Polymerization of Primary Aromatic Diamines to Azopolymers by

44 Polymerization of Primary Aromatic

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Diamines to Azopolymers by Oxidative Coupling HARTWIG C. BACH Monsanto Co., Textiles Division, Technical Center, P. O. Box 1507, Pensacola, Fla. 32502 W. BRUCE BLACK Chemstrand Research Center, Inc., Durham,N.C.

Catalyzed oxidative coupling of primary aromatic diamines yields linear polymers containing the oxidatively formed aromatic azo linkage. The generally applicable coupling reaction is conducted under mild conditions with oxygen (air) as the oxidant and a cupric ion/nitrogen base complex as the homogeneous catalyst. Tertiary amines such as pyri dine as well as Ν,Ν-disubstituted amides such as dimethyl acetamide or hexamethyl phosphortriamide serve as activat ing catalyst ligands. Fully aromatic azopolymers exhibit good thermal stability to temperatures above 300°C.; they are colored and highly crystalline. A fiber of one of the compositions was spun. It showed good tensile properties (tenacity, 4.4 grams/denier; elongation, 9.0%; tensile modu lus, 93 grams/denier) which were retained to a compara tively large degree at elevated temperatures and under the degrading influence of light.

"p\uring the last decade, catalyzed oxidative coupling has gained widespread interest as a novel and general polymerization method. Various polymer systems of scientific and commercial interest have been prepared by this novel technique. Such polymer systems include: poly phenylene oxides) (9, 13, 25), of which poly (2,6-dimethylphenylene oxide) (PPO) is produced commercially; polyphenylenes (21); poly679 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

680

ADDITION A N D CONDENSATION POLYMERIZATION

PROCESSES

acetylenes (10, 11, 17, 18); polydisulfides (12), and polyazoaromatics (1, 2, 3, 19, 20, 30). The role of oxidative coupling in the biosynthesis of lignin (8), a naturally occurring polymer, has been recognized. The reactions leading to the formation of these polymers—except polyphenylene—have one feature in common, although they otherwise differ greatly in mechanism: the crucial step in the reaction sequence is a one-electron transfer from the monomer to a transition metal ion serv ing as an electron acceptor. In addition to being an electron acceptor the transition metal ion is probably also involved in the coupling reaction by complexation of radical-like intermediates produced. Most of the polymer-forming oxidative coupling reactions known are catalytic processes by virtue of the reoxidation of the transition metal ion with an oxidant, preferably oxygen (air). Cupric-cuprous complexes serve most prominently as catalysts (1, 2, 3, 9, 10, 11, 12, 17, 18, 19, 20, 30); manganese (24) and cobalt (6) complexes have also been used. In the last few years, the catalyzed oxidative coupling of primary aromatic diamines has been investigated extensively in our laboratories (1,2, 3) and also by Kotlyarevskii (19, 20, 30). Results of these investi gations are reviewed in this chapter as well as the properties of the class of aromatic azopolymers. Experimental Monomers not commercially available were prepared by known methods: 4,4'-bis(p-aminophenyl)-2,2'-bithiazole (22), 2,5-bis (p-amino phenyl)oxadiazole (22), N,N -bis(p-aminophenyl)isophthalamide (27), 2V,N'-m-phenylenebis(m-aminobenzamide) (23), and 1,4-bis(p-aminophenyl)butadiene (16). Cuprous chloride was obtained by ascorbic acid reduction of C u C l according to Stathis (26). All polymerizations were conducted in a stirred, closed system, with oxygen at atmospheric pressure as the gas phase. Oxygen absorption was measured by an oxygen buret, which formed an integral part of the system. Depending on the reactivity of the diamine used, the reactions were run at room temperature or at slightly elevated temperatures (up to 9 0 ° C . ) in pyridine, dimethyl acetamide, hexamethyl phosphortriamide, or mixtures of these. In a typical example 0.5 gram (0.0025 mole) of C u C l was oxidized with oxygen in 50 ml. pyridine. After adding 3.46 grams (0.01 mole) of 4,4'-bis(p-aminophenyl)isophthalamide, the stirred reaction mixture ab sorbed 255 ml. of 0 in 195 minutes at 25°C. (theory: 244 ml. of 0 at 2 5 ° C ) . The polymer was isolated in quantitative yield by coagulation in water. Inherent viscosities were determined at 30°C. with solutions of 0.5 gram polymer in 100 ml. of concentrated H S 0 , dimethyl acetamidepyridine, or dimethyl acetamide. /

2

2

2

2

2

2

4

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

44.

