IMPROVED MULTI-STAGE PROCESS FOR PRODUCING MULTI-MODAL ETHYLENE POLYMER COMPOSITION

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), 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)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 polyethylene, whereby the process shows a reduced change of comonomer response with polymerization time and a more stable comonomer conversion with polymerization time compared to multistage processes using Ziegler-Natta catalysts with 100% Ti. La présente invention concerne un procédé de polymérisation multi-étages pour la production d'un polyéthylène basse densité linéaire multimodal dans au moins deux réacteurs en étages raccordés en série comprenant au moins (i) la polymérisation dans un premier étage en phase de suspension épaisse de monomères d'éthylène et facultativement d'un ou plusieurs comonomères d'alpha-oléfine, en présence d'un système de catalyseur de polymérisation Ziegler-Natta pour obtenir un premier composant de fraction de polyéthylène (A) (ii) la polymérisation dans un deuxième étage en phase gazeuse ou de suspension épaisse de monomères d'éthylène et d'un ou plusieurs comonomères d'alpha-oléfine, en présence d'un système de catalyseur de polymérisation Ziegler-Natta pour obtenir un