The role of tetrahydrofuran as Lewis base intitanium-magnesium catalysts for ethylene polymerization
The subject of the investigations were titanium-magnesium catalysts based on MgCl2, TiCl4 or TiCl3, and Et3Al or Et2AlCl. These catalysts, containing tetrahydrofuran (THF) as aninternal Lewis base (i.e. THF added to MgCl2 or to the titanium component), polymerize ethylene under low pressure with high yields. The presence of THF accelerates ball-milling of precatalyst and favours chemical anchoring of TiCl4 to MgCl2, which leads to an increaseof active center concentration in comparison to pure MgCl2. THF reacts not only with MgCl2 but also with organoaluminium compounds used as cocatalysts (activators). THF used as an internal Lewis base in form of acomplex with TiCl4, i.e. [TiCl4(THF)2], partly reduces the catalyst activity in comparison with that of the complex [MgCl2(THF)2] (compare Lp. 5 with Lp. 7 and Lp. 6 with Lp. 8 in Table 2). It is doubtful whether THF is a part of active centre in the catalyst, that is whether it is directly bonded with titanium atom in active centre. THF used as an external Lewis base (i.e. added together with organoaluminium compound or separately) completely deactivates the catalyst.
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