Activation of vanadium-based Ziegler-Natta catalysts by halocarbons for ethylene polymerization: results and mechanism
Polimery 1994, No 10, 585
The reasons for the low productivity of the heterogeneous and homogeneous V-based catalysts in the synthesis of LLDPE were discussed and some routes of improving their activity and stability were proposed. Ethylene polymerizations were performed in the isododecane solutions at 160°C and under constant ethylene pressure of 5 bars. One Ti-based catalytic system (TiCl3-0.3AlCl3) and two V-based systems (TiCl3-0.3AlCl3 and VCl4) were investigated. The main reason of activity loss is a rapid reduction of V4+and V3+ to inactive V2+ form. AlR3 cocatalysts are also involved in the deactivation process. The effect of addition to the system of various alkyl halides (a. o. of CCU, CH3Cl3, CF3CCCl3, CHCl3, CHBr3, CH2Cl2, CH2BrCl and CH2Br2) on the yield of polyethylene was investigated. The alkyl halides act as efficient activators for the heterogeneous and homogeneous vanadium catalysts in the high temperature ethylene polymerization and in its copolymerization with 1-hexane. The effect of the presence of CHCl3 on the short chain branching and themolecular weight distribution (GPC) of these copolymers was also investigated. Halocarbons do not act as chain transfer agents. The peak molecular weight remains almost unchanged, but a narrowing of molecular weight distribution is observed due to the suppression of the amount of high molecular- weight fraction in polymer.
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