Foamed polymeric materials are of great importance for many applications, for example in thermal isolation or as light-weight materials. Foam density, cell size, cell density and morphology are the main characteristics of the final product and a lot of studies have been performed to improve these features. Especially in foaming process, the strain hardening and dynamic dilution effects of branches have a distinct influence on the foaming behavior and final properties. Investigating the rheological and foaming properties and correlating it to molecular topological parameters like the degree of branching requires controlled synthesized polymers with model structure. A series of comb-PS from loosely grafted to bottlebrush-like structures were synthesized by the combination of anionic polymerization and grafting-onto method to obtain comb-PS with the same backbone, similar branch length but different branching density. Shear rheological and extensional properties were investigated and correlated with the molecular structure. Additionally, the samples were foamed using a batch foaming setup with supercritical CO2 as blowing agent. This research revealed that all comb-PS with the same length of side chains had similar cell density and size. However, there was an optimum number of branches per backbone resulted in a maximum volume expansion ratio (VER) at minimum zero shear viscosity (ZSV) and a maximum strain hardening factor (SHF) > 200 for Hencky strains below 4. Such a high strain hardening is of great fundamental and technical importance in extensional processes and could be directly correlated to the foaming results. We will present the shear and extensional rheological data in the framework of the ZSV and SHF as a function of number of side chains and correlate them to foam characteristics, as well as giving a first outlook on the influence of chain mobility and gas solubility on the foaming behavior. Literature: Abbasi, M.; Faust, L.; Riazi, K. & Wilhelm, M. Macromolecules, 2017, 50, 5964-59