Makromol. Chem. 191,2017-2026 (1990) 2017

New polymer syntheses, 4 5 a )

Soluble and meltable poly(benzobisthiazo1e)s derived from substituted terephthalic acids

Hans R. Kricheldorp, Jiirgen Engelhardt

Institut fur Technische und Makromolekulare Chemie der UniversitSlt Hamburg, Bundesstr. 45, D-2000 Hamburg 13, FRG

(Date of receipt: February 19, 1990)

SUMMARY: A not perfectly pure 1.4-bis(trimethylsilyllylamino)-2,5-bis(trimethylsilylthio)benzene was

prepared as starting material for all condensations. Condensations with Cchlorobenzoyl chloride under a variety of reaction conditions revealed formation of benzobisthiazoles at temperatures 2250 "C. Condensation with various aryloxy or bisalkoxyterephthaloyl chlorides in an one-pot procedure yielded substituted poly(benzobisthiazo1e)s. All poly(benzobisthiazo1e)s were soluble in mixtures of dichloromethane or chloroform and trifluoroacetic acid (TFA). Inherent viscosities up to 2,O dl/g were found in this solvent system. Neither homo- nor copoly(benzobisthiazo1e)s formed a freely flowing melt up to 450°C, where thermal degradation begins to affect the properties. Thermogravimetric analyses in air proved that aryloxy-substituted polymers possess a significantly greater thermostability than alkoxy-substituted ones.

Introduction

Synthesis and properties of the poly(benzobisthiazo1e) 1 derived from terephthalic acid and 2,5-diamino-l ,Cbenzenedithiol were the object of numerous studies This polyheterocycle is of interest because it possesses a high thermostability, a stiff main chain and a good electroconductivity after doping with suitable oxidizing reagents. Unfortunately, any application as engineering plastic, as component of blends or as conducting material is plagued by the problem that this poly(benzobis- thiazole) is infusible and only soluble in concentrated sulfuric acid. Therefore, several substituted poly(benzobisthiazo1e)s were synthesized from methyl" or phenyl- substituted ') terephthalic acids and from a terephthalic acid with benzothiazole side chain8p9). In recent years alkoxy and aryloxyterephthalic acids became available, and from these substituents a further improvement of solubility and meltability may be expected.

However, aryloxy groups are sensitive to electrophilic substitution and alkoxy groups to acidic cleavage. Therefore, the classical syntheses of poly(benzobisthiazo1e)s in highly acidic reaction media at elevated temperature are not useful for these substituents. In a previous part of this series it was demonstrated that poly(benzobis- thiazo1e)s with pendent aryloxy groups can be prepared from silylated 2,s-diamino- hydroquinonelO). Thus, the present work was aimed at studying the synthesis of aryloxy- (2 b) and alkoxy-substituted (3) poly(benzobisthiazo1e)s via the "silyl method'!

a) Part 44: cf. H. R. Kricheldorf, D. Lubbers, Makromol. Chem., Rapid Commun. 11,261 (1990).

@ 1990, Hilthig 62 Wepf Verlag, Base1 CCC 0025-1 16)</90/$03.00

2018 H. R. Kricheldorf, J. Engelhardt

3

Results and discussion

Synthesis of a model compound

In the synthesis of substituted poly(benzobisthiazo1e)s via the "silyl method" two crucial steps were forseeable: first, the isolation of silylated 2,S-diamino-I ,Cbenzenedi- thiol(4), and second, the formation of the thiazole ring. From the analogous synthesis of substituted poly(benzazole)slO) it was learned that it is advisable to study the reaction pathway at first by preparation of a low-molecular-weight, easily character- izable model compound. Therefore, the reaction sequence outlined in Eqs. (1)-(3) was studied in the present work.

The silylation of 2,5-diamino-l ,Cbenzenedithiol dihydrochloride was conducted either with chlorotrimethylsilane and triethylamine alone or in a two-step procedure. The second silylation step was then conducted with hexamethyldisilazane and chloro- trimethylsilane and gave an easier isolatable reaction product (4). None the less, even in this case, the result was not satisfactory, because the main reaction product was a red viscous oil of unknown structure The desired product 4, a yellow solid, was obtained as the higher boiling fraction. Even after repeated distillation, a perfect separation from the red oil was not achieved, because, owing to the high temperatures, a fractionation column could not be used. All steps of the silylation and distillation process were accompanied by the awkward smell of hexamethyldisilthiane. This product is typically formed upon silylation and decomposition of thioureas However, this by-product was also detectable after the second distillation of 4.

