ABSTRACT This review describes the application of solvent stabilized transition metal(II) complexes as initiators for polymerization reactions. Cationic complexes with transition metal(II)-metal(II) bond stabilized by solvent molecules are excellent initiators for the polymerization of cyclopentadiene, and phenylacetylene and for the ring-opening metathesis polymerization of norbornene. Substitution of certain solvent molecules by ligands with different charges and donor properties allows the synthesis of initiators with adaptable activity. The crucial aspect of the activity of the metal-metal complexes is the presence of labile ligands in equatorial coordination sites. Several mononuclear transition metal(II) cations stabilized by acetonitrile molecules, also behave as initiators for the polymerization of cyclopentadine. In general, the observed activity of the examined monomeric complexes correlates well with the ligand field stabilization energy as a function of the number of d electrons for octahedrally coordinated transition metals in the oxidation state +II. In addition, grafting of the metal(II) complexes on to a silica surface also leads to catalytic active materials in polymerization reactions. However, the activities of these materials are, in general, somewhat lower than that of the free complexes. It has been shown, that in the case of the quadruply bonded dimolybdenum(II,II) complexes grafted onto the surface of hexagonal MCM-41 mesoporous silica, the metal-metal bond remains intact after the heterogenisation process.
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