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-tetradehydrobenzo[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 intramolecular backbiting into the butynyl end-group.