A vinyl-containing macroinimer was prepared by utilizing sulfoxide chemistry and allowed for the one-pot synthesis of hyperbranched polymers (HPs). Sulfoxide-protected haloalkanes were prepared, and their transformation into vinyl-functionalized haloalkanes through thermal sulfoxide elimination was investigated. The protected haloalkanes were employed as an initiator for atom transfer radical polymerization to prepare polymers. Subsequent thermal treatment yielded the macroinimers while preserving the high chain-end functionalities. When the temperature was elevated during the linear polymerization, HPs were afforded in a one-pot process via the in situ generations of the macroinimers. This strategy is expected to aid in the synthesis of HPs with a tunable distance between the branch points. Besides, when we applied this sulfoxide chemistry to monomer form, we confirmed the thermally controlled crosslinking behavior through the multi vinyl protection of the polymer backbone.