Chelation crosslinking, 185

Metal chelates have also been used in photografting and crosslinking of different types of polymers [61,65-67]. 

Commonly Used Salts, Crosslinkers, and Chelating Agents... 

Partially hydrolyzed polyacrylamides, carboxymethylcellulose, polysaccharides, and acrylamido methylpropane sulfonate have been screened to investigate the performance of aluminum citrate as a chelate-type crosslinker. An overview of the performance of 18 different polymers has been presented in the literature [1646]. The performance of the colloidal dispersion gels depends strongly on the type and the quality of the polymer used. The gels were mixed with the polymers at two polymer concentrations, at three polymer-to-aluminum ratios, and in different concentrations of potassium chloride. The gels were quantitatively tested 1,7, 14, and 28 days after preparation. 

O. Barkat. Rheology of flowing, reacting systems The crosslinking reaction of hydroxypropyl guar with titanium chelates. PhD thesis, Tulsa Univ, 1987. 

Open-chain ligands were the first evaluated for complexation studies with indium and yttrium. The use of diethylenetriaminepentaacetic acid (DTPA) anhydride permitted early evaluation of labeled chelate-conjugates (Figure 2).80 The use of this activated chelating agent was quite popular, until the drawbacks associated with its crosslinking of proteins became apparent. 

Certain bifunctional metal chelating agents have been used to investigate protein interactions by virtue of their ability to generate reactive oxygen species that affects protein structure in the immediate vicinity of their modification site. The following sections discuss two applications of such chelate labels, one of which cleaves peptide bonds while the other one causes covalent crosslinks to occur between interacting protein structures. 

Figure 28.21 The reactions of R u (11) pby 3 + are catalyzed by light at 452 nm that begins by forming an excited state intermediate. In the presence of persulfate, a sulfate radical is formed concomitant with the oxidative product Ru(III)bpy33+. This form of the chelate is able to catalyze the formation of a radical on a tyrosine phenolic ring that can react along with the sulfate radical either with a nucleophile, such as a cysteine thiol, or with another tyrosine side chain to form a covalent linkage. The result of this reaction cascade is to cause protein crosslinks to form when a sample containing these components is irradiated with light.

Amphiphilic resin supported ruthenium(II) complexes similar to those displayed in structure 1 were employed as recyclable catalysts for dimethylformamide production from supercritical C02 itself [96]. Tertiary phosphines were attached to crosslinked polystyrene-poly(ethyleneglycol) graft copolymers (PS-PEG resin) with amino groups to form an immobilized chelating phosphine. In this case recycling was not particularly effective as catalytic activity declined with each subsequent cycle, probably due to oxidation of the phosphines and metal leaching. 

The action of zinc in increasing the efficiency and rate of crosslinking is thought to involve chelation of zinc with the accelerator as well as species XVIII and XIX. Zinc polysulfide compounds such as XX are also likely intermediates. Zinc chelated to sulfur or as zinc sulfide bonds probably facilitate cleavage of sulfur-sulfur bonds in the concerted reactions described by Eqs. 9-20 and 9-21. 

Gregor et al66 reported the Cu-complex formation of poly(methacrylic acid) (PMA) resins crosslinked with 1% or 9% divinylbenzene. The formation constants of the Cu complexes with the resins were smaller than that of noncrosslinked PMA. The stepwise formation constants decreased from Ki to KA in the resin system, which was the opposite of the noncrosslinked PMA system. The rigidity of the polymer-ligand chain was considered to hinder chelate formation. Formation constants of the PMA resin were also reported by Gustafson era/.67. The formation constant of the noncrosslinked PMA decreased for various metal ions ... 

Poly(ethyleneimine)(PEI) bridged by alkylene dihalide has been used as a chelating resin for Cu and Co68. Dingman et al,69 studied the adsorption of metal ions on PEI resins crosslinked with toluene diisocyanate. The amount of metal ions adsorbed decreases with the degree of crosslinking. These crosslinked PEI resins... 

We used partially crosslinked poly(4-vinyipyridine)(DBQP) as the polymer ligand. PVP was crosslinked by alkylation of the pyridine groups in PVP with 1,4-dibromobutane to yield insoluble DBQP resins. The free and unquaternized pyridine groups in DBQP could be used to form the polymer chelate, as represented in 33. 

---