The present technology relates to a process for polymerizing or copolymerizing ethylenically unsaturated monomers in the presence of free-radical polymerization initiators, wherein the polymerization is carried out in a continuously operated tubular reactor at temperatures from 100° C. to 350° C. and pressures from 180 MPa to 340 MPa, with a specific reactor surface area Asp of 2 m2/(t/h) to 5.5 m2/(t/h), and the tubular reactor has a specific ratio RDsp of 0.0050 MPa−1 to 0.0069 MPa−1 and an inner surface which has a surface roughness Ra of 2 μm or less.