Ethylene polymerization with supported vanadium-magnesium catalyst: Hydrogen effect

Full Paper: Ethylene polymerization on supported vanagdium-magnesium cataly sts with the composition VCl4/ MgCl2 and VOCl3/MgCl2 was studied in the pre sence of hydrogen as the chain transfer agent. Vanadium-magnesium catalysts (VMC) are known to exhibit a high reactivity towards hydrogen. This reacti vity is manifested as a sharp decrease in polyethylene (PE) molecular weigh t accompanied by a dramatic (2-6-fold) loss of catalyst activity, when hydr ogen is introduced. Catalyst deactivation in the presence of hydrogen is re versible, Catalyst activity is restored, when hydrogen is removed from the reaction medium. With this regard we have studied the effect of the concent ration of hydrogen and of Al(i-Bu)(3) and Al(i-Bu)(2)H, used as co-catalyst s, on the activity of VMC. We have also identified the reaction products fo rmed after polymerization reaction in the presence of hydrogen. Based on ex perimental data we suggest reaction the scheme leading to catalyst deactiva tion in the presence of hydrogen and its reactivation, when hydrogen is rem oved from the reaction medium. According to his scheme, catalyst deactivati on 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 a ctive sites. It is shown that the deactivation process may be diminished, i f some modifiers able to bind with alkylaluminumhydride are introduced into the catalyst composition. Magnesium dichloride introduced into the catalyt ic system or aluminum trichloride introduced into the catalyst composition may be:used for modification. Using this approach we have obtained highly a ctive supported VMC with an activity up to 50 kg/gV . h . bar C2H4 (80 degr eesC) at polymerization in. the presence of hydrogen. [GRAPHICS] Effect of addition of hydrogen during ethylene polymerization. Polymerizati on without hydrogen (stage A) with consequent H-2 addition (stage B) and H- 2-degassing and then ethylene addition (stage C). Polymerization conditions : Catalyst VCl4/ MgCl2 . nAlCl(3) (1.7 wt.-% of V), 80 degreesC, [TIBA] = 7 .6 mmol/L, stage A - ethylene pressure 2.4 bar, Stage B - ethylene pressure ; 2.4; bar, hydrogen pressure 0.25 bar,- stage C - ethylene pressure 2.4 ba r.