Polyphenylene Oxides by Oxidative Coupling - Advances in

43 Polyphenylene Oxides by Oxidative

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Coupling GLENN D. COOPER and ARTHUR KATCHMAN Plastics Department, General Electric Co., Selkirk, Ν. Y. 12158

The oxidative coupling of 2,6-disubstituted phenols to poly (arylene oxides) is a polycondensation reaction, in which polymer molecules couple with other polymer molecules as well as with monomer. Unstable quinone ketals formed by coupling of a polymeric aryloxy radical at the para position of the phenolic ring of a second radical are believed to be intermediates for the reaction. The ketals may be con verted to polymeric phenols either by a series of intramolecular rearrangements or by disproportionation to aryloxy radicals, leading to a mobile equilibrium between polymer molecules of varying degree of polymerization. Both proc esses have been shown to occur, with their relative impor tance determined by the reaction conditions.

T n 1959 Hay (19) reported that 2,6-xylenol reacts with oxygen in the *·• presence of a pyridine-cuprous chloride catalyst to yield a high mo lecular weight poly(l,4-arylene oxide) (Reaction 1).

Other 2,6-disubstituted phenols react in the same manner, provided the substituents are not too large; oxidation of phenols with bulky ortho substituents, such as 2,6-di-ferf-butylphenol, yields the diphenoquinone as the major product (16) (Reaction 2). 660 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

43.

COOPER AND K A T C H M A N

Polyphenylene Oxides

661

Many amine-copper complexes, as well as a few amine complexes of other metals, and certain metal oxides have since been shown to induce similar reactions (17, 18, 22, 23, 30). This chapter is concerned largely with the mechanism of oxidative polymerization of phenols to linear polyarylene ethers; most of the work reported has dealt with the copperamine catalyzed oxidation of 2,6-xylenol, which is the basis for the com mercial production of the polymer marketed under the trade name PPO, but the principal features of the reaction are common to the oxidative polymerization of other 2,6-disubstituted phenols. Mechanism It has generally been accepted that aryloxy radicals are intermediates in the polymerization, largely because the effective reagents are those capable of one-electron transfer. This assumption has been confirmed recently by the identification of both monomeric and polymeric aryloxy radicals in the ESR spectra of polymerizing solutions of 2,6-xylenol (21). The first step in the reaction is the oxidation of the phenol to the aryloxy radical by Cu(II). Carbon-oxygen coupling of two aryloxy radicals yields the cyclohexadienone, which tautomerizers to the dimer (II) (Reaction 3).

The simplest explanation for the formation of high molecular weight poly mer through oxidative coupling of aryloxy radicals involves the successive addition of monomer units to the radicals derived from polymer phenols (Reaction 4).

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

ADDITION A N D CONDENSATION POLYMERIZATION PROCESSES

662

Ο

H

(4)

OH

Η Jn + 1

Although reactions of this type must occur, there are at least two observa tions which cannot be explained on this basis. Endres and Kwiatek (12) reported that the dimer reacts to yield a polymer identical with that obtained from xylenol; the mixture of low molecular weight oligomers isolated from the reaction mixture at early stages of the polymerization behaves similarly. Since no xylenol is present, and hence there is no apparent source of the monomer radical, Reaction 4 cannot explain the polymerization completely. The variation in polymer molecular weight with conversion is also inconsistent with simple stepwise addition of monomer units. When the degree of polymerization is plotted against fractional polymer yield (Figure 1), a sharp increase is observed near the end of the reaction. This type of behavior is typical of polycondensation reactions in which polymer molecules couple with each other as well as with monomer molecules: Χ™ + Χ + % 0 - * X Λ

2

m + w

+ H 0 2

(5)

Proposed Mechanisms for Coupling of Polymer Molecules In polyesterification and other polycondensation reactions, polymer molecules condense with one another by the same mechanism by which they add monomer units. There is no immediately apparent explanation, however, for the coupling of two polyarylene ether molecules of struc ture I. Three possible routes were first outlined by Finkbeiner (13), and each has received some support since. Endlinking. The most obvious path for Reaction 5 is by the attack of the phenolic oxygen of one polymer molecule at the para position of the terminal ring of another. This cannot be simply a homolytic aromatic


43.

