Polyethylene prepared with supported late transition metal catalyst systems
The present invention relates to the field of ethylene polymerization with a supported late transition metal catalyst system.
1. A process for preparing a polyethylene that is characterised by simultaneously having a content of short chain branching of at least 20 short chain branches per 1000 methylene groups and an activation energy of less than 50 kJ/mole, the process comprising:
a) supporting a late transition metal catalyst component onto an activating support to obtain an activated catalyst system, wherein the activating support comprises at least 10 wt % of aluminium, based on the total weight of the activating support;
b) injecting said activated catalyst system in a polymerisation reactor with ethylene monomer, in the absence of added comonomer;
c) injecting a scavenger selected from trialkylaluminium in the polymerisation reactor simultaneously with or before or after step b);
d) maintaining at a polymerisation temperature of at least 70° C.;
e) retrieving the polyethylene product.
2. The process of claim 1 wherein the activating support comprises silica.
3. The process of claim 2 , wherein the silica is characterized by a surface area of at least 150 m 2 /g and of at most 700 m 2 /g.
4. The process of claim 2 , wherein the silica is characterized by an average pore diameter of at least 100 Å and of at most 500 Å.
5. The process of claim 1 , wherein the activating support has an aluminium content ranging between 10 and 20 wt %.
6. The process of claim 1 , wherein the late transition metal catalyst component is represented by formula LMX n wherein L is a neutral bidentate ligand, M is a metal of group 8 to 10 of the periodic table, each X is the same or different and is a halogen, alcoholate or hydrocarbyl, and n is the valence of M.
7. The process of claim 6 wherein M is Ni, Pd, Fe or Co.
8. The process of claim 7 , wherein M is Ni.
9. The process of claim 1 wherein the activating support comprises an activating agent that is an aluminoxane.
10. The process of claim 1 wherein the trialkylaluminium is triisobutylaluminium.
11. The process of claim 1 , wherein the polyethylene has a content of short chain branching of at most 80 short chain branches per 1000 methylene groups.
12. The process of claim 1 , wherein the polyethylene has a molecular weight distribution, defined as M w /M n , of at least 2.5, determined by size exclusion chromatography.
13. The process of claim 1 , wherein the short chain branches of the polyethylene consist of methyl groups in an amount of at least 70% of the total short branches.
14. The process of claim 1 , wherein the polyethylene has an activation energy is of at most 40 kJ/mole.