The reactive extrusion of maleic anhydride grafted polypropylene (PP-g-MAH) with ethylenediamine (EDA) as coupling agent is carried out in a corotating twin-screw extruder to produce long chain branched polypropylene (LCBPP). Part of PP-g-MAH is replaced by maleic anhydride grafted high-density polyethylene (HDPE-g-MAH) or linear low-density polyethylene (LLDPE-g-MAH) to obtain hybrid long chain branched (LCB) polyolefins. Compared with the PP-g-MAH, PE-g-MAH, and their blends, the LCB polyolefins exhibit excellent dynamic shear and transient extensional rheological characteristics such as increased dynamic modulus, higher low-frequency complex viscosity, broader relaxation spectra, significantly enhanced melt strength and strain-hardening behaviors. The LCB polyolefins also have higher tensile strength, tensile modulus, impact strength and lower elongation at break than their blends. Furthermore, supercritical carbon dioxide (scCO(2)) is constructively introduced in the reactive extrusion process. In the presence of scCO(2), the motor current of the twin extruder is decreased and LCB polyolefins with lower melt flow rate (MFR), higher complex viscosity and increased tensile strength and modulus can be obtained. This indicates that the application of scCO(2) can reduce the viscosity of melt in extruder, enhance the diffusion of reactive species, and then facilitate the long chain branching reaction between anhydride group and primary amine group. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 3384-3392, 2011