JOURNAL OF POLYMER SCIENCE Polymer Chemistry Edition VOL. 15,2739-2747 (1977)

Preparation and Properties of Polyamides and Polyimides Containing Phenoxathiin Units

MITSURU UEDA, TATSUO AIZAWA, and YOSHIO IMAI, Department of Polymer Chemistry, Faculty of Engineering, Yamagata University,

Yonezawa, Yamagata 992, Japan

Synopsis

Novel polyamides and polyimides having phenoxathiin units have been prepared. Polyamides with inherent viscosities in the range of 0.5-2.9 were readily prepared by the polycondensations of phenoxathiin diamines with aromatic diacyl chlorides and of aromatic diamines with new phenox- athiin diacyl chlorides. The polyimides were synthesized from phenoxathiin diamines and py- romellitic dianhydride by using a two-step procedure. The polyamic acids which formed in the first step had inherent viscosities ranging from 1.0 to 1.6, and they were converted to the polyimides by thermal cyclodehydration. Some of the phenoxanthiin-containing polyamides were highly soluble in polar amide solvents and dimethyl sulfoxide. A series of novel polymers containing phenoxathiin units were much more thermostable than the corresponding polymers having open-chain diphenyl ether linkages.

INTRODUCTION

A wide variety of polymers resistant to high temperatures, in which aromatic and heterocyclic rings are linked together in chains1,2 have been synthesized in the past decade. In most cases, the connecting groups are less thermally stable than the rings, and the stability of the polymers is largely determined by the nature of these groups. It is also recognized that thermal stability of the poly- mers increased markedly with decreasing number of single connecting groups in the polymer backbone, in other word, on introducing double-strand hetero- cycles.

In view of this stabilizing effect on incorporation of heterocycles, it was con- sidered of interest to prepare aromatic polyamides and polyimides with phe- noxathiin rings in place of open-chain diphenyl ether linkages. The objective of the present work was to synthesize a series of novel polymers from 2,8-diam- inophenoxathiin, 2,8-phenoxathiindicarbonyl chloride, and their analogs, and to examine the effect of introducing such heterocyclic units on the resulting polymer properties. Recently, phenoxathiin polyimides have been disclosed in a German patent.3

2739

0 1977 by John Wiley & Sons, Inc.

2740 UEDA, AIZAWA, AND IMAI

EXPERIMENTAL

Materials

2,s-Diaminophenoxathiin (DAP). Phenoxathiin was prepared by a known procedure4 in 78% yield by reacting diphenyl ether with sulfur in the presence of aluminum chloride; mp 5143°C (lit.4 mp 57.5-58OC).

2,8-diaminophenoxathiin was synthesized according to the method of Nobis et aL5 in three steps starting from phenoxathiin. 2,8-Diacetylphenoxathiin (I) was prepared in 59% yield by the reaction of phenoxathiin with acetyl chloride in the presence of aluminum chloride in carbon disulfide; mp 181-183°C (lit.5 mp 184-186°C). 2,8-Diacetylphenoxathiin dioxime was obtained in 60% yield by treating the

diacetyl compound with hydroxylamine in refluxing ethanol; mp 223-224°C (lit.5 mp 220-221°C).

2,8-Diaminophenoxathiin was synthesized in 45% yield by the reaction of the dioxime with phosphorus pentachloride in benzene. The crude product was purified by recrystallization from aqueous methanol to give pale brown needles, mp 173-174°C (lit.5 mp 171-173°C). The infrared spectrum (KBr) showed absorptions at 3400 and 3300 cm-l (VN-H), and 1220 cm-l (vc-0-c).

2,s-Diaminophenoxathiin 10,lO-Dioxide (DAPD). This compound was synthesized by the method of Nobis et al.5 in two steps from phenoxathiin. 2,8-Dinitrophenoxathiin 10,lO-dioxide was prepared in 38% yield by the nitration and simultaneous oxidation of phenoxathiin with fuming nitric acid and con- centrated sulfuric acid in glacial acetic acid; mp 261-263°C (lit.5 mp 283- 286°C).

