Diethyl dipropargylmalonate (1) was cyclopolymerized by various well-defined molybdenum-based Schrock initiators to produce polymers exclusively based on 1,2-cyclopent-1-enylenvinylene units. For this purpose, variations in both the imido and alkoxy ligand in molybdenum alkylidene complexes of type Mo(NAr')(CHCMe2Ph)(OR)(2) (Ar'= 2,6-i-Pr2C6H3; 2,6-Me2C6H3; OR = OCH(CF3)(2), OCH(CH3)(2), OC(CH3)(3)) were carried out. Polymers containing > 95% 1,2-cyclopent- 1-enylenvinylene units were obtained by low-temperature-initiated cyclopolymerization of 1 by Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OCH(CH3)(2))(2) (2). In the presence of quinuclidine, >95% five-membered ring structures were realized at room temperature using Mo(N-2,6-Me2C(6)H(3))(CHCMe2Ph)(OC(CH3)(3))(2) (3) and Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OC(CH3)(3))(2) (4). In contrast, molybdenum initiators containing electron-withdrawing alkoxides such as Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OCH(CF3)(2))(2)(quinuclidine) (5) or the nonfluorinated analogue Mo(N-2,6Me(2)C(6)H(3))(CHCMe2Ph)(OCH(CH3)(2))(2)(quinuclidine) (6) do not exclusively generate five-membered ring structures, either in the presence of quinuclidine or at low temperatures. Molecular structures of 3 and 5 were determined by X-ray crystallography. The exact ratio of five- to six-membered rings was based on C-13 NMR data of 4,4-bis(ethoxycarbonyl)cyclopent-1-ene (7) and 4,4-bis(ethoxycarbonyl)cyclohex-1-ene (8), which served as model compounds. 4-(Ethoxycarbonyl)-1,6-heptadiyne-(9) and di(1S,2R,5S)-(+)menthyl dipropargylmalonate (10) were cyclopolymerized by 4 in the presence of quinuclidine to investigate the influence of bulky 4-substituents on the polymerization. 4-(Ethoxycarbonyl)-4-(1S,2R,5S)-(+)menthoxycarbonyl-1,6-heptadiyne (12) was synthesized from ethyl-(1S,2R,5S)-(+)-menthyl malonate (11)and cyclopolymerized by 4 in the presence of quinuclidine to determine the configuration of the double bond (i.e., the cis/trans ratio) and the tacticity of the polyene backbone. Poly-12 consists of > 95% five-membered rings and possesses an alternating cis-trans structure. In contrast to cyclopolymers of 1 containing both ring sizes, poly-1, consisting solely of five-membered rings, is virtually insoluble in THF. A degree of polymerization (DP) dependent UV-vis absorption is found. The absorption maximum lambda(max) for poly-1 with DP greater than or equal to 50 is 591 nm, and the value for the maximum effective conjugation length (N-eff) that can be calculated therefrom is 52. Molecular weights and polydispersity indices (PDIs) of the polymers were determined in CHCl3 by GPC vs PS. Complementary, light-scattering (LS) data were collected at lambda = 690 nm and MALDI-TOF mass spectroscopy was applied in order to calculate absolute molecular weights. A linear plot of number of monomers added (n) vs molecular weights as determined by LS suggests that the cyclopolymerization of 1 proceeds in a living manner. At least a class V living system was confirmed by the stepwise synthesis of poly-1. Molecular weight distributions (PDIs) of 1.16-1. 37 result from ratios of the rate constants for-polymerization (k(p)) to initiation (k(i)), k(p)/k(i), > 1. These values were determined by NMR for the reaction of 1 with all relevant initiators and were 9, 14, 114, and 347, respectively, for initiators 3, 2, 4, and 5. Influences of temperature, the base and steric and electronic effects of the arylimido and alkoxy ligands on polymer structure were investigated.