Polypropylene (PP) foam is a potential high-temperature insulating porous material with high mechanical properties and service temperature. However, the development of insulating and multifunctional foams based on PP still faces challenges. In this study, grafting by ultraviolet (UV) radiation was successfully used to introduce ester-based groups into PP molecular chains. The grafted product was then blended with pure PP in a specified proportion and pelletized, and then subjected to chemical foaming to obtain the modified rigid PP foams. The grafted PP effectively reduced the melt flow rate and heterogeneous nucleation during foaming, improved foaming efficiency, and promoted the formation of nanometer- and micron-sized cells. The newly introduced ester-based groups also effectively absorbed near- and far-infrared radiative energy
meanwhile, the nanometer- and micron-sized cells effectively enhanced the Knudsen and Phonon Scattering Effects, resulting in a significant reduction in the thermal conductivity, from 186.7 to 65.3 mW/(m·K). Moreover, the obtained foam exhibited well mechanical and hydrophobic properties under complex environmental conditions. The ester-based multiscale porous PP foam demonstrated simultaneous reduction in the radiation heat transfer coefficient, solid thermal conductivity, and gas thermal conductivity, thereby providing a new strategy for further reducing the thermal conductivity of polymer-based foams and achieving excellent insulation. This work realized the preparation of difficult-to-make rigid PP foams, laying the foundation for further diversification of PP foams and expanding their application areas.