<p>Multiblock copolymers consisting of isotactic polypropylene (<i>i</i>PP) and ethylene–propylene rubber (EPM) were successfully synthesized by cross-metathesis between these polymers bearing a small number of C = C bonds in the main chain. The C = C bonds for metathesis were incorporated into the polyolefins by copolymerization with butadiene. This polymerization rate was greatly improved by the coexistence of a small amount of ethylene with the butadiene, probably because π-allyl metal species were rapidly converted to alkyl metal species by the insertion of ethylene. The cross-metathesis reaction occurred not only at the in-chain C = C bonds but also at the vinyl groups formed by the 1,2-insertion of butadiene, resulting in the formation of a long-chain branched polyolefin. Compared with the polymer blend of <i>i</i>PP and EPM, the obtained product showed excellent physical properties because of the partial compatibilization of the two phases. In particular, the tensile absorption energy was improved by a factor of 50 by block formation (2.8 ± 1.9 MJ/m³ vs. 141 ± 28 MJ/m³), successfully demonstrating the potential of this material as a polyolefin-based elastomer.</p>