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

摘要

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(OR 1 ) 2-n (R 1 ) n , wherein each R 1 independently represents a C 1 -C 20 hydrocarbyl group and 0 ‰¤ n < 2 and may or may not be an integer;
b) an aluminium compound of the formula (II)
Al(R 2 ) m X 3-m , wherein each R 2 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 C 1 -C 20 -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,
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.