Influencing plastic materials by reactive processing of polymer particles, as seen with extrusion processes for example, may be performed in an especially simple way and is described by taking the reactive processing of standard high-density polyethylene (HDPE) plastic material on a twin-screw extruder in the presence of a vinyl monomer as an example. In particular, the investigation deals with the influence of relatively low quantities-if compared with the comonomer portion-of 2-ethylhexyl acrylate on HDPE properties during polymerization. Furthermore, the effect of a powerful chain-transfer agent on the process of reactive processing and on the thermal oxidative properties of the products made by use of them is shown. Doing this, carbon tetrabromide proved to be an efficient transfer agent. When presenting the properties of the formed extrudates, the mechanical and thermal characteristics and application-related tests, such as those on stress-cracking resistance, is emphasized. The findings of this investigation are discussed on the basis of correlating them to the lamellar thickness of crystallites and the density of tie-molecules.