COOPER A N D K A T C H M A N

663


substitution with the polymeric aryloxy radical attacking the terminal ring of a polymeric phenol; if this were the case, aromatic ethers should act as chain stoppers, but no reduction of polymer molecular weight was observed when the reaction was carried out in 2,6-dimethyl anisole or in the methyl ether of the dimer as solvent (13). The coupling, if it occurs, therefore must be between two polymeric aryloxy radicals. The objection to this head-to-tail combination of polymeric aryloxy radicals is that no satisfactory resonance structure can be written which places the unpaired electron in the terminal ring. Bolon (4) prepared the stable radical from 2,6-di-terf-butyl-4-phenoxyphenol and found that there was no detectable spin transmission through even a single diaryl ether linkage.

200

(90

0.6

180

170 30

20

10

FRACTIONAL POLYMER YIELD

Figure 1. Variation of P with conversion during oxidation of 2,6-xylenol n

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 radi cals from which it was formed or two different aryloxy radicals, as shown


664

ADDITION A N D CONDENSATION POLYMERIZATION

PROCESSES

below for two dimer radicals:

or, for any pair of radicals, X n ~~* X m + 1 "Ι" ^ n - 1

( Ό

where the subscripts indicate the number of aromatic rings in each radical. This scheme must include steps allowing the reaction of aryloxy radicals with phenols to generate new aryloxy radicals, either directly or through oxidation-reduction reactions involving the catalyst. ArO* + Ar'OH -> ArOH + Ar'O* ArO' + Cu - » ArO" Cu +

ArO" Cu

2+

(8) (9)

2+

+ ArOH -> ArOH + ArO* + Cu

+

(10)

Reaction 7 does not change the average degree of polymerization of the system. If one of the radicals is a monomer radical, however, coupling according to Reaction 4 can occur, reducing the total number of molecules and increasing the average molecular weight. The net result of the formation of a monomer radical by disproportionation, followed by coupling with a polymer radical, is the same as would be produced by head-to-tail coupling of two polymer radicals, although the only growth step involves the addition of a single monomer unit. X

M

+

X

Xm + l + Xm +

X

n

m

Xn - 1 ^

Xn ^

X»t + n - l +

+

i

+

Χ _χ Λ

Xm +2 +

Xm +n - 1 Xi

Xn-2

Xl

Xm + n


(H)

43.



665

The sequence accounts satisfactorily for the production of polymer from low oligomers and for the shape of the DP-conversion curve in the oxida tion of 2,6-xylenol. Formation of quinone ketals similar to III is, fur thermore, a known reaction of hindered aryloxy radicals (I, 14). The major objection to this proposal is the great number of steps required to produce a monomer radical from two polymeric radicals of a high degree of polymerization. Quinone Ketal Rearrangement. This mechanism likewise assumed the formation of quinone ketals as intermediates. Examination of models of III shows that the carbonyl oxygen of the ketal is within bonding distance of the para position of the next succeeding ring. Formation of a new carbon-oxygen bond, accompanied by the breaking of one of the ether linkages, leads to a new quinone ketal, in which the second ring has become the cyclohexadienone ring (Reaction 12).

A second rearrangement can place the third ring at the head of the sequence; if rearrangement proceeds until one of the terminal rings becomes the cyclohexadienone ring, enolization yields a polymeric phenol identical in every way, except for the order of succession of the rings, with that which would be produced by the direct head-to-tail coupling of aryloxy radicals (Reaction 13). The formation of a polymeric phenol Ο

OH


666


QUINONE KETAL REARRANGEMENT MECHANISM

W v V

+V v V

— - ν ν \ ν τ : Α Λ

VWAW\

\WvWvT

(

" VWAW

0 H

Figure 2. Coupling and rearrangement of polymeric aryloxy radicals by coupling and rearrangement of two polymeric aryloxy radicals is illustrated schematically in Figure 2. When Finkbeinerfirstproposed the ketal rearrangement mechanism, this type of reaction was unknown, but similar rearrangements of cyclohexadienones have been demonstrated since. Miller (25) has shown that the rearrangement of 4-anilinocyclohexadienones to diaryl ethers is an intramolecular reaction, with a geometry almost identical to that of Reaction 12.


