Chromium catalysts active in olefin polymerization

A process for the polymerization or the co-polymerization of ethylene for the production of low molecular weight polyethylene employing a catalyst obtained by contacting at least one organoaluminium compound with a chromium tridentate complex of formula (I)

• wherein:
• Z is a group comprising at least a nitrogen, phosphorous, oxygen or sulphur atom which is able to coordinate to the chromium atom;
• Each R1 is independently selected from the group consisting of H and a monovalent radical comprising from 1 to 30 carbon atoms and from 0 to 5 heteroatoms independently selected from the group consisting of Si, N, P, O, F, Cl and B atoms;
• Each R2 is independently selected from the group consisting of H and a monovalent radical comprising from 1 to 30 carbon atoms and from 0 to 5 heteroatoms independently selected from the group consisting of Si, N, P, O, F, Cl and B atoms;
• Each R2 can independently join together with either Z or R1 forming one or more aromatic or aliphatic ring optionally containing heteroatoms;
• Each A is independently selected from the group consisting of nitrogen, phosphorous, As and Sb;
• Each E is independently selected from the group consisting of carbon, phosphorous, and nitrogen;
• each p is independently 1 or 2;
• Each n is independently 0, 1 or 2;
• and
• Each X is independently an atom or group covalently or ionically attached to the chromium atom.