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The history of the development of ring-opening metathesis polymerization (ROMP), and the related reactions of olefin metathesis (OM), ring-closing metathesis (RCM), asymmetric ring-closing metathesis (ARCM), and acyclic diene metathesis (ADMET) polymerization, is outlined. These reactions of olefins proceed by a chain mechanism involving the alternate formation of transition-metal carbene and metallacyclobutane complexes as propagating species, sometimes directly detectable by NMR spectroscopy. Initiation by metal carbene complexes can yield living polymers of narrow molecular weight distribution. Successive addition of different monomers may produce block copolymers of well defined morphology. In principle the ROMP of any cyclic olefin, or the metathesis reaction of any diene, will give rise to an equilibrium mixture of linear and cyclic species. The position of equilibrium is very dependent on the ring size and on the nature and location of substituents. The route to equilibrium can vary with the monomer and the initiator. Linear polymer is usually formed first, but cyclic oligomers are occasionally the initial products. RCM of dienes only occurs if intramolecular cyclization is favoured over intermolecular ADMET polymerization. For simple dienes RCM is favoured when the ring formed is 6-membered; conversely the ROMP of cyclohexene is thermodynamically impossible. When the conformations of the bonds between the two C=C bonds are restricted by the presence of multiple bonds, rings or hydrogen bonds, the balance may be tipped towards the formation of much larger rings (12 or more members). By the use of catalysts with chiral ligands it is possible to carry out ARCM reactions to give products of very high optical purity. When the substrate contains both an unsaturated ring and an unsaturated substituent, it is sometimes possible to effect tandem ring-opening and RCM. Great advantage has been taken of the RCM reaction in recent years, especially using the Grubbs catalyst Ru (=CHPh)Cl2(PCy3)2, for the synthesis of numerous compounds of biological importance. Current work in various laboratories is concerned with improving ruthenium carbene complexes so as to make them even more stable and more active, and to make them capable of polymerizing water-soluble monomers. |
