A novel polyaddition of terminal acetylenes bearing electron-withdrawing groups with dithiols, via a phosphine-catalysed double conjugate addition, is described. From the model reaction of methyl propiolate (1) with benzylmercaptan in the presence of tri-n-butylphosphine catalyst, the corresponding dithioacetal was obtained in high yield when the reaction was carried out under higher concentrations of the catalyst and the reactants. By using aliphatic dithiols having a rigid spacer [i.e. p-xylene-alpha,alpha’-dithiol (5)], the polyaddition of 1 took place smoothly to yield a polymer (6) bearing dithioacetal moieties in the main chain in 84% yield, whose M-n and M-W/M-n were estimated to be 8100 and 1.29, respectively. Further, the polymer (8) having dithioacetal moieties was also obtained quantitatively by the polyaddition of an ynone (7) with 5.