Extra-Coordination as a Spatial and Electronic Anomaly of the Polyhedron

Extra-Coordination as a Spatial and Electronic Anomaly of the Polyhedron 

The relatively low stability of some MCMs and polymers based on MCMs may be atributed to coordination imsaturation of the central metal, which facilitates a series of side processes. The coordination number of a metal can be increased complex formation for example, the structure of NF, Co and Cr acrylates can be expanded to an octahedral geometry. Specifically introduced compounds like solvent molecules, (most often water) can act as the additional ligands. After polymerization, not all the addends chemically bonded to the metal are removed fi om the product. Thus they serves as a somce of anomalous units in the polymers. Since the structure of coordination units in the metal-containing polymer is difficult to study due to a multiplicity of oxidation states, this type of unit variability is more convenient to examine in metal-containing monomers. 

Of other solvents, alcohols such as ethanol are used most often for polymerization. Since the structure of nonsolvated Cu acrylate is unknown, it may be expected that upon desolvation of its complex with ethanol, some of the bidentate carboxyl groups become tridentate (see above). Thus they can act simultaneously as a ligand and a solvation agent, which is absent. The increase in the dentate number of the P-diketonate groups leads to a distortion of the oxygen environment about Cu in particular, the Cu-O distances and, hence, the C-O bond lengths. 

Additional coordination can also involve other components of the polymerizable system and can exert a substantial influence on the polymerization kinetics and the structures of the products formed. In particular, the formation of MCM complexes with an initiator can both accelerate (generate the initiating species) and retard its decomposition and leads to a set of products that can be incorporated in the polymer as terminal groups. Thus the unusual kinetics (the order of the reaction with respect to the monomer is -0.5) of radical polymerization of Ti +-containing monomers are due to the occurrence of parallel reactions (scheme 27).  

The Ti -coordinated radicals thus formed can participate both mainly in chain termination and in chain growth. The metal ion acts as a sort of radical trap. The radical that is formed becomes passivated with respect to the initiation of new chains. These complexes can be relatively stable. As the first approximation, the situation is modeled by stable radicals (e.g., iminoxyl radicals). In fact, the initiating radical species in the reaction with the monomers of ri -(hexatrienyl)tricarbonyl-iron is accompanied by the formation of stable radicals, which are inefficient for chain initiation (scheme 28). 

---