B A C H AND B L A C K

681

Oxidative Coupling

Results and Discussion Oxidative Coupling Reaction. Catalyzed oxidative coupling of pri mary aromatic diamines with oxygen as the oxidant yields linear poly mers according to the following scheme: H N—Ar—NH 2

2

2

2

+°2, -2 Η Θ ^ [catalyst] 2

[ L

_

A

r

_

N

=

=

N

_

]

J

Ar: aromatic, heterocyclic, aromatic-heterocyclic, aromatic-aliphatic bi valent radical In studying this novel oxidative polymerization we investigated the following points in particular: (a) scope of the reaction, (b) side reac tions, (c) catalysis, (d) molecular weights obtainable, (e) properties of aromatic azopolymers in bulk and fabricated form. Catalyzed oxidative coupling is generally applicable to simple pri mary aromatic diamines as well as to diamines of a complex structure. The scope of the reaction is exemplified by some of the polymers prepared: —

* - 0 - -

.

-

Ar—N=N

Ό

—

—

Ο

-

. - 0 - - 0 -

-®- ~C^. -Ο"*"-"··-®"· -CÔ" o

0

-{Qj-NHCO-jQ)-

TQr

c0NH

-Q-

5

CONH-Λ-

NHC0

-(Qr

This oxidative polymerization also proved to be applicable to long chain prepolymers of a molecular weight of several thousand having terminal primary aromatic amine groups. For example, the following prepolymer


682


PROCESSES

having an inherent viscosity of 0.28 (30°C., 0.5 gram of polymer in 100 ml. dimethyl acetamide/5% LiCl) was prepared by low temperature solution polycondensation of isophthaloyl chloride with a 10% molar excess of 4,4'-diaminodiphenyl ether. This prepolymer was treated in dimethyl acetamide (DMAc)/pyridine (6:1) solution with oxygen in the presence of the cupric-cuprous redox couple. By rapid low temperature oxidative polymerization the linear, high molecular weight azoblock copolymer

was obtained having an inherent viscosity of 2.8 (30°C., 0.5 gram of polymer in 100 ml. of D M Ac/5% LiCl). A clear, strong and hot-drawable film could be cast from the polymer solution obtained. Similarly, pre dominantly aliphatic prepolymers with terminal aromatic amine end groups were prepared by successive reaction of aliphatic polyester or polyether diols with aromatic diisocyanates and aromatic diamines HO—A—OH + 2 OCN—R—NCO -> OCN—R—NHCOO) A 2

OCN—R—NHCOO) A + 2 H N — A r — N H -> 2

2

2

H N—Ar—NHCONH—R—NHCOO) A 2

A: poly[ethylene(80%)/propylene(20%)] adipate or poly(tetramethylene glycol)

2

MW = MW =

1900 1100

Addition of cuprous chloride to these prepolymers in a DMAc/pyridine solution and subsequent oxygenation led to the formation of high molecular weight polymers from which strong, elastic and hot-drawable


44.

BACK A N D B L A C K

Oxidative Coupling

683

films were obtained by casting of the polymerization dope. The prepolymer solutions cast as controls did not yield coherent films. Side reactions in the oxidative coupling of primary aromatic diamines to azopolymers could severely limit the molecular weights of polymers obtainable and disrupt their ordered, linear structure. To investigate this point the following polymer

