Well-defined high molar mass poly(1,4-butadiene)-g-poly(epsilon-caprolactone) (PBu-g-PCL) graft copolymers were prepared through the grafting-from route by the combination of ring-opening metathesis polymerization (ROMP) and organocatalyzed ring-opening polymerization (ROP). The synthesis route relies on the ROMP of cis-4-benzyloxymethyl-3-hydroxymethylcyclobutene initiated by ruthenium-based Grubbs' catalysts followed by organocatalyzed ROP of epsilon-caprolactone initiated by the hydroxyl side groups of the backbone using 1,5,7-triazabicydo[4.4.0]dec-5-ene (TBD) as the catalyst. The reported strategy provides PBu-g-PCL having a strictly poly(1,4-butadiene) backbone with the highest molar mass reported up to now (M-n > 10(6) g mol(-1)). Self-assembling properties of the resulting PBu-g-PCL graft copolymer were investigated using small-angle X-ray scattering (SAXS) in toluene solution and in the solid state.