Abstract: A process is described for the polymerization of ethylene or copolymerization of ethylene with a C.sub.3 to C.sub.15 alpha olefin in solution, suspension or in the gas phase at 20.degree. to 250.degree. C. and 1 to 100 bars in the presence of a mixed catalyst comprising a separately produced halogen-containing titanium(III) compound and an organoaluminum compound while controlling the molecular weight with oxygen, wherein a mixture of organoaluminum compounds is employed comprising a halogen-containing aluminum alkyl and a trialkyl aluminum compound in a molar ratio of 1:0.05 to 1:20, the combined amount of organoaluminum compounds being present in an amount of 0.01 to 20 moles per one mole of halogen-containing Ti(III) compound.

Abstract: An improvement in a process for the preparation of an ethylene homopolymer or a copolymer of ethylene with an .alpha.-olefin containing 3 to 15 carbon atoms by contacting the monomer or mixture of monomers in solution, suspension or in gas phase at 20.degree. to 250.degree. C. at a pressure of 1 to 50 bar with a mixed catalyst comprising a chlorine containing titanium (III) compound and an organoaluminum compound in the presence of hydrogen, the improvement residing in employing as the organoaluminum compound a trialkylaluminum compound in an amount of 0.1 to 20 mol per mol titanium (III) compound and including in the polymerization mixture oxygen at a concentration of 0.001 to 0.1 volume percent based upon the amount of ethylene. By carrying out the polymerization by including oxygen in the polymerization reaction mixture and employing trialkylaluminum compound as the organoaluminum compound the mean molecular weight of the polymer can be regulated while obtaining a desired molecular weight distribution.