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NH

667

2

An even closer parallel is found in the work of Kreilick (33, 34), who prepared the ketal derived from 2,6-di-feri-butyl-4-acetoxyphenol. From a study of the effect of temperature on line broadening in the NMR spectrum of this compound he concluded that rearrangement occurs both by dissociation to radicals and recombination and by a direct intramo lecular process. The former corresponds to the key reaction of the redistribution mechanism, while the latter is entirely analogous to the ketal rearrangement mechanism (Reaction 15).

Studies of Polymer Coupling Mechanism The principal evidence in favor of Mechanism 1—endlinking of polymer radicals—is provided by Price's observation that when 2,6xylenol labeled in the 4-position with tritium was oxidized, 23% of the



668

label was retained in the polymer (7). From this it was argued that polymer radicals coupled to give a ' phenonium ion."

Proton migration before tautomerization would cause some of the label to be retained. However, migration could occur equally well during the addition of monomer units, a step common to all the proposed mecha nisms (23). It must be concluded that this mechanism, although attrac tive because of its simplicity, lacks experimental verification. Much of the evidence for the mechanism of polymer coupling rests on identification of the intermediate products of oxidation of low

Figure 3.

Composition of product from oxidation of 2,6xylenol dimer


43.


669


oligomers. If either endlinking or ketal rearrangement operated exclu sively, oxidation of the dimer would yield only tetramers, hexamers, and other species with an even number of aromatic rings. The products actually observed in this oxidation are shown in Figure 3 (10). The initial products are largely monomer and trimer; as the reaction proceeds, the concentration of monomer increases until at 50% completion mono mer is by far the most abundant single species present. Oxidation of trimer yields a mixture of monomer, dimer, trimer, and other low oligomers; monomer has been detected among the initial prod ucts of oxidation of a low molecular weight polymer fraction consisting largely of oligomers having six to nine rings. The formation of monomer, followed by its eventual disappearance, is strong evidence that the redistribution sequence outlined in Reaction 11 is a major route to polymer-polymer coupling in these instances. The importance of the redistribution processes is confirmed by the observation that adding monomer or a low oligomer to a polymerizing solution of 2,6-xylenol causes a rapid reduction in solution viscosity, as the degree of polymeri zation of the polymer is reduced by redistribution with the monomer (Figure 4) (8).

10

20

30 40 TIME (MINUTES)

50

60

70

Figure 4. Effect of adding monomer to polymerizing solution of 2,6-xylenol


670


PROCESSES

More information is available by examining the products of oxidation of "mixed dimers'—i.e., dimers having different substitution patterns in the two rings. Cooper (9) showed that the initial products from oxida tion of 2,6-dimethyl-4-phenoxyphenol (V) consisted largely of phenol and two trimers (Reaction 17). OH

OH

(VI)

(VII)

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). Similar results were obtained by Cooper and by Mijs et al. (24) from a number of other substituted dimers. With lightly substituted dimers, however, Mijs observed that at low temperatures the initial products consist almost entirely of tetramers; the tetramer, moreover, is that corresponding to the quinone ketal rearrangement, rather than to head-to-tail coupling (Reaction 20). Recently, White (32) observed that under certain conditions (—15°C., large excess of amine base) oxidation of the dimer derived from 2,6-xylenol with a limited amount of preoxidized copper complex yielded the tetramer as the major product, with small amounts of hexamer and only traces of monomer and trimer. The two rings are indistinguishable in this case, but there is little doubt, in view of the results of Mijs, that


43.


671


Ο

the tetramer was formed by rearrangement of the quinone ketal rather than by endlinking. The experiments cited above show that redistribution, presumably via a quinone ketal intermediate, occurs during the oxidative polymeriza tion 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


672


PROCESSES

OH OH

Ο Ο

ox (20) Ο

Ο

during the rearrangement so that redistribution need not occur solely by transfer of a single unit as in Reaction 7; rearrangement followed by dissociation provides a path by which any number of monomer units may be transferred in what is essentially a single step: (21) 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. 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 possi bility 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. The experiments discussed above deal with the copper-amine cata lyzed oxidation of 2,6-xylenol, but oxidation of xylenol by metal oxides takes place by the same mechanism (22). The oxidative coupling of other 2,6-disubstituted phenols has the same characteristics as the oxida-


43.



