We have developed a strategy for olefin ROMP that has the potential to fill this gap. Our approach replaces the classic "Chauvin mechanism" 12 for olefin metathesis involving [2+2] cycloadditions/cycloreversions of metal alkylidenes with a [3+2] manifold involving 1,3-dipoles, the so-called "Huisgen mechanism" (Figure ). This strategy makes use of readily available hydrazine catalysts that condense with aldehydes to form hydrazonium ions (protonated azomethine imines), which then achieve double bond metathesis via reversible 1,3dipolar cycloadditions (Figure ). We have shown that this alternative metathesis strategy can effect a range of doublebond metathesis reactions including ring-opening and ringclosing carbonyl-olefin metathesis. Recently, we also demonstrated the use of this strategy for the ROMP of cyclopropenes. However, we observed significant monomer decomposition at the temperatures required for efficient polymerization, precluding our assessment of this polymerization as a controlled process. It was thus unclear whether hydrazine-catalyzed ROMP could be used to make larger, more well-defined polymers derived from less strained but more stable monomers. Here, we demonstrate that hydrazine-catalysis can be highly effective for the wellcontrolled ROMP of cyclobutene monomers.