Ring‐Opening Metathesis Polymerization: Mechanisms

This article covers the basic principles of modern, well‐defined group 6 and group 8 metal alkylidene catalysts and their use in ring‐opening metathesis polymerization (ROMP). In this context, the expression “well‐defined” refers to catalytic systems that are characterized by a uniform and stoichiometric composition and for which the actual propagating species is well known and characterized. Such catalysts allow for the controlled, sometimes living, ROMP, which is of advantage for a variety of purposes. Generally, one particularity of ROMP is the formation of either cis ‐ or trans ‐double bonds along the polymer chain. Their formation during ROMP is strongly related to the catalyst used. In addition, tactic ROMP‐derived polymers are accessible. The influence of the alkylidene configuration and chirality of the catalysts on the microstructure of the resulting polymers as well as the different mechanisms that lead to tactic structures are outlined.