Poly(alkyl 1-cyanotrimethylene-1-carboxylate)s (CH2CH2C(CN)(COOR))(n) are the next higher chain homologues of poly(α-cyanoacrylate)s. They were synthesized via the anionic ring-opening polymerization of the corresponding alkyl 1-cyanocyclopropanecarboxylate monomers initiated with thiophenolate salts (PhSM, with M = Li, Na, K, and NBu4) and at temperatures above 50 &DEG; C. Precipitation of the growing polymer chains at an early stage during the polymerization did not prevent further propagation, probably via polymerization in the solid or at the solid surface. The propagating cyanoacetate carbanion is very stable in a normal air atmosphere, surviving for long periods of time in the absence of strong acids. Although monodisperse polymers were obtained in most experiments ((M) over bar (w)/(M) over bar (n) < 1.10), strong experimental evidence for a living polymerization could only be obtained when K+ and NBu4+ counterions were used. Nonquantitative initiations characterized polymerizations initiated by PhSLi and PhSNa. Attempted polymerizations of ethyl 1-cyanocyclobutanecarboxylate failed under similar or slightly more energetic experimental conditions, with the thiophenolate initiator attacking the pendant ester rather than the cyclic methylene group.