2,8-Diaminophenoxathiin 10,lO-dioxide was obtained in 51% yield by the re- duction of the dinitro compound with zinc dust and concentrated hydrochloric acid in refluxing ethanol. The crude product was recrystallized from ethanol to afford pale brown plates, mp 249-251°C (lit.5 244-247.5"C). The infrared spectrum (KBr) showed absorptions at 3400 and 3300 cm-' ( ~ N - H ) , 1270 and 1140 cm-l (vso2), and 1220 cm-' (vc-o-c). 2,s-Phenoxathiindicarboxylic Acid (11). 2,8-Diacetylphenoxathiin (I) (1.4

g, 5 mmole) was dissolved in 40 ml of pyridine at 50°C. To the solution was added 42.6 ml(28 mmole) of 5% aqueous sodium hypochlorite, dropwise and with stirring at 40-50°C over a 1-hr period. When the addition of the hypochlorite was complete, the reaction mixture was allowed to stir a t room temperature for an additional 2 hr. At the end of this time, the excess hypochlorite was decom- posed by adding 0.5 g of sodium bisulfite. After filtration of the mixture, the filtrate was acidified with excess hydrochloric acid to form a precipitate. Pu- rification was accomplished by repeated dissolution of the solid with aqueous sodium bicarbonate followed by precipitation with the acid. The yield was 1.4 g (52%). The product was recrystallized from dimethylformamide and water to give pale yellow, small crystals, mp 410°C (by DTA). The infrared spectrum (KBr) exhibited bands at 1690 cm-l (YC=O) and 1260 cm-I (vc-0-c).

ANAL. Calcd for C14H805S: C, 58.33%; H, 2.80%. Found: C, 58.6%; H, 3.3%.

2,s-Phenoxathiindicarboxylic Acid 10,lO-Dioxide (111). In a flask was placed 680 ml(0.45 mole) of 5% aqueous sodium hypochlorite, which was then heated to 85°C. To this was added with stirring 7.5 g (0.026 mole) of 2,8-dia-

cetylphenoxathiin in small portions over a period of 1 hr. The clear solution was then stirred a t 85OC for 8 hr, allowed to cool overnight, and the excess hypo- chlorite was destroyed by the addition of aqueous sodium bisulfite. Acidification of the solution with excess hydrochloric acid gave a precipitate, which was col- lected and redissolved in aqueous sodium bicarbonate. The product was again precipitated with hydrochloric acid, filtered, and dried. It weighed 5.8 g (700h); mp 394°C (by DTA) (lit.6 mp 392-394OC). The infrared spectrum (KBr) showed bands at 1690 cm-l (vc=o), 1300 and 1170 cm-l (vso2), and 1270 cm-'

2,8-Phenoxathiindicarbonyl Chloride (PDC). A mixture of 4.0 g (0.014 mole) of 2,8-phenoxathiindicarboxylic acid (11) in 30 ml of thionyl chloride and 2 drops of dimethylformamide (DMF) as a catalyst was refluxed until a clear solution was obtained (2 hr). Excess thionyl chloride was removed using a water pump and the residual acid chloride was recrystallized from benzene. It weighed 3.0 g (66%). Further recrystallization afforded yellow needles, mp 153-154OC. The infrared spectrum (KBr) exhibited absorptions at 1740 cm-l (vc=o) and 1250 cm-' (vc-0-c).

(vc-0-c).

ANAL. Calcd for C14H6C1203S: C, 51.71%; H, 1.86%. Found: C, 52.02% H, 1.34%.

2,8-Phenoxathiindicarbonyl Chloride 10,lO-Dioxide (PDCD). The compound was prepared in 71% yield by refluxing 4.4 g (0.014 mole) of 2,8- phenoxathiindicarboxylic acid 10,lO-dioxide (111) for 2 hr in 20 ml of thionyl chloride and 2 drops of DMF. The acid chloride was recrystallized twice from benzene to give white plates, mp 159-161OC. The infrared spectrum (KBr) showed bands a t 1750 cm-l (vc=o), 1310 and 1150 cm-l (vsoz), and 1270 cm-l (vc-0-c).

ANAL. Calcd for C14H,jC120& C, 47.08%; H, 1.69%. Found C, 46.95%; H, 1.71%.

Reagents and Solvents. Bis(4-aminophenyl) ether, which was provided by Sumitomo Chemical Co., was purified by recrystallization from tetrahydro- furan. Isophthaloyl and terephthaloyl chlorides were synthesized by the con- ventional method from the corresponding diacids and thionyl chloride. Py- romellitic dianhydride, which was supplied by Toyo Soda Manufacturing Co., was recrystallized from acetic anhydride prior to use. N-Methyl-2-pyrrolidone (NMP, provided by Mitsubishi Chemical Industries Ltd.) was purified by vac- uum distillation and stored over 4 A molecular sieves.

