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In polymer chemistry, ring-opening metathesis polymerization (ROMP) is a type of chain-growth polymerization involving olefin metathesis. 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 some fine chemical syntheses rely on the homogeneous catalysts.[1] Catalysts are based on transition metals such as tungsten, molybdenum, rhenium, rubidium, and titanium.[2]
ROMP reaction giving polynorbornene. Like most commercial alkene metathesis processes, this reaction does not employ a well-defined molecular catalyst.
Ring-opening metathesis polymerization of cycloalkenes has been commercialized since the 1970s.[1] Examples of polymers produced on an industrial level through ROMP catalysis are Vestenamer or trans-polyoctenamer, which is the metathetical polymer of cyclooctene. Norsorex or polynorbornene is another important ROMP product on the market. Telene and Metton are polydicyclopentadiene products produced in a side reaction of the polymerization of norbornene.[3]
The ROMP process is useful because a regular polymer with a regular amount of double bonds is formed. The resulting product can be subjected to partial or total hydrogenation, or can be functionalized into more complex compounds.[3]
The most common homogeneous catalyst for ROMP is also the best understood. In particular, the third generation Grubbs' catalyst (known as G3) has excellent functional group tolerance, air-stability, and fast initiation and propagation rates.[4][5][6] The fast initiation rates of G3 results in a living polymerization with a narrow polymer molecular weight dispersion. This has made ROMP a popular choice for making advanced polymer architectures and functional polymer products.[7] It is common to quench these reactions with ethyl vinyl ether to generate the free polymer chain and an inactive Ru Fischer carbene complex.[8]
Mechanism
The mechanism of ROMP is similar to any olefin metathesis reaction. Initiation occurs by formation of an open coordination site. Propagation occurs via a metallacyclobutane intermediate.
Frontal ring-opening metathesis polymerization
Frontal ring-opening metathesis polymerization (FROMP) is a variation of ROMP in which it is a latent polymerization system that react fast, only upon ignition.[9] One example of this system is the FROMP of dicyclopentadiene with a Grubbs' catalyst initiated by heat.[10]
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