was prepared by two independent routes: (a) oxidative coupling of bis(p-aminophenyl)isophthalamide, and (b) solution polymerization of 4,4'-diaminoazobenzene with isophthaloyl chloride. The polymers ob tained were identical in their infrared spectra (1, 2,3) and other proper ties; this is strong evidence that azolinkages are formed exclusively in the oxidative coupling of aromatic diamines. The described oxidative polymerization is truly a catalyzed reaction. p-Phenylenediamine—an easily oxidizable aromatic amine—was sub jected to oxygenation with and without cupric ion/nitrogen base com plex present. Only the oxygenation in the presence of the catalyst yielded polymeric material; oxygen absorption was minimal without the catalyst. In the oxidative coupling of primary aromatic diamines to azopoly mers cupric ion/nitrogen base complexes serve as homogeneous catalysts. Such complexes are best prepared in situ by oxygenating a cuprous species, preferably CuCl or CuoO/HCl, in the presence of the nitrogen base. Cupric salts investigated except for cupric acetate yielded inactive species. The selection of the nitrogen base which also forms part of the solvent system is critically important. A suitable nitrogen base must fulfill the following requirements: (a) It must yield an active catalyst which is soluble in the reaction medium. (b) It must be nonoxidizable by the cupric ion at the reaction con ditions, therefore excluding secondary or primary amines. (c) It must have at least some solubilizing power for the azopoly mers prepared. Pyridine, a tertiary amine, yields a very active catalyst complex, [ P y C u ( O H ) C l ] (7), for the coupling reaction giving high rates of oxidation at low temperatures. However, molecular weights of the azo polymers preparable in this medium proved to be somewhat limited (see Table I) owing to the generally low solubility of the azopolymers formed. 2

n


ADDITION A N D CONDENSATION POLYMERIZATION PROCESSES

684

Use of D M Ac (J, 2, 3)—an excellent solvent for aromatic condensa tion polymers—as a cosolvent gave polymers of considerably higher molecular weight. Some comparative results are given in Table I. Be cause of the greater polymer solubility and the consequently higher molecular weights possible, strong films of several compositions could be prepared directly from the polymerization solution. Table I.

Aromatic Condensation Polymers

Polymer

I

III

-^3~

IV

p

~~

N = N

ÎULn' \ -^f3V

-

Reaction Medium

N H C O

=

/

\=/

l[ J

V

V=/

N =

0.23

DMAc/Py (4:1)

0.41

DMAc/Py (4:1)

1.1 0 3 7

DMAc/Py (1:1)

0.81

s

r

Α-Ζ^Λ^/

N=N

Py DMAc/Py (3:2)

0.64 2.1

DMAc/Py (4:1)

2.0

DMAc/Py (4:1)

0.18

N-

- V=/

_/f~V _/ V VI

y

a

T^VCONH-^~VN=N-

CH=CH)-/7A_ V

p

y

IV

-

-T Ar—NH · Cu (OH)Cl n

2

2

n

This first step would involve the displacement of a pyridine ligand by the aromatic amine. Some evidence points to this being the rate-deter mining step. A strict positive correlation between the basicity of the diamine relative to pyridine and the oxidative coupling rate (as measured by the oxygen absorption) was established (3). (2) Electron Transfer: Ar—NH · C u ( O H ) C l - * A r — N H n

2

2

+

· CuÔHJCl

This step leads to an amine radical-cation or possibly to a complexed amine radical A r — N H · Cu Cl. T

(3) Coupling: 2 Ar—NH

2

+

· CuÔHJCl -> Ar—NHNH—Ar + 2 H Q 2


686

ADDITION A N D CONDENSATION POLYMERIZATION PROCESSES

The intermediately formed hydrazine linkages would be oxidized to azolinkages in a subsequent reaction considerably faster than the oxidation of the amine as shown by Terent'ev (28, 29) for diphenylhydrazines. (4) Catalyst Reoxidation : 4 C ^ C l + 0 + 2 H 0 - » 4 Cu (OH)Cl 2

2

n

Aromatic Azopolymers. Through the described catalyzed oxidative coupling of primary aromatic diamines a great variety of aromatic azo polymers has become easily accessible. Therefore, an investigation of their properties in bulk as well as fabricated form seemed warranted. C O L O R . All aromatic azopolymers are colored owing to the strongly chromophoric azogroup. Even the azoblock copolymer derived from a phenyl oxide-isophthalamide backbone (discussed earlier) which has only one azogroup per repeat unit of a molecular weight of 3 5 0 0 is bright yellow. Naturally, the shade of color of individual polymers depends on the structure of the repeat unit; as expected, fully conjugated polymers essentially black.

