JOURNAL OF POLYMER SCIENCE: Polyma Chamklq Edition VOL. 12. 1301-1311 (1974)

Poly( arylene Sulfidee) with Pendant Cyano Groups a8 High-Temperature Laminating Resine. I1

IBRAHIM HADDAD and C. S. MARVEL, Department of Chemistry, The University of Arizona, T w m , Arizona 86721

synopsis Poly(pheny1ene sulfides) containing various. amounts of pendant cyano groups were

synthesized from m-benzenedithiol and the corresponding amounts of pdibromobenzene and 3,5-dichlorobenzonitrile. The polymers prepared by the use of 10, 15, 20, and 25% of the nitrilecontaining dichloro compound were slightly off-white with melting ranges below 100°C and had inherent viscosities of about 0.15 dl/g in hexamethylphosphoric triamide a t 30°C. The polymers prepared from m-benzenedithiol and the stoichi* metric amounts of 2,4-dichlorobenzonitrile or 3,5-dichlorobenzonitrile looked similar to those described above, yet they possemed much higher melting ranges. The poly- (phenylene sulfide) prepared by the use of 2,4-dichlorobenzonitrile had an inherent viscosity of 0.06 dl/g while the polymer prepared from the 3,5-dichloro isomer had an inherent viscosity of 0.38 dl/g. All the polymers listed above were crowlinked by heating alone or in the presence of anthracene-9,lO-bisnitrile oxide to give black resinous polymers that were insoluble in hexamethylphosphoric triamide in which the original polymers dissolved quite readily.

INTRODUCTION Thermally stable laminates possessing good mechanical propertiee can be

prepared by the use of polymers that can be cured to form crosslinked polymers. The laminating polymers used should be thermally stable, possess low melting ranges or high solubility to wet the laminated surface and be capable of forming crosslinked sites of known thermal stability. The crosslinking process, moreover, should proceed without the evolution of any gaseous side products that can form voids which can be detrimental to the mechanical properties of the final laminates.

In a previous paper,' work on the synthesis of several poly(ary1ene sul- fides) in a basic medium of potassium carbonate in dimethylacetamide or dimethylformamide, was reported. In that work i t was found that poly- m-thiophenylenes and poly-m,p-thiophenylencs possess low enough melting ranges (around 100°C) and high enough thermal stability to be used as laminating resins. The laminating resins were obtained as poly-m,p- thiophenylenes with pendant cyano groups prepared by the use of 5 mole-% 2,4-dichlorobenzonitrile or 3,5-dichlorobenzonitrile. The crosslinked poly- mers were then obtained by heating, under nitrogen, the nitrile-containing polymers, alone or in the presence of zinc chloride. These crosslinked

1301

@ 1974 by John Wiley & Sons, Inc.

1302 HADDAD AND MARVEL

polymers were found to exhibit undesirable rubbery behavior. It was also found that the polymers crosslinked in the presence of zinc chloride decom- posed more readily upon heating in air and showed a large number of voids due to water evaporating while heating the highly hygroscopic zinc chlo- ride.

In this paper we report on the continuing effort to prepare poly(pheny1ene sulfides) that can be cured to give crosslinked polymers with better me- chanical and thermal properties. Since the rubbery behavior was bclieved to be the result of insufficient crosslinking site8 (nitrile groups), poly- (phenylene sulfides) containing more nitrile groups along the polymer chains were prepared. Crosslinking was performed by heating the nitrile- containing polymers alone or in the presence of anthraccne-9,lO-bisnitrile oxide. Crosslinking the polymers by heat alone is believed to be the result of the possible trimerization of the nitrile groups to form s-triazines. Heat- ing in the presence of the dinitrile oxide is believed to produce crosslinked polymers through the formation of 1,2,4-oxadiazole ring^.^^^

RESULTS AND DISCUSSION

Polymers from mlenzenedithiol, p-Dibromobenzene, and 3,5-Dichlorobenzonitrile

All polymerization reactions were run in a medium of potassium carbo- nate in dimethylacetamide (DMAc) as reported earlier. Several poly-m,p- thiophenylenes containing various amounts of nitrile groups were prepared from m-benzenedithiol (mBDT) and the corresponding amounts of p- dibromobenzene (pDBB) and 3,5-dichlorobenzonitrile (DCBN) . These polymers were prepared in the hope of determining the polymer composition that might yield the crosslinked polymer with the optimum mechanical properties. The composition of these polymers along with the reaction conditions and results of polymerizations are given in Table I . Elemental analyses of the various polymers are given in Table 11.

