Molybdenum carbyne complexes [RC=Mo(OC(CH3)(CF3)(2))(3)] featuring a mesityl (R = Mes) or an ethyl (R = Et) substituent initiate the living ring-opening alkyne metathesis polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehyoltobenzo[a,e] [8]annulene, to yield fully conjugated poly(o-phenylene ethynylene). The difference in the steric demand of the polymer end-group (Mes vs Et) transferred during the initiation step determines the topology of the resulting polymer chain. While [MesC Mo(OC(CH3)(CF3)(2))(3)] exclusively yields linear poly(o-phenylene ethynylene), polymerization initiated by [EtC Mo(OC(CH3)(CF3)(2))(3)] results in cyclic polymers ranging in size from n = 5 to 20 monomer units. Kinetic studies reveal that the propagating. species emerging from [EtC Mo(OC(CH3)(CF3)(2))(3)] undergoes a highly selective intramblecular backbiting into