C R Y S T A L L I N I T Y . Aromatic azopolymers show a high degree of crystallinity without annealing. X-ray diffraction patterns of three representa tive polymers are shown in Figure 1 . The high degree of crystallinity of as prepared poly(azophenylene oxide) is surprising in view of the fact that other polymers such as PPO ( 1 5 ) and Bakélite polysulfone (14) having a predominance of aromatic ether linkages in the backbone show little or no crystallinity unless they are annealed for several days in a suitable solvent (4). T H E R M A L STABILITY. In contrast to their aliphatic analogs aromatic azocompounds are rather stable thermally. This stability is also charac teristic of azopolymers. Figures 2 and 3 show TGA diagrams of polymers (II, IV, IX) in nitrogen and in air. [The seemingly higher stability of II in air when compared with a nitrogen atmosphere could possibly be explained by a weight-gain arising from oxidation, thus compensating for some of the weight loss caused by expulsion of N .] As shown, the polymers are stable y more than 3 0 0 ° C ; catastrophic weight loss (^ 1 0 % ) begins in the range 3 6 0 ° - 4 2 0 ° C . Rapid thermal degradation is always accompanied by a strong exotherm. This exotherm combined with the narrow range ( 3 6 0 ° - 4 0 0 ° C . ) of its occurrence irrespective of polymer structure indicates that the first step in the degradation is the elimination of the azogroup as molecular nitrogen. 2


44.

BACH AND BLACK

Oxidative Coupling

687

Figure 1. X-ray diffraction patterns of representative polymers Top: Polymer II Middle: Polymer IV Bottom: Polymer VIII

Fully aromatic azopolymers described show no melting or softening up to the temperature of thermal degradation. Therefore, films and fibers must be prepared from solution; films cast are strong, tough and flexible.



688

PROCESSES

100 ι—ι 65 l

J

80

Ζ

ο LU

60

IUJ

oc x

40

S2 Lu

20

100

200

300

400

TEMPERATURE

Figure 2.

500 [°c]

TGA in nitrogen; heating rate: 15°C./min.

100

r-i

80

60

ζ

LLuu

40

OC I

û

20

__l 100

I 200

I 300

I 400

TEMPERATURE

Figure 3.

1— 500

[°C]

TGA in air. Heating rate: 15°C./ min.

F I B E R PROPERTIES. One of the polymers, the poly(isophthalamide) of

4,4'-diaminoazobenzene (IV), was selected for investigation offiberprop erties of aromatic azopolymers. Table III shows standard tensile data of the "as spun" as well as the hot-drawn (1.5X, 350°C.) fiber. As shown, this polymer has tensile properties fairly typical of an aromatic polyamide.


44.

B A C H AND B L A C K

Table III.

689

Oxidative Coupling

Standard Tensile Data of an Azopolymer Fiber As Spun

Tenacity, grams/denier Elongation, % Tensile modulus, grams/denier

Hot-Drawn

2.5 19.6 58

4.4 9.0 93

In view of the good stability of aromatic azopolymers at elevated temperatures, it is not surprising to find a fair retention of tensile prop erties up to about 400°C. (Table IV). Table IV. Tensile Properties at Varying Temperatures Fiber Temperature, °C.

a

Tenacity, grams/denier Elongation, % 1

250

300

350

400

450

1.8 5.7 47

1.4 8.7 37

1.5 6.6 39

1.4 4.8 40

0.6 1.4 47

Fiber held 1 minute àt temperature before test.

Figure 4 shows the retention of tenacity in percent of the original values compared with another aromatic polyamide, M3P, the polyterephthalamide of bis(m-aminobenzoyl)m-phenylenediamine (31).

I

• 300

• 400

500

[•C]

Figure 4. Retention of tenacity of polymer fibers at elevated temperature Fibers of Polymer IV show a comparatively strong resistance to light degradation (Table V ) . These values compare favorably with the


690


Table V .

PROCESSES

Resistance of Fibers of Polymer IV to Light Degradation Standard Fading Hours

0

Tenacity, grams/denier Elongation, % Tensile modulus, grams/denier

0

20

40

80

4.4 9.0 93

3.3 6.8 84

3.3 7.2 86

3.3 7.1 85

"Test according to A.A.T.C.C. Standard Test Method 16-A-1964 (Fade-Ometer, car bon arc).

lOOh 80

ζ

60 h

>-

40h

ο < ζ

20h

20

40

60

80

SFH

Figure 5. Retention of tenacity of pol ymerfibersupon exposure to light (car bon arc) strength retention (4) of a commercial aromatic polyamide as shown in Figure 5. Conclusions As shown in recent investigations, catalyzed oxidative coupling of organic molecules provides a new and general route to a variety of hitherto only difficulty accessible polymer systems. While the literature on oxidative coupling reactions in polymer synthesis is already rather extensive, by further investigations of this reaction concept, particularly studies of the catalysis involved, oxidative coupling promises to rival conventional condensation and addition polymerization in importance.