Identification of the polymers was achieved by elemental analysis and by infrared spectroscopy which gave spectra similar to that of poly-m,p- thiophenylene with additional nitrile absorptions a t 2245 cm-'. The rela- tive amounts of nitrile absorptions of the various polymcrs was quantita- tively determined by infrared spectroscopy. A measured amount of a 2% mixture of each of the polymers in potassium bromide was pressed into a pellet and the relative intensities of the nitrile groups measured. The relative intensities were found to correspond almost exactly to the amount of 3,5-dichlorobenzonitrile used.

The synthesis of polymers I and 11 have been previously reported.' Polymers 111-VI were prepared by the same method, yet only polymer I11 behaved similarly in the workup and drying processes. Polymers IV, V, and VI coagulated into a gummy mass when allowed to stir in ether to re- move the lower molecular weight fractions ; a powdery material could only

POLY(ARYLENE SULFIDES). I1 1303

be obtained by vigorous stirring in methanol/water for a t least 2 days. This powdery character was, however, lost when these polymers were dried in vacuo a t 75°C.

Polymers 111-VI were readily soluble in cold hexamethylphosphoric triamide; inherent viscosities of these polymers wcre determined by using 0.5% solutions. The results of these measurements are given in Tablc I . They show t.hat thc inherent viscosities obtained for polymers 11-VI arc lower than that obtained for poly-m,p-thiophenylenc prepared similarly. This seems to indicate that lowering the molecular weight could be the re- sult of lowering the reactivity in the polymerization reactions bv substitut- ing 3,5-dichlorobenzonitrile for p-dibromobenzcne in which the more labile bromines arc present in the more favorable para position.

Isothermal aging of polymers 111-VI was run in an atmosphere of circulating air at temperatures of 300 and 350°C. The results of this tost are given in Table 111. They show a slightly higher weight loss than for poly-m-thiophenylene. This difference in thermal stability, though minimal, is believed to be due to the fact that isothermal aging of polymers 111-VI was run in an atomosphere of circulating air whereas that of poly- m-thiophenylene was run in an at.mosphere of static air.

Polymers from rn-Benzenedithiol and 2,4-Dichlorobenzonitrile or 3,5-Dichlorobenzonitrile

Polyphenylene sulfides containing nitrile groups in every other benzene ring were prepared after polymers 111-VI gave crosslinked products tbat were quite penetrable when heated in a Vicat apparatus. Polymer VII was prepared from m-benzenedithiol (mBDD) and 2,4-dichlorobenzonitrile (DCBX) by use of the normal procedure. The polymer obtained had an inherent viscosity of 0.06 dl/g and a melting range over 300". This melting range seems t.o be very high when comparcd to that of poly-m-thiophen- ylene, yet it can be rationalized by analogy to polyacrylonitrile when com- p a r d to polyethylene. The low inherent viscosity was not expected, since, it was believed, that the presence of the chlorines in the more favor- able positions ortho and para to the nitrile, would render a more facile dis- placement and consequently a higher molecular weight polymer. Two methods to improve the molecular weight of polymer VII were undert.aken. The first used copper bronze as catalyst and the second used more concen- trated start.ing materials. B0t.h of thcsc methods, however, failed to give the desired result as polymers with identical inherent viscosity were ob- tained. KO reasonable explanation for t.his behavior could be given other than to assume that the presence of the chlorines ortho to the cyano group could retard the substitution due to crowding. The composition of polymer VII along with the conditions of the reaction and the results of polymeriza- tion is given in Table I. Effects of polymerization conditions on polymer XI1 along with the results of polymerization are given in Tablc IV.

Polymer VIII was prepared from m-benzenedithiol and 3,5-dichloro- benzonitrile after the failure to improve the molecular weight of PVII.

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1306 HADDAD AND MARVEL

TABLE I1 Elernentd Analysis Data for NitrileContaining Poly(pheny1ene Sulfides).

