Control experiments with no initiator (entry 6), or using an ammonium salt NEt3•HBF4 (entry 7) did not result in polymerization products. In the absence of initiator, 3,3methylphenylcyclopropene largely underwent thermal decomposition after being heated at 90 °C for 18 hours. In the presence of NEt3•HBF4, however, the starting material decomposed completely and gave rise to a complex mixture. While this complex mixture of side products was observed in all crude reactions, attempts to isolate and identify such products were unsuccessful. These control experiments demonstrated that the [2.2.2]-hydrazonium 6 was an active ROMP initiator for cyclopropene 3 and that the thermal degradation of cyclopropene under acidic condition likely inhibits high yielding polymerization.