673

tion of xylenol (low oligomers may be oxidized to linear polymer, a growth curve typical of a polycondensation process). Redistribution has been shown to accompany polymerization in all cases studied, so that it appears that all the oxidative polymerizations leading to linear poly (1,4arylene oxides) proceed by essentially the same mechanism. Other Condensations Leading to Poly (arylene oxides) The term "oxidative polymerization" has been used above to describe the reaction in which the substituted phenol reacts with oxygen or another oxidizing agent to eliminate a molecule of hydrogen (usually appearing as water in the product) and form a poly (arylene oxide). Another route to linear poly(l,4-arylene) oxides is by the oxidative elimination of halide ion from certain 4-halophenoxides. This reaction was studied over a number of years by Hunter (20), who obtained low molecular weight, probably branched, polymers from a number of polyhalophenols. Re cently, high molecular weight linear polymers have been obtained by this route. Two examples are of particular interest. Price (28) found that in a two-phase system consisting of aqueous base and a suitable polymer solvent 2,6-dimethyl-4-bromophenol reacted with traces of ferricyanide, benzoyl peroxide, or other initiators to form poly(2,6-dimethyl-l,4-phenylene oxide), identical except for the end groups, with that produced by oxidative polymerization of xylenol (Reaction 22).

(VIII) In a two-phase system similar to that used by Price, Stamatoff (29) obtained from 2,6-dichloro-4-bromophenol a branched polymer having approximately the statistical ratio of ortho and para ether linkages. When the reaction was carried out using the anhydrous salt of the phenol in the presence of highly polar aprotic solvents, such as dimethyl sulfoxide, the product was the linear poly(2,6-dichlorophenylene oxide) (Reaction 23). These condensations, like the oxidative coupling of phenols, pre sumably are free radical chain reactions with aryloxy radicals as inter mediates, but the gross features of the two types of reactions are quite different. At low extents of oxidation the oxidative coupling reaction


674


PROCESSES

yields only low oligomers, as expected of a polycondensation. Price and Shu, however, showed that when the oxidative dehalogenation reaction was carried out under nitrogen, an increase in initiator caused an increase in polymer yield but had little effect on molecular weight; high polymer was obtained even at low conversions (27). These are characteristics of a process involving stepwise addition of monomer units, as in radicalinitiated vinyl polymerization. Price proposed a mechanism in which the propagation step involves attack of the polymeric aryloxy radical on the monomer anion, displacing bromide ion and generating a new aryloxy radical; termination is by coupling (possibly ortho) of two radicals. ArO" -> ArO*

Initiation (24)

Ο + Br" Propagation

2ArO* - » (ArO)

2

(25)

Termination (26)

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-4bromophenol (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, η = 2) is polymerized rap idly by the same initiators which are effective with the monomer. Obvi ously, polymer growth does not occur solely by addition of monomer units in either Reaction 22 or 23; some process leading to polymer-poly mer 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


43.



120 r—

675

—11-2

20

30

40

50

TIME (MINUTES)

POLYMERIZATION

OF

ONa

Br ^ C l

Figure 5.

Yield and intrinsic viscosity of polymer in the peroxide-initiated condensation of sodium 2fi-dichloro-4-bromophenolate

the polymerization conditions in both Reaction 9 and 15, but it has not been established that it is important to the polymer growth reaction (8,15).

Consequences of Redistribution Redistribution reactions are common among condensation polymers having in the recurring unit a functional group capable of reacting with other groups or with the end groups of the polymer molecule, but it is surprising to find these reactions in the poly (arylene oxides), which have as the only functional group in the recurring unit the diaryl ether linkage, ordinarily an extremely unreactive group. The mechanistic studies cited above demonstrate, however, that redistribution of polymers of Structure I occurs with great ease.


676


PROCESSES

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-l,4-phenylene oxide) to a mixture of monomer, dimer, trimer, and other low oligomers; the compo sition 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).

ArOH + H

This procedure has been used by White (31) to prepare substituted diaryl ethers, many of them not readily available by other routes. 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):

At high temperatures redistribution of poly(arylene oxides) may occur in the absence of added initiators. This sometimes causes changes in molecular weight distribution. If the distribution is initially very broad, it is narrowed by heating; the opposite effect, broadening of an initially narrow distribution, should also be possible, but has not yet been ob served. Figure 6 shows the effect of heating at 275°C. for 3 hours on the distribution of an artificial blend made by combining two polymers of widely different molecular weight. Redistribution is obviously exten sive, although not complete, probably because of the presence of some polymer molecules not having the perfectly regular structure of I. No


43.


