Metal ion cross-linking

Figure 7.2 Schematic representation of alginate cross-linking with divalent cations and the chemical structure of the constituent repeat units, guluronate (G) and mannuronate (M). Carboxylate groups present along the backbone (largely from the G residues) interact with multivalent cations to yield metal ion cross-linked gels.

Linear amino polymers containing basic nitrogen atoms are critically reviewed with regard to their synthesis, protonation and complex formation in solution with metal ions. Cross linked resins having essentially the same structure as linear polymers, are also mentioned. As far as the proto-nation is concerned, special care has been given to thermodynamic aspects, and to the most probable protonation mechanism. Complexing abilities of these polymers have been evaluated through stability constants and spectroscopic parameters. Practical implications of the properties have been considered. 

Figure 13 Examples of interstrand metal ion cross-links in polynucleotides ...

The crystal structure of the Ni-Ni-Bz derivative is composed of layers containing metal ions cross linked by CsN groups (see Figure 8.13) [62,63]. Two ammonia... 

Figure 8.13 Schematic view of the structure of the Ni-Ni Hofmann clathrate with benzene as guest molecule. The planes of the benzene guest molecules are perpendicular to the plane of the metal ions cross-linked by C=N entities. The disordered NH3 groups are represented as a superposition of four crystallographically equivalent configurations. Reproduced with permission of the International Union of Crystallography.

Borate cross-linked fracture fluids are also believed to cause less damage to the reservoir and less likely to impair permeability than rival cross-linkers [26,50,68], This is partly due to the fact that borate cross-links can be broken down after fracturing simply by reducing pH. That is not to say that chemical (oxidative) or enzymatic means for effecting cleanup of the reservoir are not required to break down the polymer chains and flush away the fluid residues, but this process is more effective with borates because of the reversible nature of the cross-link bond. Some metal ion cross-linked gels have poor cleanup properties and soluble precipitates can be formed when they react with certain chemical breakers. ... 

Metal-Ion Cross-Linker Chemistry. The most commonly used metal-ion cross-linkers are titanates, zirconates, and borates. Titanate and zirconate gels display significantly different properties than borate gels, and these differences indicate the different nature of the cross-linking for the two types of ions. For example, gels produced by transition metal ions are more thermally stable than borate gels and do not recover viscosity after exposure to shear. 

In practice, Fe ions are not isolated, but exist in compounds. The distribution of the ions between the high-spin and low-spin states will depend on molecular interactions and the temperature. If the transition-metal ions are linked by chemical bridges involving strong interactions, it is possible to induce a cooperative interaction between the cations, so that all cross over at once. There is still a long way to go before isolated ions are used to store bits of data, but research in this area is making rapid progress. 

An interesting class of metal-induced cross-linking gel was reported by Hecht [68]. A poly(triazole-pyridine) 68 was prepared by AABB CuAAC copolymerization of a pyridyl diacetylene 69 and a chiral aryl diazide 70 (Scheme 20). Due to the repulsive interactions between the nitrogen lone-pair electrons on the triazole and pyridine units [69], the polymer tended to adopt a helical conformation in the absence of metal ions. On the other hand, metallogels 71 were formed when Zn (II), Fe(II), or Eu(III) ion was added to a solution of the polymer in acetonitrile. It was believed that the 2,6-bis(triazolyl)pyridine ligands chelated to the metal ion to form a hexacoordinate metal complex, which then led to a metal-promoted cross-linked 3D network that entrapped acetonitrile solvents. 

These monomers provide a means for introducing carboxyl groups into copolymers. In copolymers these acids can improve adhesion properties, improve freeze-thaw and mechanical stability of polymer dispersions, provide stability in alkalies (including ammonia), increase resistance to attack by oils, and provide reactive centers for cross-linking by divalent metal ions, diamines, or epoxides. 

The reactions are catalyzed by tertiary amines, quaternary ammonium salts, metal salts, and basic ion-exchange resins. The products are difficult to purify and generally contain low concentrations of acryhc acid and some diester which should be kept to a minimum since its presence leads to product instabihty and to polymer cross-linking. 

When the polymer was prepared by the suspension polymerization technique, the product was crosslinked beads of unusually uniform size (see Fig. 16 for SEM picture of the beads) with hydrophobic surface characteristics. This shows that cardanyl acrylate/methacry-late can be used as comonomers-cum-cross-linking agents in vinyl polymerizations. This further gives rise to more opportunities to prepare polymer supports for synthesis particularly for experiments in solid-state peptide synthesis. Polymer supports based on activated acrylates have recently been reported to be useful in supported organic reactions, metal ion separation, etc. [198,199]. Copolymers are expected to give better performance and, hence, coplymers of CA and CM A with methyl methacrylate (MMA), styrene (St), and acrylonitrile (AN) were prepared and characterized [196,197]. 

Polymers with a sizable number of ionic groups and a relatively nonpolar backbone are known as ionomers. The term was first used for copolymers of ethylene with carboxylated monomers (such as methacrylic acid) present as salts, and cross-linked thermoreversibly by divalent metal ions. Such polymers are useful as transparent packaging and coating materials. Their fluorinated forms have been made into very interesting ion-exchange membranes (considered further below). 

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