Control　over　polymer　architecture　and　composition　is　essential　for　disclosing　structure-property　relationships　and　developing　high-performance　materials.　Herein,　a　new　method　is　successfully　developed　to　synthesize　bottlebrush　polymer　(BP)　with　controllable　graft　density　and　side　chain　composition　by　“grafting-from”　strategy　using　in　situ　halogen　exchange　and　reversible　chain　transfer　catalyzed　polymerization　(RTCP).　The　main　chain　of　the　BP　is　first　synthesized　by　the　polymerization　of　methacrylates　containing　alkyl　bromide　as　a　side　group.　Then,　the　alkyl　bromine　is　quantitatively　converted　to　alkyl　iodide　with　sodium　iodide　(NaI)　via　in　situ　halogen　exchange　to　efficiently　initiate　the　RTCP　of　methacrylates.　By　adjusting　the　input　amount　of　NaI　and　monomers　in　sequence,　BP　named　PBPEMA-g-PMMA/PBzMA/PPEGMEMA　which　contains　three　different　kinds　of　polymer　side　chains　including　hydrophilic　PPEGMEMA,　hydrophobic　PMMA,　and　PBzMA　is　synthesized　with　narrow　molecular　weight　distribution　(Mw/Mn ≤ 1.36).　The　grafting　density　and　the　chain　length　of　each　polymer　side　chain　are　well　controlled　by　the　addition　of　NaI　in　batches　and　following　RTCP.　Moreover,　the　obtained　BP　self-assembled　into　spherical　vesicles　in　aqueous　with　hydrophilic　coronal　structure,　core　region,　and　the　hydrophobic　wall　between　the　former　two,　which　enables　to　wrap　hydrophobic　pyrene　and　hydrophilic　Rhodamine　6G　separately　or　simultaneously.　©　2023　Wiley-VCH　GmbH.