-

Polymer Repeating unit c , % H, % N, 70 S,% -

PI1 ( C l z r ~ ~ s ? ~ 1 3 H 7 N a ) , an Calcd 66.52 3.68 0.27 29.49 Found 66.33 3.80 0.25 29.43

PI11 ( ( : I ~ L S ~ I J I ~ N S & Ion Calcd 66.45 3.62 0.64 29.29 Found 66.24 3.54 0.57 B . 3 6

PIV ( C I Z H B , M ~ , H ~ N S Z ) O 12" Calcd 66.35 3.57 0.96 29.12 Found 66.11 3.47 1.00 29.73

PV ( ( : l z ~ I ~ ~ - f o ~ l r H ~ N S ~ ) ~ 10- Calcd 66.24 3.53 1.27 29.00 Found 66.30 3.42 1.21 29.32

PVI ((:IzIi~S~-f,-,h;-tC~JHINS?)O Calcd 66 14 3.49 1.60 28.80 Found 66.31 3.35 1.47 28.92

PVII (CIjFIIN&) Calcd 64.70 2 93 5.80 26..57 Found 64.67 3.07 .5.80 26.50

PVIII (CizH7Ns) Calcd 64.70 2.93 5.80 26.57 Found 64 50 3.46 5.59 26.44

. Elemental analysis data have been corrected for residue and residual halogen.

TABLE I11 Isothermal Aging of Kitrilecontaining Poly(pheny1ene Sulfides)

Weight lass, %

Polymer 8 days at 300°C 5 days at 350'C

1'1 I I 8.5 22.4 PIV 9.7 22.1 I'V 8.9 19.0 PVI 8.7 21.3

1 3 . 6 PVII - PVIII - 13.2

. Isothermal aging on PVII was run for seven days.

TABLE IV Effect of Polymerization Conditions on PVJI

Reac- Polym- mBI)T, DCBN, KZCOJ, tion Melting eriza- g g g DMAc, time, Yield, 'limb, range, tion (mole) (mole) (mole) ml hr g dl /g "C

1 1.4239 1.7207 3.2226 40 48 2.1 0.06 310-325 (0.01) (0.01)

(0.01) (0.01)

(0.02) (0.02)

2 1.42X3 1.7222 3.2028 2.5 48 2.0 0.06 295-310

3. 2.8454 3.4429 6.3535 35 24 3.7 0.06 320-330

This reaction was run in the presence of a catalytic amount of copper bronze.

POLY(ARYLENE SULFIDES). I1 1307

Polymer VIII had a higher inherent viscosity than PVII. As mentioned above, this can only be rationalized by assuming that, in this case, the steric effect is more important than the resonance effect.

Isothermal aging of polymers PVII and PVIII was run in an atmosphere of circulating air a t 350°C. The results of this experiment are given in Table 111. These results show that these polymers are fairly stable, and in the case of PVII substantial weight loss occurred within the first 96 hr.

Crosslinking Reactions

Zinc chloride was shown to be a good catalyst for the crosslinking of the nitrile-containing poly(pheny1enc sulfides), but it resulted in the formation of a large number of voids and enhanced the thermal degradation of the final product. The curing in this work was achieved by heating the nitrile- containing poly(pheny1ene sulfides), for 24 hr, alone a t 350°C or with an- thracene-9,lO-bisnitrile oxide (ABNO) a t 290°C. The method8 used in the crosslinking reactions are given in Table V, and the elemental analysis data are given in Table VI.

TABLE V Crosslinking of the Nitrilecontaining Poly(pheny1ene Sulfides).

Linear polymer Crosslinked ABNO used, ABNO required,

polymer T m e Wt, R g (wt %) g (wt %) PIX

PX

PXI

PXII

PXIII

PXIV

PXVa PXVIb

I1

I1

I11

IV

V

VI

VII VIII

0.7226 0.008

0.5090 0.273

0.5004 0.0559

0.5687 0.1085

0.5215 0.1387

0. -5676 0.1602

(1.1)

(5.4)

(11.2)

(19.1)

(26.6)

(28.2)

0.014 (3.0) 0.014

(3.0) 0.029

(6.0) 0.045

(9.0) 0.060

(12.0) 0.075

(15.0) - -

All crosslinking reactions were run under an atmosphere of nitrogen. b PV and PXVI were obtained by heating about 0.5 g samples of polymers VII and

VIII alone at 350°C.

