AbstractA novel polyolefin‐based elastomer of polyolefin‐graft‐poly(t‐butylstyrene) is synthesized from styrene moieties via graft‐from anionic living polymerization. The afforded elastomer comprises a soft poly(ethylene‐t‐1‐hexene‐t‐divinylbenzene) segment and a hard poly(t‐butylstyrene) [P(t‐BS)] segment. The polymerization proceeds via complete lithiation of the pendent styrene groups (polyolefin elastomer) and subsequent graft anionic polymerization of 4‐tert‐butylstyrene (2000–10 000 g mol−1). The graft‐from living anionic polymerization controls the grafting size with excellent efficiency by increasing the monomer concentration. To introduce functional groups into the polymer chain‐end, vinyl isoprene units are introduced into the polyolefin‐graft‐poly(t‐butylstyrene) anion to form polyolefin‐graft‐[poly(t‐butylstyrene)‐block‐polyisoprene] via living block copolymerization. Subsequent graft chain‐end multicarboxyl functionalization via thiol‐ene “click” reactions introduces carboxyl groups into the [P(t‐BS)] graft chain‐end. The water contact angle of this multicarboxyl functionalized product exhibits a sharper decrement, which affects its hydrophilicity significantly. This process produces a novel, well‐defined functionalized graft‐type polyolefin‐based elastomer with a high utility temperature and excellent mechanical properties.