Polymeric species

Chemical species involving molecular units in a repetitive structure to form a polymer are ubiquitous in soils. Multinuclear hydrolytic complexes, such as Al2(OH)2 or Fe2(OH 2, and biopolymers, such as proteins or polysaccharides, come to mind as familiar examples. The complexation reactions of hydrolytic and biological polymers are investigated in much the same way as described in Section 2.1, the principal issues being the characterization of the stoichiometry of the polymers, their metastability as aqueous species, and the effects of the close proximity among their functional groups on their reactivity.20 

The situation is different for aqueous species of humic substances, the organic matter in soil that is not identifiable as unaltered or partially altered biomass or as conventional biomolecules.21 Humic substances comprise organic compounds that are not synthesized directly to sustain the life cycles of the soil biomass. More specifically, they comprise polymeric molecules produced through microbial action that differ from biopolymers because of their molecular structure and their long-term persistence in soil. This definition of humic substances implies no particular set of organic compounds, range of relative molecular mass, or mode of chemical reactivity. What is essential is dissimilarity to conventional biomolecular structures and biologically refractory behavior. 

The Henderson-Hasselbalch, Perdue-Lytle, and Scatchard models are special cases of an affinity spectrum model, defined by23,24 

The Scatchard model23,24 is perhaps the best-known quasiparticle description of complexation by humic substances. The quasiparticles in this case are hypothetical polymeric molecules bearing one class of functional group that forms a 1 1 complex with a cation. The conditional stability constant for the complexation of 1 mol of a metal by 1 mol of a given class of Scatchard quasiparticles can be expressed as 

In addition to its relative simplicity, the quasiparticle approach has the advantage of a formal mathematical structure that is analogous to that described in Section 2.1 for complexes involving nonpolymeric ligands, such as F or C2O4. Thus, for example, the complexation reaction between Al3+ and a humic substance Scatchard quasiparticle, L, can be written by analogy with Eq. 2.5a  

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Aluminum chloride hydroxide [1327-41 -9] also called polyaluminum chloride or PAG, is made by partial hydrolysis of aluminum chloride to form a mixture of polymeric species. It is more expensive than alum on a weight basis, but has advantages over alum such as not lowering the pH as much and better cost-effectiveness in some appHcations. Residual aluminum in the water is said to be lower and performance in cold water is better (6,7). It is sold as a solution (see Aluminum compounds, polyaluminum cm ORiDEs). 

In aqueous solution, all the sodium peroxoborates dissociate for the most part into boric acid, or its anion, and hydrogen peroxide. Peroxoborate species are also present in these solutions, depending on the pH and the concentration for the species type. The nature of these species has been extensively examined by classical physicochemical methods (13), by nmr, and by Raman spectroscopy (14—17). Both monomeric and polymeric species are usually present. There is some evidence (18) suggesting that these peroxoborates are more reactive than hydrogen peroxide alone under similar conditions. 

Acrylic Polymers. Although considerable information on the plasticization of acryUc resins is scattered throughout journal and patent hterature, the subject is compHcated by the fact that acryUc resins constitute a large family of polymers rather than a single polymeric species. An infinite variation in physical properties may be obtained through copolymerization of two or more acryUc monomers selected from the available esters of acryUc and methacryhc acid (30) (see Acrylic esterpolya rs Methacrylic acid and derivatives). 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

Hydroxides. The hydrolysis of uranium has been recendy reviewed (154,165,166), yet as noted in these compilations, studies are ongoing to continue identifying all of the numerous solution species and soHd phases. The very hard uranium(IV) ion hydrolyzes even in fairly strong acid (- 0.1 Af) and the hydrolysis is compHcated by the precipitation of insoluble hydroxides or oxides. There is reasonably good experimental evidence for the formation of the initial hydrolysis product, U(OH) " however, there is no direct evidence for other hydrolysis products such as U(OH) " 2> U(OH)" 2> U(OH)4 (or UO2 2H20). There are substantial amounts of data, particulady from solubiUty experiments, which are consistent with the neutral species U(OH)4 (154,167). It is unknown whether this species is monomeric or polymeric. A new study under reducing conditions in NaCl solution confirms its importance and reports that it is monomeric (168). 8olubihty studies indicate that the anionic species U(OH) , if it exists, is only of minor importance (169). There is limited evidence for polymeric species such as Ug(OH) " 25 (1 4). 

