Synthesis and Characterization of Polyimides Containing Heterocyclic Units

PREMA SUBRAMANIAM and MAHALINGAM SRINIVASAN, Department of Chemism, Indian Institute of Technology,

Madras-600 036, India

Synopsis

Five new polyimides containing phenoxaphosphine as well as dibenzothiophene, phenoxathiin, and thianthrene units have been synthesized from lO-phenylphenoxaphosphine-2,3,7,8-tetra- carboxylic dianhydride-10-oxide and heterocyclic diamines by the cyclopolycondensation method. These polyimides had inherent viscosities in the 0.75-1.10 dL/g range in conc. H2SO4 at 30°C. AU the polymers were characterized by elemental analysis, density, solubility, crystallinity, IR spectra, and thermal methods.

INTRODUCTION

Polyimides have been exploited for their excellent thermal, mechanical, and electrical properties; some of them are commercially useful.' Heat and flame- resistant phosphorus containing polyimides have been synthesized by several ~ o r k e r s . ~ - ~

The preparation and properties of phenoxaphosphine as well as other heterocyclic unit containing polybenzoxaazinones6 and p~lyamides,~ using 2,8-dichloroformyl-l0-phenylphenoxaphosphine-l0-oxide as one of the mono- mers have been reported earlier from this laboratory. Since these polymers degraded only above 425°C and have better thermal properties than analo- gous open-chain phosphorus-containing polymers, the polyimides derived from these heterocycles would also be expected to have excellent thermal proper- ties. In this paper, the preparation and properties of new polyimides contain- ing phenoxaphosphine dibenzothiophene, phenoxathiin and thianthrene units ,are reported (Scheme 1).

EXPERIMENTAL

Infrared spectra were obtained on a Perkin-Elmer 781 spectrophotometer. X-ray diffractograms were taken on a Phillips PW 1050 instrument using Ni-filtered CuK a radiation. Thermogravimetric analysis and differential ther- mal measurements were performed on a Stanton-Redcroft thermobalance in static air. Densities were determined with a small pyknometer in hexane as the nonsolvent at 2OOC. The inherent viscosities were measured with a suspended level visconieter.

Materials

N-Methyl-Bpyrrolidone (NP) pyridine and thionyl chloride were purified by the usual procedures. 2,8-Diaminodibenzothiophene, 2,8-diaminodibenzothio-

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, 1553-1560 (1988) 0 1988 John Wiley & Sons, Inc. CCC 0360-3676/88/061553-08$04.00

1554 SUBRAMANIAM AND SRINIVASAN

NMP

O h : & O 0 0"Ph 0 + HzN-R-NHz 25'C

I c

AczO. pyridine

CONH-R-NH NMP, 130'C

PAA- 1 - 5

PI- 1-5

Polymer code 1 R

PAA-1, P1-1

PAA-2, PI-2

PAA-3, PI -3

PAA-4, PI -4

PAA-5. PI-5

Scheme 1.

phene-5,5-dioxide, 2,8-diaminophenoxathiin, 2,8-diaminophenoxathiin-10,10- dioxide, and 2,7-diaminothianthrene were prepared by published procedure.8-10. Phenylphosphorous dichloride was commercial and distilled before use.

l0-Phenylphenoxaphosphine-2,3,7,8-Tetracarboxylic Dianhydride-10-Oxide (I)

2,3,7,8-Tetramethyl-lO-phenylphenoxaphosphine (obtained from 3,3',4,4'- tetramethyldiphenyl ether" and phenylphosphorus dichloride under Friedel-Crafts conditions) was oxidized with pyridine-KMnO, to give 2,3,7,8- tetracarboxy-10-phenylphenoxaphosphine-10-oxide. Yield 78%, mp > 360°C (lit.5 > 36OoC). This tetra acid (4.68 g, 0.01 mol) was refluxed with thionyl cdoride (20 mL) for 3 h. The colorless product that separated was filtered, washed with heme , and dried. Yield 3.5 g (834%): mp 325°C (lit.5 324°C).

POLYIMIDES CONTAINING HETEROCYCLIC UNITS 1555

C=O ; 1610 cm-' (-pH); 1400 cm-' IR (KBr): 1850 and 1780 cm-'

'H-NMR (DMSO-d,/a): 7.2-8.6 (m, 9H, ArH).

i> i (-P-Ph); 1245 cm-'(C-0-C); 1180 cm-'(P=O).