Polymerization

Two typical examples of polymerization are given below. Polyamide V. In a flask 0.575 g (2.5 mmole) of 2,8-diaminophenoxathiin was

dissolved in 6 ml of NMP at ambient temperature. The solution was cooled to a mush with a Dry Ice-methanol bath. To this was added 0.508 g (2.5 mmole) of solid isophthaloyl chloride in one portion, and the cooling bath was changed to an ice-water bath. The mixture was stirred for 30 min at 0°C or below and for additional 30 min at room temperature. During this time a viscous solution of polymer formed. The polymer was precipitated by pouring the polymerization mixture into 500 ml of rapidly stirred water. After thorough washing with hot acetone and drying, i t weighed 0.89 g (99%). The inherent viscosity of the polymer in NMP was 1.05, measured at a concentration of 0.5 g/dl a t 30°C. The

2742 UEDA, AIZAWA, AND IMAI

infrared spectrum (film) exhibited absorptions at 3300 cm-l ( V N - H ) and 1660 cm-l ( V C ~ ) . Elemental analysis of the polymer is given in Table I1 (see Results and Discussion).

Polyimide XIV. In a flask 1.150 g (5 mmole) of 2,8-diaminophenoxathiin was dissolved in 9 ml of NMP, and to this was added 1.095 g (5 mmole) of solid pyromellitic dianhydride with stirring at room temperature. A viscous solution formed after 5 hr of the polymerization. The inherent viscosity of the polyamic acid in NMP was 1.61 (by dilution of the polymerization mixture to 0.5 g/dl solids).

The solution was cast into film on a glass plate with a spreading knife, and the plate was heated at 70°C for 3 hr in a forced-air oven. The film, which was re- moved from the plate, was heated at 250°C for 2 hr and then to 300°C for 1 hr under a thin stream of nitrogen. The infrared spectrum (film) showed absorp- tions at 1780 and 1730 cm-l (vc-0).

6.32%. ANAL. Calcd for C22H8N205S: C, 64.08%; H, 1.96%; N, 6.79%. Found: C, 63.82%; H, 2.14%; N,

RESULTS AND DISCUSSION

Syntheses of Monomers

Syntheses of phenoxathiin derivatives, which are useful as polymer-forming monomers, have been reported by Nobis et al.5 Thus, 2,8-diaminophenoxathiin (DAP) was synthesized through the use of the Friedel-Crafts reaction and Beckmann rearrangement in three steps starting from phen~xathiin,~ and 2,8-diaminophenoxathiin 10,lO-dioxide (DAPD) was prepared by simultaneous nitration and oxidation of phenoxathiin, followed by r e d ~ c t i o n . ~

Although the synthesis of 2,8-phenoxathiindicarboxylic acid 10,lO-dioxide (111) by the hypochlorite oxidation of 2,8-diacetylphenoxathiin (I) has been described briefly? no other detailed study has been done on this reaction. When the diacetyl compound I was treated with a large excess of aqueous sodium hy- pochlorite a t 85"C, the reaction yielded almost exclusively the diacid 111. In order to obtain previously unreported 2,8-phenoxathiindicarboxylic acid (II), which is considered to be an intermediate product for the diacid 111, the reaction was conducted repeatedly under milder reaction conditions. Finally, the diacid I1 could be synthesized successfully by the controlled oxidation of the diacetyl compound I with a slight excess of hypochlorite solution at a temperature below 50°C.

New diacyl chlorides, 2,8-phenoxathiindicarbonyl chloride (PDC) and 2,8- phenoxathiindicarbonyl chloride 10,lO-dioxide (PDCD), were prepared readily by the chlorination of the corresponding diacids I1 and 111, respectively, with thionyl chloride.

Preparation of Polyamides

Various aromatic polyamides were prepared from different combinations of diamines such as DAP, DAPD, and bis(4-aminophenyl) ether (ODA), and diacyl chlorides including PDC, PDCD, isophthaloyl chloride (IPC), and terephthaloyl chloride (TPC), as shown in eq. (1).

POLYAMIDES AND -1MIDES WITH PHENOXATHIIN UNITS 2743

H,N--Ar-NH, + CI-C-Ar'-C-Cl (1) II 0

U 0

II 0

The polycondensation was carried out by the low-temperature solution po- lymerization technique in NMP,7s8 which acts as a solvent and an acid acceptor. The results are summarized in Table I. In the cases of the polyamides VIII, XI, and XII, 5 wt % of lithium chloride based on the solvent was added to prevent precipitation of the polymer formed. Thus, all of the polymerizations proceeded in a homogeneous solution, and gave quantitative yields of high molecular weight polyamides having inherent viscosities in the range of 0.5-2.9 without difficulty. As can be seen from Table I, the polymerizations with DAPD tended to give polymers with lower inherent viscosity than those using DAP. This may be attributable to lower basic nature of DAPD, which possesses electron-with- drawing sulfone group in the molecule.

