Unit Variability Due to Different Valence States of the Transition Metal Ions

Unit Variability Due to Different Valence States of the Transition Metal Ions 

Different valence states are also a fairly widespread type of unit variability. By analogy with macromolecular complexes (Section II), it may be expected that homopolymerization and copolymerization of metal-containing monomers would prevent or retard redox processes involving participation of metal ions. Experimental data confirm the fact that a polymeric matrix stabilizes complexes of metals in low oxidation states (e.g., Pd ). Moreover, the stability of the Cu+ state during polymerization (including thermal polymerization) of copper acrylate controls the use of this method for the preparation of coordination compounds of Cu . The polymeric framework plays a stabilizing role, whereas the metal ions that are localized on the surface layer are oxidized to Cu +. However, polymerization of monomers that contain metal ions in high oxidation states is often accompanied by their reduction Fe + - Fe +, and Mo + - Mo (scheme 14). For example, polymerization of Cu and Fe + acrylates may be accompanied by intramolecular chain termination. This may be attributed to the relatively low standard reduction potentials of these metal ions (7io(Cu + Cu+) = 0.15, o(Fe - Fe ) = 0.77 V). 

In systems of this type, electron transfer occurs via a complex route and is accompanied by reduction of M to Monomolecular chain termination 

It is assumed that the set of eigen states of a dimeric copper dimethacrylate structural fragment allow delocalization of the excited rr-electron of the C=C bond to the COO group during the initiation of graft post-polymerization7 This is accompanied by the reduction of copper and partial dissociation of the monomer (scheme 15). 

An estimate of the proportions of different valence forms of copper in copper polyacrylates shows that the Cu content does not exceed 60%, and Cu is absent. Most probably, autocatalytic Cu - Cu reduction occurs. The role of the ligand and their subsequent transformations during the reduction reaction are not entirely clear, although some of the possible routes will be analyzed below. It stands to reason that the geometry of complexes also changes on reduction of metal ions. For example, square-planar Cu + complexes add an additional ligand and are converted into square-pyramidal complexes, whereas Cu complexes are normally tetrahedral. 

---