We report here the living cyclopolymerization of 1,6-heptadiyne derivatives (usually 4,4-disubstituted) using well-defined alkylidene complexes as initiators. Diethyl dipropargylmalonate (2a), di-tert-butyl dipropargylmalonate (2b), optically active di-(1R,2S,SR)-(-)-menthyl dipropargylmalonate (2c(-)), di-(1S,2R,5S)-(+)-menthyl dipropargylmalonate (2c(+)), di-(1R)-endo-(+)-fenchyl dipropargylmalonate (2d), 4,4-bis[[p-tolylsulfonyl)oxy]methyl]-1,6-heptadiyne (3b), 4,4-bis[(trimethylsiloxy)methyl]-1,6-heptadiyne (3c), the cyclic silyl ether, PhEtSi(OCH2)(2)C-(CH2C drop CH)(2) (3d), and N,N-dipropargyl-2,4,6-triisopropylbenzamide (5b) are polymerized to give soluble polymers in high yield using Mo(NAr)(CHCMe(2)Ph)(OR(F6))(2) (1a; Ar = 2,6-i-Pr2C6H3, OR(F6) = OCMe(CF3)(2)) as the initiator in 1,2-dimethoxyethane (DME). The polymers show a high degree of conjugation (lambda(max) > 500 nm) and have narrow molecular weight distributions. Poly(2a) is soluble in most organic solvents (THF, C6H6, toluene, CH2Cl2, CHCl3, DME, DMF, MeCN). The mechanism of the polymerization has been investigated-by H-1 NMR studies and by monomer, initiator, and solvent variations. Symmetric, diphenyl-capped polyenes, "pull-pull" polyenes containing p-cyanophenyl end groups, and "push-push" polyenes containing p-dimethylamino end groups have all been prepared, as have polyenes that contain optically active substituents and "push-pull" polyenes containing p-(dimethylamino)-phenyl and p-cyanophenyl end groups. 4,4-Bis(carboxyethyl)cyclopent-1-ene and 1-vinyl-3-methylene-5,5-bis-(carboxyethyl)cyclohex- 1-ene were employed as model compounds in order to quantify five- and six-membered ring structures in the polymer. The substituted polyenes are far more stable at room temperature and in air than unsubstituted polyenes of the same length. Random and block copolymers of 2a, 2,3-dicarbomethoxynorbornadiene (DCMNBD) and 7,8-bis(trifluoromethyl)tricyclo[4.2.2.0(2,5)] deca-3,7,9-triene (TCDT) were prepared and characterized. The solution electrochemistry, thin film electrochemistry, and UV/vis spectroelectrochemistry of several homopolymers and copolymers have been examined.