text: The mechanism for transition metal-catalyzed olefin metathesis has been widely researched over the past forty years. RCM undergoes a similar mechanistic pathway as other olefin metathesis reactions, such as cross metathesis (CM), ring-opening metathesis polymerization (ROMP), and acyclic diene metathesis (ADMET). Since all steps in the catalytic cycle are considered reversible, it is possible for some of these other pathways to intersect with RCM depending on the reaction conditions and substrates. In 1971, Chauvin proposed the formation of a metallacyclobutane intermediate through a [2+2] cycloaddition which then cycloeliminates to either yield the same alkene and catalytic species (a nonproductive pathway), or produce a new catalytic species and an alkylidene (a productive pathway). This mechanism has become widely accepted among chemists and serves as the model for the RCM mechanism.
  Initiation occurs through substitution of the catalyst’s alkene ligand with substrate. This process occurs via formation of a new alkylidene through one round of [2+2] cycloaddition and cycloelimination. Association and dissociation of a phosphine ligand also occurs in the case of Grubbs catalysts.  In an RCM reaction, the alkylidene undergoes an intramolecular [2+2] cycloaddition with the second reactive terminal alkene on the same molecule, rather than an intermolecular addition of a second molecule of starting material, a common competing side reaction which may lead to polymerization Cycloelimination of the metallacyclobutane intermediate forms the desired RCM product along with a [M]=CH, or alkylidene, species which reenters the catalytic cycle. While the loss of volatile ethylene is a driving force for RCM, it is also generated by competing metathesis reactions and therefore cannot be considered the only driving force of the reaction.