Zirconium [7440-67-7] is classified ia subgroup IVB of the periodic table with its sister metallic elements titanium and hafnium. Zirconium forms a very stable oxide. The principal valence state of zirconium is +4, its only stable valence in aqueous solutions. The naturally occurring isotopes are given in Table 1. Zirconium compounds commonly exhibit coordinations of 6, 7, and 8. The aqueous chemistry of zirconium is characterized by the high degree of hydrolysis, the formation of polymeric species, and the multitude of complex ions that can be formed. 

Polyborates and pH Behavior. Whereas bode acid is essentiaHy monomeric ia dilute aqueous solutions, polymeric species may form at concentrations above 0.1 M. The conjugate base of bode acid in aqueous systems is the tetrahydroxyborate [15390-83-7] anion sometimes caHed the metaborate anion, B(OH) 4. This species is also the principal anion in solutions of alkaH metal (1 1) borates such as sodium metaborate,... 

Chapter 6 Miscellaneous crown type compounds, including acetals, and compounds containing sulfur, phosphorus, arsenic, etc. Polymeric species are included here and in chapter three. 

Still lower pHs to give a pale-yellow solution. As a result of spectrophotometric studies there is general agreement that the predominant species in the initial colourless solution is the tetrahedral V04 ion and, in the final pale-yellow solution, the angular VOz ion. In the intervening orange to red solutions a complicated series of hydrolysis-polymerization reactions occur, which have direct counterparts in the chemistries of Mo and W and to a lesser extent Nb, Ta and Cr. The polymerized species involved are collectively known as isopolymetallates or isopolyanions. The determination of the equilibria involved in their formation, as well as their stoichiometries and structures, has been a confused and disputed area, some aspects of which are by no means settled even now. That this is so is perfectly understandable because ... 

In the thermal production of gold coatings on ceramics and glass, paints are used which comprise Au chloro-complexes and sulfur-containing resins dissolved in an organic solvent. It seems likely that polymeric species are responsible for rendering the gold soluble. 

The polymeric species containing silole nuclei also form the Fe(CO)3 complexes -coordinated via the heteroring on interaction with Fe(CO)5 [93JOM(456)35 990M1717],... 

During synthetic studies toward pynrolo-fiised indoles, the polymeric species 218 has been suggested as aproduct from the diformylation of 219 under Vilsmeier-Amold-Haack conditions, followed by heating with 35% aqueous sulfuric acid (Scheme 33) (9171,5035). With the deeper knowledge of the reactions of 3-formylIndoles now available (99CHE561), this assignment should be reconsidered. 

The dicationic ditriflate salt of 1,2,4-trimethyltriazolium with silver acetate gives the bis-carbene complex 185 (00JOM(600)l 12). In excess silver acetate, the one-dimensional polymeric species with alternating silver ions and 1,2,4-triazol-3,5-diylidene carbenes result, where both carbon atoms of each heteroring are engaged in coordination. 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

All of these initiation processes have one feature in common in each of them a stable X—M bond is formed on one end of a bifunctional monomeric unit, while a reactive center is created on the other end. Hence, as the addition of further monomeric units proceeds a polymeric species growing on one end only is produced, the other end being blocked by the stable X—M bond. 

The log CLD method can sometimes provide better quality data than the conventional Mayo method. It is less sensitive to experimental noise and has application in measuring the transfer constant to polymeric species where the distributions of the transfer agent and the polymer product partially overlap.24... 

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