Perfectly regioregular alpha,omega-telechelic poly(1,4-butadienes) (1,4-PB(lin) or B) bearing terminal alkoxyamine functionalities are synthesized by chain transfer ring-opening metathesis polymerization (ROMP-CT) of trans,trans,cis-1,5,9-cyclododecatriene in the presence of an acyclic olefin chain transfer agent. We demonstrate that these polydisperse, telechelic 1,4-PB(lin) polymers are macromolecular initiators for nitroxide-mediated polymerizations (NMP) of styrene and isoprene to produce SBS, IBI, and SIBIS block copolymers (S = poly(styrene) and I = poly(4,1-isoprene)). Selected SBS and SIBIS multiblock copolymers were hydrogenated to produce microphase-separated multiblock copolymers containing polydisperse, high-density poly(ethylene) (E(lin)) segments inaccessible by other polymerization techniques.