The infrared spectra obtained for all of the prepared polymers were consistent with expected polyamide structures. All spectra showed absorption bands at near 3300 cm-l and 1660 cm-l corresponding to an N-H stretching and an amide carbonyl stretching, respectively. Table I1 lists the elemental analyses of the various polyamides, and they were found to be generally in good agreement with the calculated values.

TABLE I Preparation and Properties of Polyamidesa

Polymer

IV V VI VII VIII IX X XI XI1

Diamine

ODA DAP DAPD DAP DAPD ODA ODA DAP DAPD

Diacyl chloride Binhb

IPC 1.20 IPC 1.05 IPC 0.82d TPC 2.91d TPC 1.62d PDC 1.19 PDCD 0.67 PDC 1.60 PDCD 0.5od

Decomposition temperature, "C"

In air In NI

450 450 460 490 470 480 490 525 510 530 475 485 470 485 495 515 490 490

a Polycondensation was carried out with 2.5 mmole of the reactants in 6 ml of NMP at -20 to 20°C for 1 hr.

Measured at a concentration of 0.5 g/dl in NMP at 30°C. A 10% weight-loss temperature observed by TGA. Measured in concentrated sulfuric acid.

2744 UEDA, SATO, AND IMAI

TABLE I1 Elemental Analvses of Some Polvamides

Calcd Found polymer C, 96 H, % N, 96 C, 96 H, Yo N, %

V 66.66 3.36 7.77 66.30 3.57 7.97 VIII 61.22 3.08 7.14 61.03 3.25 7.15 IX 69.02 3.56 6.19 68.74 3.72 6.32 X 64.46 3.33 5.78 64.26 3.55 5.77

Preparation of Polyimides

Aromatic polyimides were prepared by reacting pyromellitic dianhydride (PMDA) with the diamines described above. The two-step polymerization is shown in eq. (2) and involves ring-opening polyaddition forming soluble polyamic acid, followed by thermal cyclodehydrati~n.~J~

1 0

II 0

In the first step, the ring-opening polyaddition was carried out in NMP at room temperature. Figure 1 shows the rates of the polymerization of various diamines in terms of inherent viscosity of the polymers. The polymerizations proceeded fairly rapidly and went essentially to completion within 3 hr. Therefore, diamine components had no marked influence on the reaction rates, although they af- fected the molecular weight attained by the polymers appreciably.

In the second step, thermal cyclodehydration of various polyamic acids in the form of films was conducted by heating at 250-300°C under a nitrogen atmo- sphere to afford polyimides quite readily.

The formation of polyimides was confirmed by means of infrared spectroscopy and elemental analyses. The spectra of these polymers showed the strong imide carbonyl absorption bands at 1780 and 1730-1720 cm-l, while the absorption bands characteristic of the amic acid at around 3300 and 1660 cm-l had entirely disappeared, suggesting that the thermal imidization had proceeded complete- ly.

SYNTHESIS OF POLYAMIDES 2745

I . . . . . . . . . . . ' . I 0 2 4 6 8 1 0 1 2

REACTION TIME ( hr

Fig. 1. Polymerization of PMDA with the diamines in NMP a t room temperature: (A) po- lymerization with DAP; ( B ) polymerization with DAPD.

Properties of Polyamides

Qualitative solubilities of the polyamides in excess solvent were determined, and the results are summarized in Table 111. The most striking aspect of the present results was that the introduction of rigid and bulky phenoxathiin ring in place of open-chain diphenyl ether linkage into the polymer backbone did not influence the solubility of the polyamides, while that of phenoxathiin dioxide ring did in most cases. Therefore, the polyamides V, IX, X, and XI, which were soluble in polar aprotic solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), gave films by casting from the DMF solutions. All films were transparent, varying in color from almost colorless to slight yellow, and showed a high degree of flexibility. All of the polyamides were also soluble in concen- trated sulfuric acid. Among them, the polyamides containing phenoxathiin unit 'in the main chain imparted an intense blue color in the strong acid, which might be caused by the protonation at the sulfur atom in the heterocycle. These phenomena have been recognized concerning monomeric phenoxathiin com- pounds in many years."

