Redistribution reactions, sequence

The sequence of redistribution reactions leading to the polymer shown in reaction (18) is illustrated by the following ... 

The isolated products are not perfectly pure owing to further redistribution reactions. Several reactions steps in the sequence do not give rise to observable net reaction, e.g.,... 

A further complication is due to bond redistribution reactions as, for example, in the following sequence ... 

Now some remarks should be made about the chemical and mechanistical realization of this specialized reaction sequence. The formation of a crypto-hydroxyl radical has been discussed already implicitly in Ref.95). It can be accomplished in two ways a) If the central manganese ion of a storage place is coordinated directly with a water molecule, then a univalent oxidative valence change of the manganese by electron transfer to Chl-an can lead to an electronic redistribution between the central ion and the inner sphere water ligand in the form ... 

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. 

A chemical transform is a chemical structural change, or redistribution of electrons, generally described in the analytical direction (the inverse of the synthetic direction. Note that it is not necessary that a transform correspond to a complete reaction. We have defined three different levels of representation the ab initio level, name reaction level, and common reaction sequence level. At the ab initio level a transform represents an electron-pushing step or sequence (equations 1 and 2). 

The issue of transmetalation reactions, or metathesis or redistribution reactions (all terms have been used to describe this) is an important one in the chemistry of organostannanes The typical electrophilic chemistry of allylstannanes involves reaction with an electrophile in an Se2 process. Since all of the Lewis acids we are using to mediate this process are electrophilic, one can easily envision the sequences outlined in Fig.2. Thus a... 

Additional steps in the reaction sequence involve the normal redistribution reactions O and loss of halide anion eventually leading to high polymers. With monomers I and VII, extensive crosscoupling reactions would produce random copolymers. nmr and nmr spectra suggested that copolymers did form, and that they were random (i.e., the general structure was XI where x and y are 0, 1, 2.). Polymer XI was identical to the product that... 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

There are, however, other possible routes to block copolymers successive addition of units of the reactive monomer to the polymer already present, Reaction 5 termination reactions between polymer molecules —side reactions of unknown nature lead to loss of reactive hydroxyl groups (18) possible reactions are ortho carbon-carbon coupling followed by dimerization, addition of amine or water to the ketal intermediate, etc. Block copolymers might even be formed by polymer-polymer redistribution assuming that such redistribution in polymers of greatly different reactivities (such as DMP and DPP), takes place almost exclusively in one type of polymer sequence—that is, that bond scission in a "mixed ketal such as IV occurs always in the same direction—to produce the aryloxy radical corresponding to the more reactive monomer. None of these possible sources of block copolymer can be ruled out on the basis of available evidence. All could produce homopolymer in addition to block copolymer. All of the polymers produced in this work, except for those characterized as completely random copolymers, probably contained at least small amount of one or both homopolymers. 

The kinetics of such redistributions as exemplified in Fig. 11 for the reactions of CH3SiBr3 + CH3GeCl3 and CH3SiCl3 + CH3GeBr3 indicate that a sequence of reactions occurs. The first reaction of the left-hand graph is that of Eq. (104). 

This sequence explains Price s observations adequately and seems to be required in this particular case. The oxidative elimination of halide ion from salts of phenols does not always follow this course, however. In the peroxide-initiated condensation of the sodium salt of 2,6-dichloro-4-bromophenol (Reaction 23) molecular weight continues to increase with reaction time after the maximum polymer yield is obtained (Figure 5) (8). Furthermore, Hamilton and Blanchard (15) have shown that the dimer of 2,6-dimethyl-4-bromophenol (VIII, n = 2) is polymerized rapidly by the same initiators which are effective with the monomer. Obviously, polymer growth does not occur solely by addition of monomer units in either Reaction 22 or 23 some process leading to polymer—polymer coupling must also be possible. Hamilton and Blanchard explained the formation of polymer from dimer by redistribution between polymeric radicals to form monomer radicals, which then coupled with polymer, as in Reaction 11. Redistribution has indeed been shown to occur under... 

In many systems, however, the analysis of the cationic copolymerization of heterocyclic monomers is complicated by two factors (1) at least some of the homo- and cross-propagation reactions may be reversible (2) redistribution of the sequences of comonomers within the chain may occur as a result of chain transfer to polymer. Therefore, the conventional treatment involving four irreversible propagation steps is rarely applicable in cationic ring-opening copolymerization. Instead, the diad model should involve four reversible reactions, i.e., eight rate constants... 

The most illuminating consequence of multi-dimensional vibrational dynamics in polyatomic molecules are fluctuations of resonance widths. In particular, narrow resonances can often be found far above the first dissociation threshold. We have seen that in systems with one degree of freedom the sequence of resonance states is rather short. Since the excitation energy is deposited directly into the reaction coordinate, the complex breaks apart very quickly and the resonances become broad even close to the dissociation threshold. In polyatomic molecules, energy can be temporarily stored in additional degrees of freedom. The lifetime is then determined not only by the total energy, but also by the rate with which the excitation can be redistributed and transferred to the dissociation bond (see the discussion of the classical phase space structure in Sect. 8). 

The inaccessibility of reaction interfaces to investigation means that indirect methods must be used to explore the chemical reactions that occur within these active zones. The determination of the sequence of bond redistribution steps that results in the transformation of a crystalline reactant into a (usually different but often crystalline) product phase is the fimdamental objective of mechanistic studies [74]. All intermediates and the factors that determine the rate and energetics of the transformation (reactivity) must be identified. 

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. 

When applicable, a common method for controlling a redistribution process is to initiate the reaction with a catalyst. Control may then be achieved by quenching the catalyst at the desired extent of reaction. Certain types of redistribution catalyst may thermally decompose under controlled processing conditions that make quenching unnecessary. In these cases, a predominance of block copolymer may be formed that serves as an effective compatibilizer for an immiscible polymer blend. Just as importantly, only a relatively small proportion of the polymer chains actually participates in the redistribution process so that phase separation and the properties attributable to the original sequence distribution are maintained. 

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