Self-catalyzation

Using excess ethylene glycol is the usual practice because it drives the equilihrium to near completion and terminates the acid end groups. This results in a polymer with terminal -OH. When the free acid is used (esterification), the reaction is self catalyzed. However, an acid catalyst is used to compensate for the decrease in terephthalic acid as the esterification nears completion. In addition to the catalyst and terminator, other additives are used such as color improvers and dulling agents. For example, PET is delustred hy the addition of titanium dioxide. 

SY Ng, T Vandamme, MS Taylor, J Heller. Controlled drug release from self-catalyzed poly(ortho esters). Ann NY Acad Sci 831 168-178, 1997. 

Self-baking electrodes, 12 305, 755 Self-bonded reaction-sintered silicon nitride, 17 210, 211 Self-catalyzed polyols, 25 464 Self-cleaning materials, 22 108-127 problems and outlook for, 22 123-124 surface characteristics of, 22 108-109... 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

Because TPA has a melting point of >300 °C and poor solubility in PDO, direct esterification is preferably carried out in the presence of a heel under a pressure of 70-150kPa and at 250-270 °C for 100-140min. A heel is an oligomeric PTT melt with a degree of polymerization (DP) of 3 to 7, purposely left in the reaction vessel from a previous batch to improve TPA solubility and to serve as a reaction medium. The esterification step is self-catalyzed by TPA. 

In the case of the esterification of the diacid, the reaction is self-catalyzed as the terephthalic acid acts as its own acid catalyst. The reverse reaction, the formation of TPA and EG from BHET is catalytic with regard to the usual metal oxides used to make PET, but is enhanced by either the presence of hydroxyl groups or protons. In the case of transesterification of dimethyl terephthalate with ethylene glycol, the reaction is catalytic, with a metal oxide needed to bring the reaction rate to commercial potential. The catalysts used to produce BHET are the same as those needed to depolymerize both the polymer to BHET and BHET to its simpler esters. Typically, titanium, manganese and zinc oxides are used for catalysts. 

The reduction of a-carbonyl diphenyldi-thioacetals [212] was reported to be self-catalyzed (with formation in the course of the cathode process of the couple... 

The main issue wifh fhese maferials is oxidative stability at potentials above 1.0 V. The development of graphihzed blacks to address this is covered elsewhere in this chapter. The thermal stability of high area blacks is also an issue. Sfevens ef al. showed fhaf high area blacks such as BP2000 suffer excessive gas-phase oxidahon af 150°C in dry air when high loadings of Pt are deposifed onto fhem.i Oxidation rates increase in the presence of H2O/ air mixtures, indicating that Pt/C can self-catalyze the water-gas shift reaction at temperatures as low as 150°C. 

Autoxidation. Self-catalyzed oxidation in the presence of air. Autoxidation can be initiated by heat, light, or a catalyst. The commercial production of phenol and acetone from cumene is autoxidation. Other examples include the degradation of polymers exposed to sunlight for long periods of time gum formation in lubricating oils and gasoline and the spoilage of fats. 

Strategies for functionalization have been developed that exploit the naturally occurring amino acids as well as the non-natural ones. Post-synthetic modifications have been reported that are based on reactive sites that self catalyze the incorporation of the new functionality at the side chains of Lys residues [24,25] and on the chemoselective ligation reaction [26-29]. These developments in combination with new methodology for the synthesis of large proteins [30] provide access to a highly versatile pool of new polypeptides and proteins. 

Fig. 2-1 Third-order plot of the self-catalyzed polyesterification of adipic acid with diethylene glycol at 166°C. After Solomon [1967] by permission of Marcel Dekker, New York) from the data of Flory [1939] (by permission of American Chemical Society, Washington, DC).

In the absence of an added strong acid, the reaction is self-catalyzed by a second molecule of the adipic... 

In the case of the Cu-type enzyme from A. cycloclastes, it has been shown (Averill and Tiedje, 1990 Jackson et al., 1991) that nitrite can be reduced to N2O in the presence of NO in what resembles a self-catalyzed reaction. At the outset, NO was overwhelmingly the major product in the reduction of nitrite. 

With amine initiators the so-called self-catalyzed polyols are obtained, which are used in the formulation of rigid spray foam systems. The rigidity or stiffness of a foam is increased by aromatic initiators, such as Mannich bases derived from phenol, phenolic resins, toluenediamine, or methylenedianiline (MDA). In the manufacture of highly resilient flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. 

