A conjugated polymer containing a phosphorus atom in the main chain, poly(vinylene-phosphine), was synthesized by ring-collapsed radical alternating copolymerization (RCRAC) of a cyclooligophosphine and phenylacetylene (1a). Under a nitrogen atmosphere, pentamethylcyclopentaphosphine (cyclo-(MeP)(5)) and 1a were reacted in the presence of a catalytic amount of 2,2'-azobis(isobutyronitrile) (AIBN) at 78 degrees C in deaerated benzene for 8 h. Poly(vinylene-phosphine) (2) was obtained as a yellow powder after purification by reprecipitation into n-hexane three times and freeze-dried for 10 h. The number-average molecular weight of polymer 2 was 2500, which is estimated by gel permeation chromatographic (GPC) analysis (chloroform, PSt standards). The structure of 2 was supported by H-1, C-13, and P-31 NMR spectroscopies, suggesting that trans isomers of diphosphaalkene units were mainly formed in the main chain. The UV-vis absorption spectrum of polymer 2 in chloroform showed small absorption at a visible region derived from the n-pi* transition in the main chain as well as strong absorption at a UV region derived from aromatic groups. The fluorescence spectrum of a dilute chloroform solution of 2 showed an emission peak at 470 nm, which derived from the n-pi* transition along the polymer backbone.