Conceptually related to olefin metathesis, carbonyl-olefin metathesis has emerged as a prominent area of research over the past decade. In 2012, our group reported the first catalytic strategy for carbonyl-olefin metathesis using hydrazine catalysts. In this chemistry, hydrazonium intermediates and [3+2]-cycloadditions / cycloreversions play an analogous role to the metal alkylidenes and [2+2] processes found in traditional olefin metathesis. While closure of the catalytic cycle for this carbonyl-olefin metathesis reaction requires hydrolysis of the metathesized hydrazonium intermediate 1 (Figure ), we recognized that preventing this step could allow for the engagement of an additional olefin substrate in the cycloaddition / cycloreversion sequence. In this case, the transformation would be an olefin metathesis, and iteration would result in a ROMP reaction. Such a process would be closely analogous to the metal alkylidene based approach but would utilize only a simple organic initiator. In this Communication, we demonstrate the feasibility of this proposal with the hydrazonium-initiated ring-opening metathesis polymerization of cyclopropenes. Despite an interesting polymeric thermal profile and a large amount of ring-strain, cyclopropene monomers have been underutilized for ROMP. Schrock, Binder, Buchmeiser and Xia demonstrated the formation of homopolymers with unique elastomeric properties from 1,1-and 1,2-disubstituted cyclopropenes, but only a few examples were shown. In a more recent case, Xia engaged 1-aryl-1-(benzoyloxymethyl)-and 1-methyl-1-carboxylate-cyclopropenes in an alternating ROMP reaction with cyclohexene and norbornene; however, this process was predicated on the cyclopropene undergoing only single monomer addition and not homopolymerization. Thus, the intriguing physical properties of polycyclopropenes and the relative paucity of methods to access them warrant further investigations into cyclopropene ROMP.