There are many other variations of the heat test.101 They are sometimes called stability tests, but most of them, it will be noted, involve the self-catalyzed decomposition of the sample in an atmosphere of air or of red fumes. They indicate the comparative stability only 6f materials which are physically alike. True indications of the stability of nitric esters are to be secured only by studying the decomposition of the substances in vacuum. For this purpose the 120° vacuum stability test is most generally preferred. 

The same reaction occurs at lower temperatures 0.665% of a given portion of the material decomposes in 3 years at 20°, 2.43% in 1 year at 35°, 0.65% in 10 days at 50°, and 100% during 14 hours heating at 100°. The decomposition is not self-catalyzed. The product, hexanitrosobenzene, m.p. 159°, is stable, not hygroscopic, not a primary explosive, and is comparable to tetryl in its explosive properties. 

Aminosilanes contain the catalyzing amine function in the organic chain. The reaction of aminosilanes with silica gel in dry conditions is therefore self-catalyzed. They show direct condensation, even in completely dry conditions. Upon addition of the aminosilane to the silica substrate, the amine group may form hydrogen bonds or proton transfer complexes with the surface silanols. This results in a very fast adsorption, followed by direct condensation. This reaction mechanism of APTS with silica gel in dry conditions, is displayed in figure 8.9. After liquid phase reaction, the filtered substrate is cured, in order to consolidate the modification layer. 

Further information about the position of attack and the nature of the rate-determining step was obtained by Grimson and Ridd98 who studied the kinetics of iodination of some deuterio derivatives of imidazole. The kinetic data on the iodination of 2,4,5-trideuterio-imidazole enabled them to calculate the isotope effects for both the uncatalyzed (fcH/fcD = 4.36) and the self-catalyzed (fcH/fcD = 4.47) reactions. A comparison of the istope effects observed in the iodination... 

Figure 2 Chemical reaction models for (a) isodesmic and (b) nucleated supramolecular assembly. 1 and K 2>1 are equilibrium constants for the elongation reactions, and Ka 1 and K a those for the conversion between assembly active and inactive forms of the monomer units. If /f aKa, then the nucleated assembly is self-catalyzed ( autosteric ) and if K a = Ka this is not so.

Figure 3 Fraction of material in the polymerized state, f, as a function of the mass action variable X relative to its value X at the half-way point f =1/2. Indicated are predictions for the isodesmic and the self-catalyzed nucleated polymerization models. Activation constant of the nucleated polymerization Ka = 10-4.

Let us start with the nucleated assembly that is not self-catalyzed. It turns out useful to distinguish between the mean aggregation number of all the material in the solution, N, from that in which only the activated species is considered and that we denote by Na. If we define the equilibrium constant K = exp [—.%] with g as the binding free energy, and introduce the nucleation constant Ka = exp[—/3ya], then under conditions of thermodynamic equilibrium, mass action gives (Aggeli, 2001 Ciferri, 2005 Nyrkova et al., 2000 Tobolsky and Eisenberg, 1960)... 

If the nucleated assembly is self-catalyzed, then this modifies only the role of the equilibrium constant K in the non-self-catalyzed model and has to be replaced by the ratio K/Ka. This means that we again obtain for the fraction polymerized material f=KaNa/(1 + KaNa) but that the degree of polymerization averaged over the active material only now obeys (Ciferri, 2005)... 

The self-catalyzed model is a simplification of the actin polymerization model of Oosawa and Kasai (1962) [and more recent elaborations of it (Niranjan et al., 2003)], and both equilibrium constants K and Kn, and hence the free energies g and ga can in principle be obtained by fitting the theory to assembly experiments. Typical values of g for, for example, actin vary between —10 and —20 kBT and ga between +2 and +3 kBT (Oosawa and Asakura, 1975 Oosawa and Kasai, 1962). For the biomimetic compound oligo(phenylene vinylene) similar to compound 2 shown in Figure 6, dissolved in alkane solvents, similar values were found for g but much larger ones for ga of +8 to +10 kBT, making the supramolecular polymerization of this compound an extremely highly co-operative process (Jonkheijm et al., 2006). 

IfX 1, disordered (nonhelical) assemblies do not form in any appreciable quantities. For h< 0, there is a polymerization transition from monomers to helical assemblies that is of the self-catalyzed nucleation type provided flj 3> 1. In the language of the coarse-grained self-catalyzed nucleated assembly model, the transition takes place near Xp exp [f3h] and we are able to assign an activation constant Ka exp [ 3j + 311], The theory of Section 2 approximately applies. 

Perez-Vilar J. and Boucher R.C. (2004). Mucins form disulfide-linked multimers through a self-catalyzed pH-dependent mechanism. Pediatr Pulmonol. 38, 238(144)... 

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