The first C5 polymerization is reported, where the main-chain is growing by five carbon atoms of the monomer at a time. Three dienyltriphenylarsonium ylide monomers were synthesized and polymerized with triethylborane as initiator leading to random terpolymers (C1, C3, C5) with mainly C5 repeating units (up to 84.1%). It has been found that the methyl group (electron-donating substituent) on the conjugated double bond of ylides facilitates the formation of C5 segments. A mechanism was proposed based on NMR characterization and DFT calculations. The high C5 content ensures that we are on the right track for pure C5 homopolymerization.