Process for polymerising ultra-high molecular weight polyethylene

A process for polymerising ethylene in the presence of an olefin polymerisation catalyst comprising titanium, magnesium and halogen in at least one polymerisation stage where ethylene is polymerised in slurry, the process comprising the steps of:

• a. introducing the polymerisation catalyst into a pre-treatment step operated at a temperature of from 25 to 40° C. and a pressure of from 1 to 20 bar;
• b. introducing an olefin into the pre-treatment step wherein the olefin introduced into the pre-treatment step is a vinyl unsubstituted olefin selected from the group consisting of propylene, 1-butene, 4-methyl-1-pentene and vinyl cyclohexane;
• c. polymerising the olefin on the polymerisation catalyst in the pre-treatment step to form a pre-treated catalyst comprising a single olefin polymer on the polymerisation catalyst so that the ratio of the weight of the olefin polymer to the weight of the original solid catalyst component is from 0.1 to 10 g/g;
• d. recovering the pre-treated catalyst from the pre-treatment step;
• e. introducing ethylene, a diluent and the pre-treated catalyst into a slurry polymerisation step operated at a temperature of from 30 to 110° C. and a pressure of from 1 to 100 bar to polymerise ethylene on the pre-treated catalyst to produce polyethylene having a viscosity average molecular weight of from 700000 to 5000000 g/mol.