Polymer coupling, redistribution

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. 

Continuation of this process, with monomer produced by redistribution and then removed by coupling, would lead to a random copolymer. Alternatively, if polymer-polymer coupling were to proceed solely by rearrangement, without dissociation at any stage, either of the ketals I or III would produce only block copolymer. 

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). 

Although redistribution and coupling can be observed separately, oxidative polymerization under ordinary conditions involves both reactions and redistribution of oligomers to form monomer followed by removal of the monomer by coupling is an important mechanism of polymer growth. Redistribution in dimethylphenol polymerizations is extremely rapid. Addition of monomer to a polymerizing solution causes an immediate drop in the solution viscosity almost to the level of the solvent, as redistribution of polymer with monomer converts the polymer already formed to a mixture of low oligomers. 

The experiments cited above show that redistribution, presumably via a quinone ketal intermediate, occurs during the oxidative polymerization of 2,6-xylenol and must be responsible at least partially for the polycondensation characteristics of the reaction. Although the conditions under which Mijs and White demonstrated rearrangement are different from those usually employed for oxidative polymerization of xylenol, it appears certain that this process also contributes to the coupling of polymer molecules. Redistribution and rearrangement are complementary reactions. Dissociation into aryloxy radicals can occur at any point... 

Although it is not ordinarily possible to separate the two processes, both rearrangement and redistribution undoubtedly occur during the oxidative polymerization of xylenol, with the relative contribution of each to polymer coupling determined by reaction conditions. The possibility that other reactions, such as the endlinking of Reaction 17, also contribute cannot be excluded, but no other reactions are required to explain the experimental observations. 

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... 

Redistribution is a free-radical chain reaction that does not consume oxygen or change the overall degree of polymerization. However, the net result of redistribution between polymeric phenols to form a monomeric phenol or phenoxy radical, followed by coupling of the monomer as in reaction (5) is the same as if two polymer molecules combined in a single step. 

Exactly the same result was obtained when the homopolymers were oxidized at — 25°C with a N,N,N, N -tetraethylethylenediamine-cuprous chloride catalyst, conditions which have been reported to cause coupling of DMP homopolymers solely by rearrangement (14). The NMR spectrum of this polymer is shown in Figure 3, together with the spectra of a mixture of homopolymers and of a random copolymer formed by simultaneous oxidation of the monomers. Apparently, dissociation and redistribution occur often enough to determine the structure of the product in this system, even under conditions that favor coupling of polymer molecules by the rearrangement mechanism. 

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 structure of copolymers produced by oxidative coupling is determined largely by the rate and other characteristics of the redistribution reaction, as is true of polyesters and other types of polymers which are... 

Sequential Oxidation of DMP and DPP. The usual approach to formation of block copolymers is by the sequential polymerization of two or more monomers or by linking together preformed homopolymer blocks. In view of the importance of the redistribution process in the oxidative coupling of phenols there can be no assurance that successive polymerization of two phenols will yield block copolymers under any conditions. It is certain, however, that block copolymers can be formed only if the conditions are such that polymerization of the second monomer is much faster than redistribution of the added monomer with the polymer previously formed from the first. The extent of redistribution is followed conveniently by noting the effect of added monomer on solution viscosity, as indicated by the efflux time from a calibrated pipet. 

B. Redistributive Coupling of Bis(silyl)phenylenes to Hyperbranched Polymers. 159... 

Several comprehensive reviews on the preparation, structure, properties, photochemistry, and redistribution of polysilanes prepared by the Wurtz coupling method have been published.1516 31 33 There are also several excellent reviews on the transition-metal-complex-catalyzed dehydrocoupling of hydrosilanes to polysilanes.34 36 The present section reviews the more recent work on the dehydropolymerization of hydrosilanes to polysilanes, catalyzed by transition-metal complexes under homogeneous conditions. Extensions of the methodology to the dehydropolymerization of hydrogermanes, hydrostannanes, and hydrophosphanes to the corresponding polymers are also reviewed. The literature is covered up to early 1997. 

Since the driving force T and the frictional force F are not in line, they form a clockwise couple, which has to be balanced by an anticlockwise couple. The other two forces acting arc the weight W, providing the normal load, and the reaction R. They can provide an anticlockwise couple only if surface A tilts so that the reaction force acts forward of the weight W. When surface A tilts, the reaction takes place clo.se to the forward edge of surface A. (If the surface A is a soft polymer, then the shift in the po.sition of the reaction force is likely to be taken up by distortion of the polymer, with a consequent redistribution of pressure, rather than by an observable tilt.)... 

A drawback of A and B homogeneous catalytic systems is their broad molecular mass distribution, coupled to a relevant amount of a fraction with a low molar mass. Very interestingly, the clay-immobilized catalysts displayed not only a much higher activity due to a maximum dispersion of active sites within the silicate host and a longer polymerization lifetime, but also an increase of high molar mass fraction polymers because of a decrease of chain transfer rates or a redistribution of the active center populations (Figure 6.13). 

An important feature of the electroactive polymer materials is the presence of two different kinds of mobile charge carriers, electronic and ionic. These polymers constitute mixed conductors. As in the case of ionic transfer phenomena in electrolyte solutions, there are generally two reasons for coupling between fluxes of different charge species. First creation in the concentration gradient of a charged species must be compensated by a redistribution of other components to retain the local electroneutrality of the system. Another factor is an electric field inside the polymer phase whose distribution is adjusted to the fluxes of charged species in a self-consistent way. 

Although reactions of this kind must occur, there are two observations which cannot be explained on this basis. Firstly, the dimer (I) forms a polymer identical with that obtained from 2,6-xylenol since no monomer is present in this case, the above scheme cannot explain polymerization completely. Secondly, when 2,6-xylenol is polymerized, there is a sharp increase in average molecular weight near the end of the reaction this type of behaviour is typical of stepwise reactions in which polymer molecules couple with each other (Section 1.4.5). However, there is no inunediately apparent way by which two polyarylene ether molecules can be coupled. There is experimental evidence (see Reference 8 for an account of this work) that coupling takes place through two processes, namely redistribution and rearrangement with the relative contribution of each depending on reaction conditions. 

Charge redistribution within the polymer sample modulates its refractive index. A future application of photorefractivity in optical computing is demonstrated by a two-beam coupling experiment. When a poled polymer film is irradiated with two incident laser beams crossing each other in the polymer, energy transfer between the two beams occurs that is, one beam loses power, the other gains an equal amount of power. 

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