Redistribution reactions mechanisms

The first step for the synthesis of a melt spinnable polysilane is the alkoxylation and distillation of the residue (Figure 1). 1,2-dimethyltetramethoxydisilane and 1,1,2-trimethyltrimethoxydisilane are mixed in a special ratio and a poly silane will be obtained by a catalytic redistribution reaction. Catalysts for this reaction are alkali alkoxides like sodium methoxylate. Phenylmethoxydisilanes [22] or phenylchloride are used as additives. A mixture of methyltrimethoxysilane and dimethyldimethoxy-silane distils off as a byproduct of the redistribution reaction. Figure 2 shows the mechanism of the catalytic redistribution. 

The redistribution reaction of MeSnCl3 to give Me2SnCl2 and S11CI4 in solution at 50 °C follows second-order kinetics, and is faster in coordinating solvents and in the presence of alcohols or amines, suggesting a nucleophile-assisted electrophilic mechanism.330... 

Redistribution in Polymer Coupling. Monomer-polymer redistribution occurs most easily when the monomeric phenol and the phenol of the polymer are identical or, at least, very similar in reactivity (2). The homopolymers of DMP and MPP obviously redistribute very rapidly with either of the two monomers, so that sequential oxidation of DMP and MPP can produce only random copolymer. The redistribution reaction and its relation to the overall polymerization mechanism have been the subject of many previous investigations (2, 10, 13, 14), but the extraordinary facility of redistribution in the DMP-MPP system leads to results that could not be observed in other systems examined. 

The formation of random copolymer, even when the starting materials are preformed homopolymer blocks, as was observed with DMP and MPP, is reasonably explained by the monomer-polymer and polymer-polymer redistribution reactions of Reaction 3 and 9. Block copolymers are accounted for most easily by polymer-polymer coupling via the ketal arrangement mechanism (see Reaction 15, p. 256). 

As we have seen, the mechanism of a reaction is the stepwise process by which reactants are converted to products. Moreover most steps in a reaction mechanism involve the movement and redistribution of electrons in the reactants or intermediates until the electronic configuration of the product is obtained. The electronic changes which are often depicted by curved-arrow notation result in bond making and/or bond breaking needed to get from the reactant to the product. 

These few studies make it appear very likely that mechanisms involving four-center transition states will be common in redistribution reactions. However, it is not yet possible to predict a mechanism on an a priori basis. 

In the last decade, an immense amount of experimental material has been generated describing the preparation and the chemical and physical properties of transition metal n complexes and coordination compounds. Recently great emphasis has been placed on the study of the kinetics and the reaction mechanisms involving such compounds. Although redistribution reactions as defined earlier in this review and as exemplified specifically by the reaction of Eq. (168) (M = transition metal, L=coordinated ligand)... 

In the course of developing the idea of the enzymatic catalysis mechanism Poltorak [99] stated the uniformity of enzymatic catalysis mechanisms in the framework of suggested notion of linear chain of bond redistribution (linear CBR). Actually, this idea laid the foundation for the catalase reaction mechanism suggested by Poltorak. In this mechanism, owing to composition of linear CBRs he showed the means for effective proton transfer between... 

The authors suggested a mechanism via the intermediacy of a reactive pentacoordi-nated hydrosilyl anion,50c which is formed by the addition of hydride (H-) on the silanes, for the redistribution reactions. 

A major problem in postulating silylenoid metal complexes as intermediates in the redistribution reactions is simply that good model compounds are lacking, and the decomposition mechanisms of silyl transition metal complexes have not been systematically investigated. While there is evidence for transient R2Si species produced by thermal or photochemical means (80-83), there are no known monomeric silylene metal complexes. Several monomeric stannylene and germylene complexes are... 

The mechanism snfficiently explains how prodnctive condensation metathesis occms since terminal alkenes are contained in every step of the productive cycle (Scheme 5). In this kinetically controlled regime (high concentration of terminal alkene present), alkylidene reactions with unsubstituted (terminal) alkenes are favored over interchain redistribution reactions or other degenerate eqnilibria. However, when terminal alkene concentration decreases near the end of the polycondensation reaction, competing... 

The arrows that are used to show the redistribution of electron density are drawn from a position of high electron density to a position that is electron-deficient. Thus, arrows are drawn leading away from negative charges or lone pairs and toward positive charges or the positive end of a dipole. In other words, they are drawn leading away from nucleophiles and toward electrophiles. Furthermore, it is only in unusual reaction mechanisms that two arrows will lead either away from or toward the same atom. 

Many metal carboxylates are prepared in the form of hydrates and water is lost in a lower temperature range than that required for the onset of anion decomposition [86]. The removal of water from the structure must be accompanied by substantial bond redistribution within the anhydrous phase so formed, and this can sometimes lead to structural reorganization within the solid [142]. Nickel salts, prepared in the form of crystalline hydrates, however, show a low degree of lattice order after water removal [116,118,121]. The crystal structures of few dehydrated metal carboxylates are known in any detail, an omission that must be remembered in formulating reaction mechanisms. 

The term reaction mechanism specifies the sequence of chemical steps through which reactants are transformed into products. In the collision model of homogeneous reactions the steps are described in terms of their molecularity. However, the sequence of bond redistributions and other processes (diffusion, recrystallization, etc.) by which a solid reactant is converted into products will generally be far more complex (see Chapter 18) and the information required to characterize contributing steps is far less accessible. Description of these steps. 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

While cationic catalysts 16 are an effective and important class of catalysts, their electrophilicity still hampers their ability to tolerate polar functions. The less electrophilic neutral 17-19 and zwitterionic 20 nickel catalysts have been examined in an effort to enhance compatibility with polar monomers and co-monomers. Again, bulky ligands are necessary not only to promote olefin insertion, but also to discourage ligand-redistribution reactions resulting in deactivated bis-ligand complexes and decomposition products. For a typical neutral Ni(ii) system, such as 19, Brookhart has shown that the mechanism involves (i) associative displacement of the PR3 ligand by ethylene to... 

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