The present invention relates to a middle-pressure polymerization process for the preparation of a liquid ethylene copolymer which comprises in polymerized form 20 to 60 wt % of ethylene; and at least 20 wt % of an acrylate, which is selected from C1-C22 alkyl (meth)acrylate, where a monomer mix comprising the ethylene and the acrylate is polymerized at a pressure of 50 to 400 bar and in the presence of at least 2 wt % of a chain transfer agent. The invention further relates to a liquid ethylene copolymer obtainable by the polymerization process; and to a lubricant comprising the liquid ethylene copolymer obtainable by the polymerization process; and to a method for reducing friction between moving surfaces comprising the step of contacting the surfaces with the lubricant or with the ethylene copolymer.