To date, olefin metathesis polymerizations using Grubbs catalysts have been widely used to produce well-defined polyalkenomers. Herein, we demonstrate the first example of cascade polymerization by combining olefin metathesis and metallotropic 1,3-shift reactions to form unique conjugated polymers consisting of sequence-specific polyenynes (triene-yne). By rationally designing of a series of monomers, tetradeca-1,6,8,13-tetrayne moieties, we achieved highly selective cascade transformations via ring-closing/metallotropic 1,3-shift/ring-closing reactions to produce polyenynes with high molecular weights. After careful optimization, living polymerization was realized to give controlled molecular weights and narrow dispersities (Ð). Detailed kinetic investigations of the polymerization mechanism conducted using in situ NMR analysis confirmed that the metallotropic 1,3-shift was a fast process and the stability of the propagating carbene improved upon addition of a pyridine ligand. Thus, block copolymers were successfully synthesized, making this cascade polymerization approach a useful tool for preparing a new class of conjugated polymers.