Based on the design of various microstructure for both chemical and physical branching, a series of high melt strength polypropylenes (HMSPPs) including long chain branched polypropylene of diamine-modified, silane-modified, ionic liquid-modified and polypropylene ionomers on the presence of physical cross-links were prepared by the reactive extrusion with assisted supercritical carbon dioxide (scCO2). Moreover, the significantly improved foamability with scCO2 and its rheological behaviors were investigated. The new insight into the foaming action have been proposed.