Ring-opening metathesis polymerization
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) and a wide variety of catalysts have been discovered.
The catalysts used in the ROMP reaction include a wide variety of metals and range from a simple RuCl3/alcohol mixture to Grubbs' catalyst
The ROMP reaction is catalyzed primarily through 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 ROMP catalytic cycle requires a strained cyclic structure because 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 play a vital role in the formation of the carbene species. One example of such interactions was reported by Basset, et al. regarding RuCl3 and the effects of various alcohols on its catalytic activity. Depending upon the alcohol used, the mechanistic pathway resulted in either a reactive ruthenium-hydride species or the formation of a ruthenium-carbene. Experimental results demonstrated that by altering the solvent, the molecular weight of the polymer produced was either increased or decreased. This observation could result in increased diversity of the catalytic system enabling the production of polymers of various strengths, as polymers with higher molecular weights are typically stronger than polymers of low molecular weights. Drastic differences in the rate of the reaction were also observed, thereby supporting the conclusion that the solvent plays a role in the formation of the ruthenium-carbene.
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