The regio- and stereoselective cyclopolymerization of a series of 1,6-heptadiynes and 1,7-octadiynes, i.e., of 4,4-bis(ethoxycarbonyl)-1,7-heptadiyne (M1), 4-ethoxycarbonyl-4-(1R,2S,5R)-(-)-menthyloxycarbonyl-1,6-heptadiyne (M2), (R,S)-4,5-bis((1R,2S,5R)-(-)-menthyloxymethylcarboxymethyl)-1,7-octadiyne (M3), and (S,S/R,R)-4,5-bis((1R,2S,5R)-(-)-menthyloxymethylcarboxymethyl)-1,7-octadiyne (M4), by the action of the molybdenumimidoalkylidene N-heterocyclic carbene (NHC) complexes (Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OTf)(2)(IMesH(2)) (I1), Mo(N-2,6-Me2C6H3)(CHCMe3)(OTf)(2)(IMesH(2)) (I2), (Mo(N-2,6-Me-2-C6H3)(CHCMe2Ph)(OTf)(OCH(CF3)(2))(IMesH(2)) (I3), Mo(N-2,6-Cl2C6H3)(CHCMe3)(OTf)(2)(IMes) (I4, IMesH(2) = 1,3-dimesitylimidazolidin-2-ylidene, IMes = 1,3-dimesitylimidazolin-2-ylidene) and by the molybdenum-imidoalkylidene monoaryloxide monopyrrolide (MAP) complexes Mo(N-2,6-iPr(2)-C6H3)(CHCMe2Ph)(2,5-Me-2-pyrrolide)(O-2,6-Me-2-C6H3) (MAP1), Mo(N-2,6-iPr(2)-C6H3)(CHCMe2Ph)(2,5-Me-2-pyrrolide)(O-2,6-Mes(2)-C6H3) (MAP2), and Mo(N-2-tBu-C6H4)(CHCMe2Ph)(2,5-Me-2-pyrrolide)(O-2,6-Mes(2)-C6H3) (MAP3) is described. With initiators I1-I4, high regioselectivity >96% was observed in the polymerization of M1-M4, allowing for virtually selective a-insertion. These initiators displayed also high stereoselectivity, allowing for all-trans polyenes with up to 96% syndioselectivity. By contrast, the MAP initiators MAP1-MAP3 showed poor regioselectivity (26-62% a-selectivity). A polymerization mechanism is proposed that explains for the high stereo- and regioselectivity of the Moimidoalkylidene NHC complexes.