Abstract: A process for the production of a powdery, thermoplastic copolymer of ethylene and butene-(1) having a density of up to about 0.930 g/cm.sup.3 which comprises copolymerizing a mixture of ethylene and butene-(1) at 50.degree.-95.degree. C. and 10-100 bar in the presence of a catalyst which comprises (1) the reaction product of a chlorine and/or alkoxy-containing vanadyl (V) compound and an organic aluminum compound selected from the group consisting of ethyl aluminum dichloride, diethyl aluminum chloride, isobutyl aluminum dichloride, diisobutyl aluminum chloride, and mixtures thereof, and (2) an aluminum alkyl activating compound therefor, selected from the group consisting of aluminum trialkyls, alkyl aluminum sesquichloride, alkyl aluminum dichloride, and mixtures thereof, wherein the polymerization reaction is carried out in a solvent which comprises liquid butene-(1).