Polyphenylene Oxides POLYMER BLEND

ACETYLATED AND HEATED

Figure 6. Gel permeation chromatograms showing redistri bution of poly(2,6-dimethyl-l,4-phenylene) oxide further changes in distribution were observed on continued heating. This thermal redistribution, like the other types mentioned above, is eliminated completely when the terminal hydroxyl groups are converted to ester or ether groups. Facile redistribution requires: (1) 2,6-disubstitution, (2) 1,4-arylene ether linkages, and (3) a free terminal hydroxyl group (8). Literature Cited (1) Becker, H. D., J. Org. Chem. 29, 3068 (1964). (2) Blanchard, H. S., Cooper, G. D., unpublished work. (3) Blanchard, H. S., Finkbeiner, H. L., Endres, G. F., SPE Trans. 2, 110 (1962). (4) Bolon, D. Α., J. Am. Chem. Soc. 88, 3148 (1966). (5) Bolon, D. Α., private communication. (6) Bolon, D. Α., Gilbert, A. R., Polymer Letters 5, 277 (1967). (7) Butte, W. Α., Price, C.C.,J. Am. Chem. Soc. 84, 3567 (1962).


678

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES

(8) Cooper, G. D., Natl. Conf. Redistribution Reactions, Ν.Y. Acad. Sci., June 1967. (9) Cooper, G. D., "Abstracts of Papers," Western Regional Meeting, ACS, December 1965. (10) Cooper, G. D., Blanchard, H. S., Endres, G. F., Finkbeiner, H. L.,J.Am. Chem. Soc. 87, 3996 (1965). (11) Cooper, G. D., Gilbert, A. R., Finkbeiner, H. L., ACS, Div. Polymer Chem. Preprints 7, 166 (1966). (12) Endres, G. F., Kwiatek, J., J. Polymer Sci. 58, 593 (1962). (13) Finkbeiner, H. L., Endres, G. F., Blanchard, H. S., Eustance, J. W., SPE Trans. 2, 110 (1962). (14) Forrester, A. R., Hay, J. M., Thomson, R. H., "Organic Chemistry of Stable Free Radicals," p. 281, Academic Press, New York, 1968. (15) Hamilton, S. B., Blanchard, H. S., J. Org. Chem., in press. (16) Hay, A. S., J. Polymer Sci. 58, 581 (1962). (17) Hay, A. S., U. S. Patent 3,306,875 (1967). (18) Hay, A. S., Advan. Polymer Sci. 4, 496 (1967). (19) Hay, A. S., Blanchard, H. S., Endres, G. F., Eustance, J. W., J. Am. Chem. Soc. 81, 6335 (1959). (20) Hunter, W. H., Dahlen, M. Α., J. Am. Chem. Soc. 84, 2459 (1932). (21) Huysmans, W. G. B., Waters, W. Α.,J.Chem. Soc. (B) 1967, 1163. (22) Lidgren, B. O., Acta Chem. Scand. 14, 1203, 2089 (1962). (23) McNelis, E., J. Org. Chem. 31, 1255 (1966). (24) Mijs, W. J., von Lohuisen, O. E., Bussink, J., Vollbracht, L., Tetrahedron 23, 2253 (1967). (25) Miller, B., J. Am. Chem. Soc. 86, 1127 (1964). (26) Muller, E., Ley, K., Ber. 87, 922 (1954). (27) Price, C. C., Shu, N. S., J. Polymer Sci. 61, 135 (1962). (28) Staffin, G. D., Price, C. C., J. Am. Chem. Soc. 82, 3632 (1960). (29) Stamatoff, G. S., U. S. Patent 3,257,358 (1966). (30) Van Dort, H. M., de Jonge, C. R. H. I., Mijs, W. J., J. Polymer Sci., Pt. C 22, 431 (1968). (31) White, D. M., J. Org. Chem. 34, 297 (1969). (32) White, D. M., "Abstracts of Papers," 155th Meeting, ACS, April 1968, T103. (33) Williams, D. J., Kreilick, R., J. Am. Chem. Soc. 89, 3408 (1967). (34) Ibid., 90, 2775 (1968). RECEIVED

March 26, 1968.


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