Phenols, redistribution

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

In equation 7, ttimer radical (4) is produced when (3) dissociates. Whenever (4) couples with the other product of equation 7, ie, the 2,6-dimethylphenoxy radical, the tetramer is produced as described. These redistribution reactions of oligomers that proceed by ketal formation and subsequent dissociation ultimately generate terminal quinol ethers which enolize to the more stable terminal phenol (eq. 8). 

A second process that occurs concurrently with the dissociation— redistribution process is an intermolecular rearrangement by which cyclohexadienone groups move along a polymer chain. The reaction maybe represented as two electrocycHc reactions analogous to a double Fries rearrangement. When the cyclohexadienone reaches a terminal position, the intermediate is the same as in equation 8, and enolization converts it to the phenol (eq. 9). 

It has also been shown that xylenol dimer (XVII) can be redistributed with other phenols in the presence of initiators (6, 63). Bolon (6) has arranged a number of phenols into a relative reactivities scale (Table 12) by reacting xylenol dimer with two different phenols and measuring the relative amounts of the two new dimers (XXIII, XXIV). 

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. 

Aryloxy radicals react with phenolic species, either by direct hydrogen transfer or by oxidation-reduction reactions with the catalyst as carrier, to form new aryloxy radicals, which continue the redistribution. 

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. 

Spontaneous Redistribution of DMP and MPP. The stretching frequency of the phenolic hydroxyl group in DMP homopolymer occurs at 3601 cm-1 and that of MPP homopolymer at 3552 cm-1, allowing the two different head groups to be distinguished. In the 1 1 DMP-MPP copolymers, 80 to 90% of the phenolic hydroxyls are of the MPP type. This can be explained, at least qualitatively, by the greater reactivity of... 

The First plastic sabots were made of glass-fiber filled diallylphthalate sheathed in nylon and they included metal reinforcements whenever it was felt necessary to redistribute the stresses. The nylon sheath was necessitated by the abrasive nature of glass-filled materials. Nylon also is used for rotating bands on projectiles and on metal sabots. Other plastics used for the structural portions of sabots include poly propylenes, polycarbonates, celluloses, epoxies and phenolics. Polyethylene, neoprene, and silicone rubbers are used for seals and obturators... 

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. 

Thus, the redistribution reaction does not change the degree of polymerization, does not consume oxygen other than that required for the initiation step, and can be observed independently of polymerization under suitable conditions (6) redistribution of high polymer with a monomeric phenol has been developed as a synthetic method for preparing substituted aryl ethers (18). 

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... 

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. 

The polymer redistribution reaction with functionalized dendrimers has been investigated by Van Aert et al. for the case of the transetherification of poly-(2,6-dimethyl-1,4-phenylene ether) (PPE) by means of phenols attached to dendrimers [143]. The number average molecular weight of the arms is controlled by the ratio of moles of PE units and the moles of added phenol. The phenols have been attached to poly(propylene imine) dendrimers by means of a tert-butyloxycarbonyl tyrosine(Scheme 19a). The redistribution rate is slow but can be increased by adding CuCl / 4-dimethylaminopyridine catalyst. Oxygen-free... 

Quinone Ketal Redistribution. This mechanism suggests that in the coupling of two aryloxy radicals the oxygen atom of one attacks at the para position of the phenolic ring of the second to yield the unstable quinone ketal. This rapidly decomposes either to yield the aryloxy radicals from which it was formed or two different aryloxy radicals, as shown... 

Trimer VI and phenol are the products predicted from the first step of the redistribution reaction trimer VII would be expected from coupling of some of the phenol with the starting dimer (Reactions 18 and 19). 

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). 

In the preparation of a 2,3,4,4,5,6-hexamethylcyclohexa-2,5-dien-l-one [304] enriched with in the carbonyl group, Isaev et at. (1969) methylated (l-i C)phenol with (CH3)20-BF, at 130°C and found a partial redistribution of the label to other positions in the aromatic ring. Prolonged heating... 

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