Tri-n-butylphosphine-catalyzed polyadditions of bifunctional acetylenes having ester groups [i.e., bis(propiolate)s (1A-C)] with dithiols (2a-e) are described. Although a gel was obtained by the polyaddition of a terminal bispropiolate (1A) with alpha,alpha’-xylene dithiol (2a) through the second addition of thiol moieties to the produced double bonds, the polyaddition of 1A with 1,4-benzene dithiol (2b) in THF (0.3 IM) using tri-n-butylphosphine (5 mol %) as a catalyst proceeded at room temperature within Ih to obtain a soluble polymer (3Ab) having beta-alkylmercaptoenoate moieties in the main chain in quantitative yield, whose M(n) and M(w)/M(n) were estimated as 20 700 and 2.12, respectively. Further, polyadditions of 1B having internal acetylene moieties with dithiols (2a-e) proceeded under mild conditions and the corresponding polymers were obtained in almost quantitative yield. For instance, the polyaddition of 1B with 2a in THF (0.5 M) proceeded at room temperature within 3 h and a polymer (3Ba) was obtained in 93% yield whose M, and M(w)/M(n) were estimated as 25 200 and 2.16, respectively. The obtained polymer was colorless and stable in air. Furthermore, 3Ba showed a good film-forming character by casting from a dichloromethane solution. On the other hand, the polyaddition of 1C with 2a gave a polymer (3Ca) that mainly contains regioisomeric units (i.e., alpha-alkylmercaptoenoate units) by means of a radical reaction in almost quantitative yield.