The regio-and stereoselective cyclopolymerization of a series of 1,6-heptadiynes and 1,7-octadiynes, i.(MAP3) is described. With initiators I1−I4, high regioselectivity >96% was observed in the polymerization of M1−M4, allowing for virtually selective α-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% α-selectivity). A polymerization mechanism is proposed that explains for the high stereo-and regioselectivity of the Mo− imidoalkylidene NHC complexes. ■ INTRODUCTIONThe cyclopolymerization of 1,6-hepta-and 1,7-octadiynes has been widely used for the syntheses of soluble, conjugated polymers possessing a rigid backbone. 1−29 Equally important, it can also be used as a probe to determine both the regio-and stereoselectivity of insertion and to come up with a polymerization mechanism for a specific initiator system. Two different pathways for the cyclopolymerization of α,ω-diynes with Schrock-type molybdenum carbenes exist (Scheme 1). 3,4 Thus, the monomer can undergo either α-or β-addition. The first addition step of the monomer to the metal alkylidene is decisive for the structure of the resulting repeat unit. In the absence of any backbiting, the polymer structure is a concise record of all previous insertion steps. For first-generation Schrock initiators, the concept of large and small alkoxides was developed by the Schrock group and allowed for the selective β-addition of monomer. 1,2 Our group was capable of extending this concept to selective α-addition by using first-generation Schrock initiators bearing small alkoxides, an additional coordinating base, and low temperatures. 18,[20][21][22][23][24]30,31 In that context not only the alkoxide but also the imido ligands played a predominant role in the control of the regioselectivity. 32,33 Recently, molybdenum−imidoalkylidene NHC bis(triflate) 34 and cationic tungsten−oxoalkylidene NHC complexes 35 have been developed in our group. Particularly molybdenum− imidoalkylidene NHC bistriflates display high reactivity for various norborn-2-enes, 7-oxanorborn-2-enes, and diynes and are also tolerant toward nitrile, amino, hydroxyl, and carboxylic acid groups. 34 We were interested in the polymerization mechanism that applies in the polymerization of α,ω-diynes with these novel initiators. Equally important, we wished to find out about the stereo-and regioselectivity of these novel complexes and therefore addressed the issues of tacticity and cis/trans selectivity in cyclopolymerization. Since the cationic species derived from Mo−imidoalkylidene NHC bis(triflate) complexes resemble to some extent molybdenum−imidoalkylidene monoalkoxide monopyrrolide (MAP) catalysts developed by the Schrock group, 36,37 and since these systems have