Multi-stage process for producing multi-modal linear low density polyethylene

Multi-stage polymerization process for the production of a multi-modal linear low density polyethylene in at least two staged reactors connected in series comprising at least

• (i) polymerizing in a first slurry phase stage ethylene monomers and optionally one or more alpha-olefine comonomers, in the presence of a Ziegler-Natta polymerization catalyst system to obtain a first polyethylene fraction component (A)
• (ii) polymerizing in a second gas or slurry phase stage ethylene monomers and one or more alpha-olefine comonomers, in the presence of a Ziegler-Natta polymerization catalyst system to obtain a second polyethylene fraction component (B),
• one of fraction component (A) or (B) being the lower molecular weight component of the linear low density polyethylene, the other being the higher molecular weight component of the linear low density polyethylene,
• wherein the second polymerization step is carried out in the presence of the polymerization product of the first polymerization step,
• whereby the Ziegler-Natta polymerization catalyst system comprises
• 1) a solid procatalyst formed by contacting at least:
• a) a Mg-alcoholate complex of the formula (I):
• Mg(OR1)2-n(R1)n, wherein each R1 independently represents a C1-C20 hydrocarbyl group and 0≦n<2 and may or may not be an integer;
• b) an aluminum compound of the formula (II)
• Al(R2)mX3-m, wherein each R2 independently represents an alkyl of up to 6 carbon atoms;
• each X is independently a halogen;
• 0≦m<3 and m and may or may not be an integer
• c) a vanadium compound and a titanium compound in portions such as to provide a molar ratio of V:Ti from 10:90 to 90:10
• in order to yield the solid procatalyst and
• 2) one or more organometallic cocatalyst(s) of the formula (III)
• wherein each R is independently a C1-C20-alkyl group, 0≦x≦2;
• 1≦y≦3;
• 0≦z≦2 and x+y+z=3;
• x, y and z may or may not be an integer,
• yielding a multi-modal linear low density polyethylene with a narrow molecular weight distribution, measured with GPC, below 20, a density according to ISO 1183 of 900 to 935 kg/m3, an MFR21 according to ISO 1133 (190° C., 21.6 kg load) of 1 to 15 g/10 min and a C3-C12-comonomer content of 1 to 15 wt %,
• whereby the linear low density polyethylene shows an improved comonomer composition distribution compared to linear low density polyethylene produced with multi-stage processes using Ziegler-Natta catalysts with 100% Ti.