Redistribution monomer-polymer

Redistribution and Polymer Structure. The structure of DMP-DPP copolymers is probably determined by the relative rates of the polymerization reaction and the monomer-polymer redistribution reaction. In the DMP-DPP system, structure may be predicted simply by observing the effect on solution viscosity of the addition of one of the monomers to the growing polymer derived from the other monomer. When DPP is added to a DMP reaction mixture, the solution viscosity drops immediately almost to the level of the solvent, as redistribution converts the polymer already formed to a mixture of low oligomers ... 

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. 

A major factor in the interaction of the two phenols during oxidation, making the dimethylphenol appear less reactive and diphenylphenol more reactive than expected, must be the monomer-polymer redistribution reaction. Redistribution of diphenylphenol with the low oligomers... 

Because the onset of monomer-polymer equilibrium can occur before the filaments achieve their own equilibrium concentration behavior, these filaments will undergo polymer length redistribution. This is a slow process in vitro that in many respects resembles crystallization (See Ostwald Ripening). 

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). 

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. 

Polymer-Polymer Redistribution. The redistribution reaction causes no change in the over-all degree of polymerization of the system. If the monomer formed by redistribution is removed continuously, however, the molecular weight of remaining polymer necessarily increases. High polymer has been prepared in this way from the dimer II, xylenol being removed either by distillation or by extraction with alkali (3, 9) ... 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

Continued oxidation of this mixture should yield a random copolymer, which was the result when method b was used. When DMP was added to growing DPP polymer, the solution viscosity did not drop but continued to increase at a faster rate than before the reactive DMP monomer was added. If redistribution between DMP and DPP homopolymer occurred, the rate was small compared with the rate of polymer growth. This at least allows the possibility of producing block copolymers, the result obtained when procedure c was followed. The effect of varia-... 

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. 

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. 

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. 

Besides, the disproportionation reaction between amide groups and amide anions represents a redistribution, too, if linear amidic groups of the polymer are concerned (scheme (g)) in this case the number of particles remains constant, whereas when the disproportionation proceeds between a monomer molecule (or its anion) and an anionic group (or resp. amide group) inside the polymer chain the number of macromolecules is increased and the viscosity is decreased... 

Oxidation of mixtures of 2,6-disubstituted phenols leads to linear poly(arylene oxides). Random copolymers are obtained by oxidizing mixtures of phenols. Block copolymers can be obtained only when redistribution of the first polymer by the second monomer is slower than polymerization of the second monomer. Oxidation of a mixture of 2,6-di-methylphenol (DM ) and 2fi-diphenylphenol (DPP) yields a random copolymer. Oxidation of DPP in the presence of preformed blocks of polymer from DMP produces either a random copolymer or a mixture of DMP homopolymer and extensively randomized copolymer. Oxidation of DMP in the presence of polymer from DPP yields the block copolymer. Polymer structure is determined by a combination of differential scanning calorimetry, selective precipitation from methylene chloride, and NMR spectroscopy. 

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. 

These redistribution reactions of polymer molecules with other polymer molecules as well as with monomer, continue throughout the polymerization and should result in randomization of the polymer. Inasmuch as dimethylphenol is among the most reactive and diphenylphenol the least reactive of the phenols which have been oxidized successfully to linear high polymers, it appears likely that oxidation of any mixture of phenols will yield random copolymers. 

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. 

White has obtained evidence for this process by examining the products of redistribution of monomer with high polymer. At low temperatures the products first formed did not consist only of dimer, as would be expected if redistribution occurred solely by Reaction 7 trimer, tetramer, and higher oligomers were initially present in more than their equilibrium ratio to dimer, indicating that several rearrangement reactions preceded the dissociation of the ketal. 

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 of Monomer with Polymer. Cooper et al. (11) showed that traces of oxidizing agents converted a mixture of equal weights of 2,6-xylenol and poly (2,6-dimethyl-1,4-phenylene oxide) to a mixture of monomer, dimer, trimer, and other low oligomers the composition was identical with that obtained from pure dimer under the same conditions. Phenols other than xylenol may be used, yielding a mixture of low oligomers having the terminal unit derived from the added phenol and all others from the polymer (Reaction 26). 

It is a well-established characteristic of polymerization reactions that a monomer changes to a polymer via an activated intermediate.3 Chemical activation of a monomer requires (1) a redistribution of electron densities in the bonds of the monomeric molecule (the intramolecular effect) and/or (2) a change of properties of existing (loose) intermolecular bonds between the reacting sites (intermolecular effect an alteration of mutual orientation may suffice). For practical purposes,... 

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