Crosslinking the nitrile-containing polymers by the use of a dinitrile oxide \vm found suitable, since the reaction of a nitrile with a nitrile oxide is known to produce 1,2,4-oxadiazole rings without the formation of any volatile side products [eq. (1)].2.a

'0'

1308 HADDAD AND MARVEL

TABLE VI Elemental Analysis Data of the Crosslinked Polymers

Polymer Repeating unit C, % H, % N, % S, % PIX

PX

PXI

PXII

PXIII

PXIV

PXV

PXVI

Calcd Found Calcd Found Calcd Found Calcd Found Calcd Found Calcd Found Calcd Found Calcd

66.65 3.65 0.41 29.17 68.45 3.67 0.21 27.76 66.73 3.67 0.62 28.62 66.68 3.55 0.42 28.51 65.82 3.61 1.21 27.65 62.92 3.51 1.36 25.11 66.92 3.55 1.75 26.76 64.73 3.80 3.51 23.76 67.01 3.55 2.26 25.92 67.94 3.44 2.20 25.28 67.48 3.46 2.75 2.5.14 68.68 3.36 2.15 24.45 64.70 2.93 5.80 26.57 65.95 2.77 5.20 26.06 64.70 2.93 5.80 26.57

Found 64.04 2.88 4.79 26.42

It has been reported' that some nitrile oxides, especially the low molecular weight ones, are energy rich compounds and highly explosive. Anthra- cene-9,lO-bisnitrile oxide was used in this work since it was found to be quite stable when heated, presumably because it contained the thermally stable anthracene moiety. Anthracene-9,lGbisnitrile oxide was prepared from anthracene-9,lO-dicarboxaldehyde according to the procedure of LaRochelle [eq. (2)].6e6

CH=NOH CBx-NOH

- I

Br

C - N 4

Because of the potential hazard in heating nitrile oxides, curing of PI1 with anthracene-9,lO-bisnitrile oxide was carried out by gradually raising the temperature to the required 290°C. The react.ing mixture of the linear polymer and the dinitrile oxide waa heated at 160°C for 4 hr followed by

POLY(MYLENE SULFIDES). I1 1309

TABLE VII Isothermal aging of the Crosslinked Polymers.

Cumulative Weight Loss, %

Crwlinked polymer 8 days at 300°C 5 days at 350°C

PIX 1 . O b I l . l b PX 2.0b 18.0b PXI 9 . 1 74.9 PXII 21.4 91.4 PXIII 9 . 3 94.3 PXIV 7 .9 95.1 PXV - 24.4 PXVI - 14.01

Isothermal aging was carried out in an atmosphere of circulating air unless otherwise noted.

b Isothermal aging was carried in an atmosphere of static air.

heating a t 185°C for 12 hr; then the temperature was raised slow71y to 290°C) where it was maintained for 24 hr. The crosslinked product ob-

' tained from PI1 and 1 w t % of anthracene-9,lO-bisnitrile oxide (PIX) was 91% insoluble and the one obtained from PI1 and 5 wt % of the dinitrile oxide (PX) was 90% insoluble in hexamethylphosphoric triamide in which PI1 dissolved readily. Both PIX and PX were shown to be quite thermally stable by thermogravimetrie analysis and isothermal aging, however, these products were found to be quite penetrable by the Ehlers-Fisch teehni-

Polymers PIII-PVI were also crosslinked by heating with anthracene-9,- 10-bisnitrile oxide. Here, a twofold excess of the theoretical amount needed of the dinitrile oxide was used in order to insure that most of the nitrile groups were crosslinked. The reaction mixtures, after heating at 290°C for 24 hr, gave dark resinous materials (PXI-PXIV) that were quite thermally stable at 300°C but disintegrated completely when heated at 350°C for 5 days (Table VII). It is believed that the excessive degrada- tion of the crosslinked product could be caused by the formation of some side products from the reaction of anthracene-9,lO-bisnitrile oxide with itself. This phenomenon was also observed by Boschan,lo when cross- linked products were heated in the presence of a polymer prepared from a dinitrile oxide. Further attempts to prepare crosslinked products by the use of the theoretically required amounts of anthracene-9,lO-bisnitrile oxide were not carried out, since polymers PXI-PXIV were found to soften markedly upon heating and thus lack the required mechanical properties.

Polymers PVII and PVIII were prepared in order to get poly(pheny1ene sulfides) with the highest nitrile content. Crosslinking of these polymer was carried out by heating in the absence of a catalyst or a crosslinking agent after i t was found that PI1 and PVI gave insoluble products when heated alone under nitrogen at 350°C. Thc black rcBinous products ob- tained, PXV and PXVI, were found to be insoluble in hexamethylphos-

que.'-9

1310 HADDAD AND MARVEL

phoric triamide and to undergo no visible softening when aged at 350" for 5 days. This observation was supported by the reeults of the Ehlers-Fisch softening m e t h ~ d ~ - ~ that showed polymer PXV to undergo only 30% penetration which is a vast improvement over results obtained earlier. The results of the isothermal aging of polymers PXV and PXVI are given in Table VII and the result of Ehlers-Fisch softening method is shown in Figure 1.

