Despite a broad range of commercial uses, applications of polyolefins have often been limited because of a lack of functionality and structural diversity, which has sparked the motivation to modify polyolefins, especially postpolymerization modifications (PPMs). The Csingle bondH insertion capability of nitrene intermediates has been of interest due to its universal applicability to organic macromolecules containing Csingle bondH bonds. In this study, a sulfonyl azide (SA) end-functionalized polystyrene (SAPS) is prepared as a model compound by synthesizing a SA containing azo-type radical initiator. SAPS is then used to modify commercial grade high-density polyethylene (HDPE) through simple melt process, affording HDPE-graft-polystyrene. The material exhibits homogeneous fractured surface, decreased crystallinity value, increased complex viscosity value, and relatively high Young’s modulus value, suggesting the formation of the polymer hybrids. This study demonstrates the capability of SA end-functionalized polymers as a noncatalytic platform for the PPM of polyolefins.