Ethylene polymerisation using Ziegler catalyst system - contg. cyclopentadienyl zirconium cpd., aluminium tri-alkyl cpd. and opt. water

New Ziegler catalyst system for prepn. of polyethylene by ethylene polymerisation at -80 to 120 degrees C. consists of (1) a cyclopentadienyl Zr cpd. having formula (cyclopentadineyl)nZrY4-n, (I), (where n is 1-4; Y is R, or -CH2-AlR2 or -CH2-CH2-AlR2 or -CH2-CH(AIR2)2 and R is alkyl or a metal-alkyl gp.), (2) an Al-tri-1 -4C alkyl, (II) and (3) opt. wateR. Mol. ratio (I):(II) is 1:0.001 to 1:20 (1:0.001 to 1:5). Components (I) and (II) are combined before charging to polymerisation and left until gas generation, starting on mixing, has subsided. Mol. ratio (II):H2O >1. (I) introduce no halogen into polymer. Mol. wt. of polymer can be regulated over a wide range e.g. from liquid oligomers at high temps. to high mol. wt. plastics at low temps. (I) may be tetracyclopentadienyl-Zr or tricyclopentadienyl-Zr-(bis-diethylamino)-ethane.