Polypropylene (PP) with a share of 19.2% on European plastic demand is the second most important commodity polymer. Nearly half of the produced PP ends up after a short period of use as post-consumer waste. Recycling rates of PP are increasing since years, but converting PP-waste into a valuable resource is still a challenge and needs scientific research from a different perspective. Long chain branching (LCB) is known as a suitable method to introduce strainhardening behaviour to virgin polypropylene and increases thereby the melt strength. Commonly, thermo-oxidative degradation, ageing and shear induced chain scission during recycling reduces viscosity and molar mass. As a result, most of recycled PP is a product of worse quality compared to virgin material, thus this process can be regarded as "down-cycling". LCB is shown as an innovative tool for value adding to PP from household post-consumer waste, therefore, one can speak of a real “up-cycling”. Within this study, we investigated the influence of polymeric impurities like polyethylene with high density (PE-HD) first. Model mixtures from PP containing 10% PE-HD were prepared, chemically modified and compared with PP without impurities. The same study was repeated with post-consumer material from household plastic waste. The melt properties were improved in any case, independent of the PE-HD in the blend. However, the mechanical properties showed mixed results, especially in the case of the post-consumer feedstock, the material suffered from the formation of highly branched gel particles and an unfavourable viscosity ratio of the PE-HD impurities. However, single polymer PP post-consumer waste did not show any limitation, so the investigation was continued on the influence of different linear PP-grades and therefore different molar masses (but similar molar mass distributions) on the LCB formation itself. The branching number for the comparison of the number of LCB per molecule was determined by high temperature size exclusion chromatography and dynamic rheology. PP-types with higher molar masses, like for pipe extrusion, show lower number of LCB per polymer compared to an injection moulding PP-type with low molar mass under the same conditions for the LCB-reaction. The higher viscosity (caused by a more entangled structure), caused by the higher molar mass inhibits the migration of V the chain fragments through the polymer melt and reduces the efficiency of the branching reaction. Due to the promising results from the experiments in laboratory scale twin-screwextruder (g), a scale-up to a bigger single-screw extruder was performed. For this the reaction parameters need to be modified for the changed processing parameters (shorter dwell time). The single-screw extruder not only gave more up-cycled PP in shorter time, also the shear stress on the polymer was less pronounced, therefore the results were better, compared to the twin-screw lab extruder. As a possible application for the up-cycled PP foaming experiments were performed in laboratory scale with scCO2 in a high pressure autoclave. The up-cycled extrusion grade PP gave the best results, the cell structure was comparable to the foam of commercial foaming-type PP produced under similar conditions.