Here we report a copper-catalyzed formal dehydration polymerization of propargylic alcohols. Copper catalysis allows for efficient in situ generation of [ n ]cumulenes ( n = 3, 5) by a soft deprotonation/β-elimination pathway and subsequent polymerization via organocopper species. Alkyne polymers ( M n up to 36.2 kg/mol) were produced with high efficiency (up to 95% yield) and excellent functional group tolerance. One-pot synthesis of semiconducting head-to-head poly(phenylacetylene) was demonstrated through a polymerization-isomerization sequence.