Vinyl-containing macroinimers were prepared in situ by utilizing sulfoxide chemistry in an unprecedented manner. They allowed for the one-pot synthesis of hyperbranched polymers. The sulfoxide group may be removed under thermal reaction conditions. Through this reaction, we transformed sulfoxide-protected haloalkanes into haloalkanes with vinyl functional groups. Macroinitiators using protected haloalkanes were prepared via supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP). After heating, the macroinitiators produced macroinimers while maintaining high chain-end functionalities. When the temperature was elevated during the linear polymerization, hyperbranched polymers were prepared from haloalkanes and vinyl groups of macroimers. This strategy will be helpful in synthesizing hyperbranched polymers that can adjust the distance between branch points, and it is expected that even hyperbranched block copolymers can be synthesized.