To parlay the findings shown in Table into a cyclopropene ROMP reaction (Table ), we sought to suppress chain-end hydrolysis by using preformed hydrazonium 6 as the initiator. Thus 6 was prepared by the condensation of the bistetrafluoroborate salt of hydrazine C with benzaldehyde (2). Tetrafluoroborate was chosen as the counterion based on the results described above and because of the high crystallinity and relative non-hygroscopicity it imparted to the hydrazonium 6. After the reaction of cyclopropene monomer 3 with this initiator 6 (40:1) at 60 °C for 60 h, the polymerization was terminated by hydrolysis at 90 °C for two hours with water (20 equiv) and TFA (2 equiv). The polymeric product 5 was then recovered via centrifugal precipitation from CH2Cl2/MeCN. GPC analysis referenced to polystyrene standards revealed the number average molar mass (Mn) to be 6.1 kDa with a dispersity (Đ) of 1.5. Despite the relatively good agreement between the theoretical and experimental Mn, repeated trials resulted in variable results (Mn as low as 3.6 kDa or as high as 9.2 kDa, Table ). [a] See ESI for experimental details. The degree of polymerization (DP), was calculated from Mn exp. Yields reflect isolated and purified polymer. Reaction time 60 h. No initiator was used. NEt3•HBF4 used instead of 6.