ANAL. Found (calcd): C, 61.11 (61.11); H, 2.08 (2.08); P, 7.17 (7.18).

General Procedure for the Preparation of Polyimides

To a solution of the diamine (1 mmol) in NMP (5 mL), I (1 mmol) was added in one portion. The polymerization was carried out for 6 h a t 25"C, with stirring. The reaction mixture was diluted with water and the resulting polyamic acid was filtered, washed with acetone, and dried. The polyamic acid (300 mg) was taken in a mixture of NMP (1 mL), acetic anhydride (2 mL), and pyridine (2 mL) and heated at 130-140°C for 5-6 h. It was then cooled and poured into a large excess of acetone. The precipitated polyimide was filtered, washed with acetone, and dried at 100°C for several hours.

RESULTS AND DISCUSSION

A series of polyimides containing heterocyclic units were prepared from I and five heterocyclic diamines by the two-step cyclopolycondensation method. In the first step, the polyamic acids (PAA's) were synthesized by the polycon- densation of the dianhydride E with the diamines by the low-temperature solution method in NMP in 87-93% yield. It was observed that the addition of the diamine to the dianhydride solution resulted in low molecular weight polymers than those from the reverse addition.

The PAA's were characterized by IR spectroscopy. The absorption a t 1660 and 1700 cm-' for the amide and acid carbonyl and the broad band in the

region 3000-3700 cm-' for the ,NH, -OH groups confirmed the product formation (Fig. 1).

\

I I I I I I I I I I 3000 moo 1600 1200 800

Wavenumber I cm-l

Fig. 1. IR Spectrum of PAA-3 and PI-3.

1556 SUBRAMANIAM AND SRINIVASAN

TABLE I Results of Polyamic Acid Synthesis

Yield ?inha Polymer code (% ) (dL/g) IR (KBr) cm-'

PAA-1 91 0.81 1660,1720,3100-3700 PAA-2 87 0.75 1670,1710,3000-3650 PAA-3 92 1.10 1660,1720,3000-3700 PAA-4 89 0.80 1660, 1710,2800-3700 PAA-5 93 0.86 1660,1700,3000-3700

'Measured at a concentration of 0.2 g/dL in DMA at 30°C.

These PAA's were soluble in polar aprotic solvents such as NMP, DMAc, DMF, DMSO, and HMPT. Inherent viscosity values of these PAA's were in the 0.75-1.10 dL/g range though both the yield and the qinh value are lower for PAA-2 and PAA-4 containing electron-drawing sulfone groups (Table I).

Polyimides (PI'S) were prepared by the cyclodehydration of the correspond- ing PAA by refluxing the PAA with a solvent mixture of NMP, acetic anhydride, and pyridine (1 : 2 : 2 ratio), for 5-6 h. The PI'S were obtained as yellow or brown powders in 84-90% yields ( Table 11). The density values of the PI'S indicate that PI-2 has lower density than PI-1 while PI-4 has lower density than PI-3. This is quite reasonable since the two oxygens of the sulfone groups in these polymer molecules inhibits their close-packing a fact supported by x-ray diffraction studies (Fig. 2).

IR spectra was used to follow the formation of the polyimides from the amic acids. The disappearance of amide carbonyl with concomitant ap- pearance of imide bands at 1770 * 10 and 1720 * 5 cm-' and C-N band at 1360 k 10 cm-' seemed to indicate chemical imidization could have proceeded

TABLE I1 Results of Polyimide Synthesis

Temperature' f ir 10%

weight loss Polymer Yield ?inha Density (W inair

Elemental analysis

PI-1 88 0.79 1.15 Calcd.: 66.58 2.46 4.59 5.24 550

PI-2 84 0.75 1.11 Calcd.: 63.35 2.34 4.36 4.98 565

PI-3 89 1.15 1.10 Calcd.: 65.17 2.39 4.47 5.11 535

PI-4 86 1.07 1.05 Calcd.: 62.00 2.28 4.26 4.86 548

PI-5 90 1.10 0.98 Calcd.: 63.55 2.33 4.36 9.97 525

Found: 67.05 2.83 4.31 4.98

Found: 62.48 2.38 4.28 4.75

Found: 65.48 2.77 4.17 5.81

Found: 62.18 3.00 4.38 4.75

Found: 63.96 2.82 4.09 9.09

"Measured a t a concentration of 0.2 g/100 mL in conc. H,SO, at 30°C. 'Heating rate, 10"C/min.