EXPERIMENTAL

Dimethylacetamide used in the polymerization reactions was dried over calcium hydride and distilled before use. Hexamethylphosphoric triamide was similarly dried and distilled under vacuum (63"C/0.3 mm Hg).

Commercially available p-dibromobenzene and 3,5dichlorobenzonitrile were used. p-Dibromobenzene was recrystallized from ethanol and 3,5- dichlorobenzonitrile was used as received. 2,4-Dichlorobenzonitrile was prepared from 2,4dichlorobenzoyl chloride

in two steps:" 2,4-dichlorobenzoyl chloride was reacted with ammonia in chloroform to give 2,4-dichlorobenzamidc (mp 187-188"C), which was de- hydrated with thionyl chloride to give 2,4dichlorobenzonitrile, mp 61-62°C.

m-Benzenedithiol was prepared by the reduction of m-benzene-disulfonyl chloride with zinc amalgam in sulfuric acid. The methods employed for the amalgam preparation and the redhction have been reported by Caeaar12 and in an earlier publication.'

't m. (OC)

100

, I I I 1 I

54 100 1YJ z[a & 3-J 5m WJ

Fig. 1. Softening under load of crosslinked polymer PXV. AT, 2.fi0C/min; atmosphere, static air.

Rate of heating

POLY(ARYLENE SULFIDES). I1 1311

Polymerization Reactions

All polymers wcrc prepared in a medium of potassium carbonate and dimethylacetamide as reported earlier. A different workup procedure was used in polymerizations IV-VI, since crystalline products could not be ob- tained after stirring in ether. In this procedure, the gummy material ob- tained after stirring in ether was separated from the solvent by decantation and stirred vigorously in 75y0 methanol-water. A tine powdery solid was obtained after filtration and air drying. This solid, however, melted into one solid mass when dried under vacuum a t 75°C. The amounts of starting materials, reaction conditions, and results of polymerizations are given in Tables I and XI.

Crosslinking Reactions

Crosslinking reactions were run in test tube whose inside walls were coated with aluminum foil and equipped with a nitrogen inlet and a side outlet. The crosslinked material was removed from the aluminum foil by dissolving the latter in dilute HCI, followed by filtration, washing and drying of the polymeric material. The conditions along with the results of these reactions are given in Tables V and VI.

We gratefully acknowledge the Air Force Materials Laboratory, Air Force Systems Command, WrighbPatterson Air Force Base, Ohio, for the financial support of this work.

We are indebted to Dr. G. F. I,. Ehlers and to Dr. R. K. Fisch, Air Force Materials Laboratory, Wright-Patterson Air Force Base, for the thermogravimetric analysis and softening curves, and to Captain LaRochelle for supplying some of the anthracene-9,lO- bisnitrile oxide.

We me also indebted to Dr. D. V. Todd of Koppers Company for the supply of m- benzenedisulfonyl chloride.

References 1. I. Haddad, S. Hurley, and C. S. Marvel, J. Polym. Sci. Polym. Chem. Ed., 11,

2. C. G. Overberger and S. Fujimoto, J. Polym. Sci. B, 3 , 73.5 (1965). 3. R. Huisgen, Angew. Chem., 75,604 (1963). 4. Z. Rappoport, The Chemistry ofthe Cyano Group, Interscience, New York, 1970, p.

5. G. A. Loughran and Captain LaHochelle, Air Force Materials Laboratory, Wright-

6. Captain LaRochelle, AFML-TF-72-239. 7. G. A. Loughran, G. F. L., Ehlers, and K. R. Fisch, Air Force Materials Laboratory

8. G. F. L. Ehlers, ASTM Bull., 236, 54 (Feb. 1959). 9. G. F. L. Ehlers and W. M. Powers, Mater. Res. Stand., 4. No. 6, 298 (1964).

2793 (1973).

791.

Patterson Air Force Base, Ohio, private communication.

Wright Patterson Air Force Base, Ohio, private communications.

10. H. H. Boschan, Hughes Aircraft Company, private communication. 1 1 . It. E. Lutz e t al., J. Org. Chem., 12, 617 (1947). 12. P. Caesar, in Organic Symhesis, CON. Vol. IV , N. Rabjohn, Ed., Wiley, New York,

1963, pp. 693-97.

Received January 18,1974 Revised March 6,1974