Polymeric metal complexes schematic

Figure 1. Schematic representation of a polymeric metal complex (left) and two stage degradation mechanism, at the metal center via macroligand dissociation (center) and along the polymer backbone (right).

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Scheme 29 (a) Schematic of the ultrasound-induced scission of a metal complex to produce an active polymerization catalyst,... 

The latter route is also known as the Pechini method and has been used to prepare multicomponent metal oxides with a high homogeneity [43]. In short, the route of the polymerizable compound (PCR) process combines the formation of metal complexes and organic in situ polymerization. The schematic representation of this process is illustrated in Fig. 13.25. 

The second mechanism of polymerization, chain-growth, involves three distinct steps initiation, propagation, and termination. An initiator reacts with a monomer to produce a new species which can then react with another monomer, and another, until the monoma is depleted or the growing polymer chain undergoes a termination reaction. This procedure is shown schematic y bdow where refers to a species that can add another monomer. This species is oft a radical, cation, anion, or attached transition metal complex that can insert a monomer between itself and the growing polymer chain. 

Sonication of 68 or 69 in the presence of transition metal ions produced coordi-natively polymerized bilayer membranes (CPBMs) as schematically illustrated in 70 26.27.130 Upon complexation with the transition metal ions, stability of bilayer membranes was remarkably improved. Amphiphiles 68 and 69 form complexes with transition metal ions with 1 1 stoichiometry. The pronounced enhancement of the stability of bilayer membranes suggests that each metal ion of the resulting CPBM is bound to two nitrogen atoms of the two adjacent azobenzene moieties as indicated by 70. Some metal ions of the CPBMs may be attached to one azobenzene unit. Even if metal ions are singly attached to the azo ligand, a polymeric cluster of metal ions bound to the dihydroxyazobenzene ligand is obtained upon sonication of the amphiphile with transition metal ions. 

Oxygen ligands form very stable complexes with hard Lewis-acid met-alloporphyrins such as Sn(IV), Zr(IV), Mn(III), Mo(V), but also with P(V)-porphyrins. Mixed-metal dimers were synthesized from Al(Me)OEP and phosphorus, arsenic or antimony porphyrins in the form of fi-oxo dimers [73]. hi most cases, polymeric structures are obtained because Sn(IV)-, Mn(III)- and Fe(III)-porphyrins can bind two axial ligands on either side of the porphyrin referred to as trans-binding. This is schematically represented in Fig. 25a. An example includes the linear trinuclear or polymers /r-frans-dioxo-MoTPP [74,75]. 

We can also classify the systems based on the states of the reactants and products. Monofunctional monomers such as benzyl acrylate are liquids and produce liquid polymers, and we call these Tiquid/liquid systems. Figure 1 shows a schematic of the changes in properties across a liquid/liquid front. Multifunctional monomers such as 1, 6 hexanediol diacrylate (HDDA) are liquid but produce a thermoset, solid product, and these we call liquid/solid systems. Finally, solid monomers such as acrylamide (8,25) and transition metal nitrate complexes of acrylamide (26-28) can be polymerized frontally in solid/solid systems. 

Figure 3 Schematic representation of two common supramolecular motifs used in supramolecular polymerizations (a) UPy quadruple hydrogen bonding unit 1, its dimerization, tautomerization, and subsequent complexation with NaPy 2 (b) terpyridine 3 and 2 1 complexation with a low-oxidation-state metal ion M. As shown, each motif demonstrates a different binding mode.

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