Ultrapure diethyl 1,1-cyclopropanedicarboxylate (1) undergoes ring-opening polymerization when placed in the presence of small amounts of sodium thiophenolate (2) at temperatures above 80 degrees C, and leads to the synthesis of a carbon-chain polymer substituted on every third carbon atom by ethyl ester substituents. 1 polymerizes also in the presence of 2 generated in situ from thiophenol and metallic sodium, but does not polymerize in the presence of metallic sodium alone. Polymerizations are slow and require high concentrations in monomer and initiator to obtain reasonable polymerization times. In spite of the high temperatures used in some experiments (up to 130 degrees C), no structural subunit resulting from side reactions was detected, presumably due to the high stability of the propagating malonate carbanion. Polymers of narrow molecular weight distribution were obtained at both 80 and 130 degrees C ((M) over bar(w)/(M) over bar(n) < 1,14), and absolute degrees of polymerization increased with polymer yields. These results were rationalized on the basis of a quasi-living polymerization mechanism. Thermal properties of poly(1) were determined by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Poly(1) is stable up to 275 degrees C and is a micro-crystalline polymer insoluble in most typical solvents, with the noticeable exception of chloroform and dichloromethane.