POLYIMIDES CONTAINING HETEROCYCLIC UNITS 1557

I I I I I

5 10 15 20 25 30 28

Fig. 2. X-ray difiactogratm of PI’s 1-5.

5

TABLE I11 Isothermal Weight Loss of PI’s in Air

IR (KBr), cn-’ code 2 h 6 h 10 h Imide >C=O ’

Percent weight 10s at 3MoC Polymer

PI-1 0.7 0.9 1.1 PI-2 0.4 0.6 0.7 PI-3 0.8 1.1 1.3 PI-4 0.6 0.7 0.9 PI-5 0.9 1.2 2.6

1780,1720 1785,1730 1780,1715 1780,1715 1775,1720

1558 SUBRAMANIAM AND SRINIVASAN

4000 3000 2000 1600 1200 800

Fig. 3. IR spectra of PI-3 (1) before aging, after aging at 350°C in air for (2) 6.5 and (3) 10 h. Wavenumber I cm-1

completely. However isothermal gravimetry of the imides at 350°C in air (Table 111) showed that the polymers lose 0.7 to 2.6% of their weight during 10 h of aging at 350°C. This clearly indicates that the core of the polymer contained uncyclized PAA units which dehydrated slowly. Also while the imide carbonyl was still seen for the aged polymer at 1780 & 5 and 1720 5 5 cm-' a small but significant peak appeared at 2200 cm-l (Fig. 3). This can arise only if there is carbon nitrogen multiple bond formation due to some structural change.

Thermal stabilities of the PAA's and PI's were further examined by TGA and DTA. The thermograms of the PAA's showed a two-step decomposition. Figure 4 shows the TGA and DTA curves of PAA-5. The first decomposition which occurred in the 200-350°C region was presumed to be due to the conversion of the PAA to PI, resulting in the loss of water molecules. The second decomposition occurring around 400-700°C is due to the decomposi- tion on the polyimide formed in the first step. DTA curve of the PAA also supports the above observation. Its differential thermogram exhibited two exothem. The first exotherm which is quite broad occurred around 200-300°C (near the first stage decomposition). The second sharp exotherm, around 575"C, is due to the decomposition of the PI formed in the first step.

Typical TGA curves of the PI's is shown in Figure 5. It can be seen that PI-2 and PI-9 containing dioxoheterocycles are more stable than analogous nondioxoheterocycle containing PI-1 and PI-3. PI-5 containing thianthrene units is the least stable of all (Table 11). A comparison of the thermal stabilities of these polyimides with the already reported aromatic poly- pyrromellitimides12 containing phenoxathiin units was made. Polyimides prepared from the diamines, viz., 4,4'-diaminodiphenylether, 2,8-diaminophen- oxathiin and 2,8-diaminophenoxathiin-lO,lO-dioxide, and pyromellitic di-

POLYIMIDES CONTAINING HETEROCYCLIC UNITS 1559

t Endo

I I I I I I J

Temperature I ’C

Fig. 4. TGA and DTA curves of PAA-5 in static air.

anhydride showed 10% weight loss at 480 and 520°C in air whereas the present polyimides, PI-3 and PI-4 containing phenoxathiin and phenoxathiin dioxide units showed 10% weight loss at 535 and 548°C under identical conditions. This observation clearly shows the superior thermal stability of the polyimides which contain phenoxaphosphine heterocycle in their back- bone.

The solubility tests of the PI’s showed that the incorporation of hetero- cyclic units did not improve the solubility characteristics of the PI’s. All of them were insoluble in organic polar solvents and dissolved only in conc. H,SO, and TFA. The PI’s gave brittle films on casting from the DMAc solution of the corresponding PAA and heating in vacuo at 200°C.

SUBRAMANIAM AND SRINIVASAN 1560

loo

40

0

\ \

I I 1 1 1 300 400 500 600 700

Temperature /‘C

Fig. 5. TGA curves of PI’S 1-5 in static air.

The authors thank IIT Madras for facilities, RSIC-IIT Madras for spectral data i d UAE for (SP) fellowship.

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Received December 12, 1986 Accepted June 29,1987