactivation, when hydrogen is removed from the reaction medium. According to this
scheme, catalyst deactivation is caused by dialkylaluminumhydride forming in the side reaction of AlR3 co-catalyst
with the V-H bonds and then being adsorbed onto the catalyst active sites. It is shown that the deactivation process
may be diminished, if some modifiers able to bind with alkylaluminumhydride are introduced into the catalyst
composition. Magnesium dichloride introduced into the catalytic system or aluminum trichloride introduced
into the catalyst composition may be used for modification. Using this approach we have obtained highly active
supported VMC with an activity up to 50 kg/gV N h N bar C2H4 (808C) at polymerization in the presence of hydrogen.