Ring opening metathesis polymerisation

Ring-opening metathesis polymerization (ROMP) is a type of olefin metathesis chain-growth polymerization that produces industrially important products. The driving force of the reaction is relief of ring strain in cyclic olefins (e.g. norbornene or cyclopentene). A variety of heterogeneous and homogeneous catalysts have been developed. Most large-scale commercial processes rely on the former while many fine chemical syntheses rely on the homogeneous catalysts.

The ROMP reaction is catalyzed the formation of metal-carbene complexes as first reported by Nobel Prize winner Yves Chauvin and his colleague Jean-Louis Hérisson although a hydride mechanism has also been reported. The initiation of the carbene species occurs through numerous pathways; solvent interactions, substituent interactions, and co-catalysts all can contribute to the production of the reactive catalytic species

The driving force of the reaction is relief of ring strain. After formation of the metal-carbene species, the carbene attacks the double bond in the ring structure forming a highly strained metallacyclobutane intermediate. The ring then opens giving the beginning of the polymer: a linear chain double bonded to the metal with a terminal double bond as well. The new carbene reacts with the double bond on the next monomer, thus propagating the reaction

The choice of solvent can be a vital role in the formation of the carbene species. The identity of alcohols affects the activation of RuCl₃-based catalysts. Depending upon the alcohol used, activation produced either a ruthenium-hydride or a ruthenium-carbene. The nature of these intermediates affects the properties of the resulting polymer. Solvent can also affect the tacticity of the polymer product.

As previously stated, ROMP catalysis is dependent on ring strain. Therefore, the best substrates are bi- and tri-cyclic rings; however, these reactions can lead to numerous products. The addition of substituents to the ring system can result in more complex or more functional polymer products. Unfortunately, substituents on the ring can react deleteriously with some of the most common catalysts. The first Grubbs’ catalyst is poisoned by nitrile or amine groups. Many common molybdenum or tungsten metathetical catalysts are affected by oxygenate or nitrogenous groups. Thus alternative catalysts, such as ruthenium carbene complexes that are not affected by these functional groups are being researched.

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