Depolymerization reaction, reactivity

Consider the effects of the N-R group on the structure and reactivity of polysilazanes as susceptible to hydrogen bonding effects. These effects alone should favor ring closure over the respective siloxane analogs. Moreover, for R = H, a new type of depolymerization reaction, analogous to reaction (4), is available as illustrated in reaction (35). Thus, reaction (35) could also contribute to the... 

The reactivity of metal ions is not always the same with DNA and RNA. One reaction that is exclusive to RNA is depolymerization of the polynucleotide structure by the cleavage of the phosphodiester bonds. This depolymerization reaction, as with other RNA hydrolyses, can be induced by metal hydroxides, Zn being one of the most effective. A simple mechanism is that the Zn" chelates to the phosphate group and the 2 -hydroxyl group of ribose (the 2 -group is absent in DNA). Electron withdrawal by the Zn ion then weakens the phosphodiester linkage. Such a mechanism, however, does not take into account the observed influence of the nature of the adjacent base and the formation of metal-dependent products. Pb is also an effective catalyst in site-specific depolymerization of tRNA. In this case the metal has been shown to bind to the bases with only weak interactions with phosphate groups. The catalytic action has been interpreted in terms of nucleophilic attack by a metal-bonded hydroxide ion.134 This may have implications for the mechanisms of other metal ions active in this reaction. 

P n is a reactive molecule of the polymer with n monomer units in the chain, and it is unimportant whether the polymerization mechanism is radical or ionic. The rate constant of the propagation step is kp. Under certain conditions, monomer units can be split off the reactive polymer molecule. Then it is necessary to consider also the depolymerization reaction (with the rate constant kd). 

Comparison of the Reactivity of Various Coals to the Depolymerization Reaction... 

If the depolymerization reaction proceeds by the mechanism of equations (1), (2) and (3) it should be possible to predict a solvent reactivity sequence. The relative reactivity is thus expected to be 3,5-xylenol > naphthol "v phenol > xylene > toluene > naphthalene > aniline-v benzaldehyde. This order, as measured by the indices for reaction used here, is not followed. 

The etch resistance of poly (butene-1 sulfone) in fluorocarbon-based plasmas can be enhanced by prior treatment of the surface in an oxygen plasma. This pretreatment inhibits or retards the depolymerization reaction that characterizes normal etching in fluorocarbon plasmas, thereby permitting formation of a surface-modified layer which exhibits a substantially reduced etch rate. Pretreating PBS in an oxygen plasma enables it to be used subsequently in selective reactive-ion etch processes involving fluorocarbon plasmas to delineate submicron, anisotropically etched patterns. 

A wide range of organic reactions have also been performed in SCW including hydrogenations, eliminations, condensations, hydrations and partial oxidations.It has also been used in depolymerization reactions of natural and synthetic polymers. However, because SCW is more reactive than NCW,... 

Depolymerization reactions can also be avoided when attaching reactive groups to the polymer backbone, which increase the cross-linking density of the polymeric precursor at ambient temperature. In this regard, the attachment of vinyl groups HC=CH2 was studied in much detail (see below). According to Scheme 18.17, vinyl-substituted polysilazanes are best synthesized by ammonolysis of chlorovinylsilanes (H2C=CH)Si(R)Cl2 (R = H, CH3). 

Methyl-, hydroxyethyl-, hydroxypropyl-, and carboxymethyl starches, starch acetates, succinates, alkenyl succinates (Fig. 2), adipates, and phosphates, are all well-known products. Furthermore, special derivatives have also been prepared, such as vinyl-, silyl-, ° or propargyl starches, as reactive intermediates for fiirther fime-tionalization. Unusual substitution patterns can also be established by highly selective deacetylation with alkyldiamines and subsequent introduction of such functional groups as sulfates. From die analytical point of view, the most important aspects are stability under alkaline (mediylation) and acidic or Lewis-acidic (depolymerization) conditions, reactivity (such as migration, rearrangement, further substitution or addition reactions, or any intramolecular reaction), and polarity (lipophilic/hydrophilic, ionic/nonionic, acidic/basic). These properties mainly determine the analytical... 

In summary, the depolymerization reactions of metal oxo- and hydroxo-oligomers have proton-assisted, base-assisted, ligand-assisted, and pH-independent pathways for bridge cleavage. These pathways depend upon whether protons or ligands are configured to the reactive site at equilibrium and whether this configuration increases the rate. 

This chapter present a state-of-the-art review of the field with examples that are presented in two parts. Part I is focused on polymerization reactions by chain addition and step growth mechanisms, while Part II describes reactive modifications of polymers via side group modifications, reactive blending, or depolymerizations reactions. Future directions and research needs are also presented. 

By assimiing the reactivities of bridging oxygen groups towards oxide ion to be independent of molecular size (cf. polymer theory above) the depolymerization reactions of a basic oxide with a silicate melt can be represented by the single reaction ... 

Zavitsas et al. account for the effects of water in their calculations. Water promotes depolymerization of the paraformaldehyde as well as the hemiformals. Their modifications correct for the apparent reduction in methylolation rate as the extent of reaction proceeds, in that the hemiformals remove formaldehyde reactivity from the reaction mixture. Their rate constants look large because they are written for phenate concentrations rather than phenol and because of the formaldehyde equilibrium adjustments. They note that unsalted phenol is a by-... 

The metal-catalyzed amplification of e.e. in small molecules, demonstrated by Soai and coworkers, along with the chiral enrichment of amino arid polymers by sequential polymerization/depolymerization steps, have shown that small enantiomeric excesses in nearly racemic mixtures can be reactively amplified to produce chiral dominance. These real chemical systems, which include plausible prebiotic reactions, experimentally demonstrate the principle of the chiral amplification of a spontaneously broken chiral symmetry in a dynamic and authentic chemical milieu. Therefore amplification to dominance of a small chiral excess of both small and polymeric molecules can be credibly incorporated into an origin-of-life model. 

This reaction is referred to as end capping or end blocking. The result is that reactive carbanion or carbocation centers do not form and depolymerization does not occur at the ceiling temperature of the polymer. The polymer chains are end blocked from depolymerization. The effective ceiling temperature is increased considerably above the ceiling temperature. Acetic anhydride is the usual capping reagent. 

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