A facile and efficient synthetic grafting-through strategy for preparing well-defined bottlebrush block copolymers (BBCPs). was developed through a combination of-living anionic polymerization (LAP) and ring-opening metathesis polymerization (ROMP). co-End-norbornyl polystyrene (NPSt) and poly(4-tert-butoxystyrene) (NPtBOS) were synthesized by LAP using terminator of chlorine moiety containing silane-protecting amine and coupled with a subsequent amidation using norbornyl activated ester. Bottlebrush homopolymers of NPSt were obtained by ROMP with ultrahigh molecular weights (MWs, M-w, = 2928 kDa) and narrow molecular weight distributions (MWDs, D = 1.07) at high degre of polymerizations (DPw = 1084). Welldefined BBCPS with ultrahigh MWs (M-w similar to 3055 kDa) and narrow MWDs (D similar to 1.13) were synthesized through sequential ROMP of NPSt with NPtBOS. The effect of ultrahigh MWs was investigated by self-assembly of the BBCPs in which the phase separated BBCPs presented periodic lamellar structures and exhibited structural colors from blue to pink.