44.

BACH AND BLACK

Oxidative Coupling

691

Acknowledgment We wish to express our sincere appreciation to J. R. Sechrist for skillful and dedicated assistance, to H . S. Morgan and J. D. Fowler for spinning thefiberreported. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)

Bach, H . C., ACS, Div. Polymer Chem., Polymer Preprints 7, 576 (1966). Ibid., 8, 610 (1967). Bach, H . C., Black, W . B., J. Polymer Sci. Pt. C, in press. Butte, W . Α., Price, C . C., Hughes, R. E., J. Polymer Sci. 61, S 28 (1962). D u Pont Co., New Product Technical Information NP-33 (1963). Finkbeiner, H . L . , "Abstracts of Papers," 155th National Meeting, A C S , April 1968, p. 206. Finkbeiner, H . , Hay, A. S., Blanchard, H . S., Endres, G . F., J. Org. Chem. 31, 549 (1966). Freudenberg, K., Science 148, 595 (1965). Hay, A. S., Blanchard, H . S., Endres, G . F., Eustance, J. W . , J. Am. Chem. Soc. 81, 6335 (1959). Hay, A. S., J. Org. Chem. 25, 1275 (1960). Ibid., 27, 3320 (1962). Hay, A. S., U . S. Patent 3,294,760 (1966). Hunter, W . H . , Olson, A. O., Daniels, Ε . Α., J. Am. Chem. Soc. 38, 1761 (1916). Johnson, R. N . et al., J. Polymer Sci. A-1, 5, 2375 (1967). Karasz, F. E., O'Reilly, J. M . , Polymer Letters 3, 561 (1965). Katz, L . , Hein, D . W . , Pretka, J. E . , Long, R. S., U . S. Patent 2,852,556 (1958). Korshak, V . V . , Sladkov, A. M . , Kudryavtsev, Yu. P., Vysokomolekul. Soedin. 5, 793 (1963). Kotlyarevskii, I. L . , Fisher, L . B., Dulov, Α. Α., Slinkin, Α. Α., Izv. Akad. Nauk SSSR, Otd. Khim. Nauk, 1960, 950. Kotlyarevskii, I. L . , Terpugova, M . P., Andrievskaya, E . K., Izv. Akad. Nauk SSSR, Ser. Khim. 1964 (10), 1854. Kotlyarevskii, I. L . , Svartsberg, M . S., Fisher, L . B., Sanina, A. S., Bardamova, M . Α., Terpugova, M . P., IUPAC Symp., Prague 1965, Preprints,

p. 497. Kovacic, P., Kyriakis, Α., J. Am. Chem. Soc. 85, 454 (1963). Preston, J., J. Heterocycl. Chem. 2, 441 (1965). Preston, J., J. Polymer Sci. A-1, 4, 529 (1966). Shono, T . , Yamanoi, K., Shinra, K., Makromol. Chem. 105, 277 (1967). Staffin, G . D . , Price, C. C., J. Am. Chem. Soc. 82, 3632 (1960). Stathis, E . C., Chem. Ind. (London) 1958, 633. Stephens, C . W., U . S. Patent 3,049,518 (1962). Terent'ev, A. P., Mogilyanskii, Ya. D . , Dokl. Akad. Nauk SSSR, 103, 91 (1955). (29) Terent'ev, A. P., Mogilyanskii, Ya. D . , Zh. Obshch. Khim. 31, 326 (1961). (30) Terpugova, M . P., Kotlyarevskii, I. L . , Andrievskaya, E . K., Izv. Akad. Nauk SSSR, 1966 (4), 713. (31) Weiss, J. O., Morgan, H . S., Lilyquist, M . R., J. Polymer Sci. Pt. C, 19, 29 (1967). (21) (22) (23) (24) (25) (26) (27) (28)

RECEIVED

April 1, 1968.


Polymerization of Primary Aromatic Diamines to Azopolymers by

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