TABLE I11 Solubility of Polyamides

Solubilitya NMP, Formic acid, DMF, tetrahydrofuran,

Polymer Sulfuric acid DMSO Pyridine rn-Cresol acetone

- - - IV ++ ++ V ++ ++ f f VI ++ VII ++ VIII ++ IX ++ ++ - x ++ ++ ++ XI ++ ++ XI1 ++

- - - - - - - - - - - - -

- f - -

- - - - - - -

* Solubility: (++) soluble a t room temperature; (+) soluble on heating; (.t) partially soluble or swollen; (-) insoluble.

2746 UEDA, SATO, AND IMAI

TABLE IV Preparation and Properties of Polyimidesa

Decomposition temperature, O C C

Polymer Diamine f)inhb In air In N2

XI11 ODA 1.01 480 490 XIV DAP 1.61 520 560 xv DAPD 1.04 520 525

a Polyamic acids were prepared by the polymerization with 5 mmole of the reactants in 9 ml of

Inherent viscosity of the polyamic acid was measured at a concentration of 0.5 g/dl in NMP at

A 10% weight-loss temperature observed by TGA.

NMP at room temperature for 5 hr.

3 O O C .

Thermal stability of the polyamides was evaluated by thermogravimetric analysis (TGA) both in air and under nitrogen. Typical TGA curves are given in Figure 2, and the thermal behavior data are summarized in Table I. Weight loss of the polyamides was gradual until a temperature of around 430°C was reached, and then the weight loss was rapid in air. The polyamides containing the heterocycles appeared to be more stable than those having open-chain di- phenyl ether linkage, as can be anticipated by the incorporation of the double- strand structures in the polymer backbone.

Properties of Polyimides

All of the polyimides were quite insoluble in organic solvents, while the cor- responding polyamic acids were soluble in polar aprotic solvents such as DMF and DMSO. Tough, transparent films of the polyamic acids were obtained by casting from solutions in an appropriate solvent and heating at 70°C. After thermal conversion and curing at 250-30O0C, the resulting polyimide films were orange-red to deep tan in color and characterized as being stiff and tough, but somewhat brittle.

" 0 100 200 300 400 SO0 600

TEMPERATURE ( 'C )

Fig. 2. TGA curves of the polymers: (A) polyamide V in air; ( B ) polyamide V in nitrogen; (C) polyimide XIV in air; (D) polyimide XIV in nitrogen.

SYNTHESIS OF POLYAMIDES 2747

Thermal stability of the polyimides was also determined by TGA method. Typical thermograms are shown in Figure 2, and the TGA data are listed in Table IV. These new type of polyimides XIV and XV exhibited an initial weight loss at around 500"C, followed by gradual weight loss above that temperature in air. Once again, the results clearly showed the polyimides containing the heterocycles t o be much more stable than the polyimide XI11 with ether linkage, the former 1

of which had step-ladder structures in the polymer backbone.

The authors are indebted to Professor Shigeru Hayama and his co-workers for the elemental analyses.

References

1. H. Lee, D. Stoffey, and K. Neville, New Linear Polymers, McGraw-Hill, New York, 1967. 2. A. H. Frazer, High Temperature Resistant Polymers, Interscience, New York, 1968. 3. F. Bentz and F. Bodesheim (assigned to Farbenfabriken Bayer A. G.), Ger. Offen. 1,811,167

4. C. M. Suter and C. E. Maxwell, in Organic Syntheses, Coll. Vol. 11, A. H. Blatt, Ed., Wiley,

5. J. F. Nobis, A. J. Blardinelli, and D. J. Blaney, J. Amer. Chem. SOC., 75,3384 (1953). 6. S. H. Edit, Iowa State Coll. J. Sci., 31,397 (1957); Chem. Abstr., 51,14729h (1957). 7. P. W. Morgan, Condensation Polymers: B y Interfacial and Solution Methods, Wiley, New

8. K. Kuze and S. Miwa, Kogyo Kagaku Zasshi, 71,443 (1968). 9. G. M. Bower and L. W. Frost, J. Polym. Sci. A, 1,3135 (1963).

(1970); Chem. Abstr., 73,46064j (1970).

New York, 1943, p. 485.

York, 1965.

10. C. E. Sroog, A. L. Endrey, S. V. Abramo, C. E. Berr, W. M. Edwards, and K. L. Olivier, J. Polym.

11. T. P. Hilditch and S. Smiles, J. Chem. Sac., 100,408 (1911). Sci. A, 3,1373 (1965).

Received October 5,1976