New polymer syntheses, 45 2019

(3)

None the less, the crude 4 was used for the preparation of a model compound by reaction with 4-chlorobenzoyl chloride (Eqs. (1)- (4)). When 4 was reacted with 4-chlorobenzoyl chloride in toluene at room temperature and the resulting amide 5 desilylated with methanol, a crude amide 7 could be isolated. Amide 7 was subjected to DSC measurements without further purification, to avoid oxidation of its SH groups. The DSC heating trace (Fig. 1 (A)) exhibits broad endotherms in the tempera- ture range of 120-220°C. A sharp melting endotherm follows at 368°C. This endotherm agrees perfectly with that of a cyclized product 6 prepared by a one-pot procedure (Eqs. (2), (3)) at 300°C (Fig. 1 (B)). Thus, the DSC measurements suggest that the cyclization of amides 5 or 7 occurs at temperatures below 250 "C.

Fig. 1. DSC measurements (heating rate: 20 "Chin) of the amide precursor 7 (A) and of model compound 6 (FS) prepared in Marlothem-S at 300 "C (see lkb. 1)

100 200 300 LOO T/OC

This suggestion is confirmed by a series of "one-pot" syntheses conducted either in 1,Zdichlorobenzene or in Marlotherm-S (lhb. I). Although the elemental analyses of the product isolated from 1,2-dichlorobenzene were quite promising, both the 'H NMR and IR spectra indicated that the cyclization was not complete. The best results were obtained in Marlotherm-S at 250 and 300 "C (Rb. I and Figs. 2,3). The weak IR band (X) in Figs. 3 (A) and 3 (B) agrees with the "amide band" of 5 or 7. Yet the absence of N-H stretching vibrations and the clean 'H NMR spectra suggest that (X) represents an aromatic vibration of the cyclized compound 6. In other words, the crude starting material 4 allowed the synthesis of a pure benzobisthiazol model compound.

2020 H. R. Kricheldorf, J. Engelhardt

'kb. 1. Synthesis of benzobisthiazole 6 under various reaction conditions

Reaction medium Rmp. in "C

1,2-Dichlorobenzene 180 Diphenyl ether 250 Marlotherm-S 250 Marlotherm-S 300 Marlotherm-S 350

Time Yield in h in Yo

3,O 61 1,5 72 1,5 64 3,O 81 3,O 60

Elemental analyses bt C H N C l S

5 8 3 0 2,64 6,82 17,87 11,31 58,94 2,70 6,95 - - 58,30 2,52 6,82 - - 58,45 2,45 6,85 17,03 14,lO 58,85 2,54 6,82 16,63 14,05

a) Calc. for the perfectly cyclized product: CnHloC12N2SzOl (413,35) C 58,12 H 2,44

b, Calc. for amide 7 C2zH,4CIzNzSz0, (449,38) C 53,46 H 3,14 N 6,23 C1 15,78 N 6,78 C1 17,15 S 15,51.

S 14,27.

h Jv- l 1 1 1 1 1 1 ~ 1 1 ~

10 9 8 7 6 5 L 3 2 1 0 6 in pprn

Fig. 2. 100 MHz 'H NMR spectrum of model compound 6 prepared at 300 "C in Marlotherm-S

Fig. 3. IR spectra (KBr pellets) of model compound 6; (A): prepared at 300 "C in Marlotherm-S, (B): prepared at 250 "C in

I 1 Marlotherm-S

" I - .- E, z

I-

LOO0 3000 2000 1600 1200 800 LOO Wovenumber in cm-'

New polymer syntheses, 45 2021

Synthesis and characterization of poly(benzobisthiazo1e)s

In analogy to the synthesis of model compound 6 the poly(benzobisthiazo1e) 8 was prepared from 4 and 4-cumylphenoxyterephthaloyl chloride. Synthesis and properties of this dicarboxylic acid chloride have been previously described 12). Four polyconden- sations were conducted by the one-pot procedure in Marlotherm-S under variation of temperature and reaction time (Tab. 2). Both the elemental analyses and IR spectra indicated that condensation at 250°C for 1,5 h does not suffice to bring about complete cyclization. However, satisfactory results were obtained at 300 and 350°C (Tab. 2, Fig. 4(A)). The IR spectra of the poly(benzobisthiazo1e)s do not exhibit a significant NH bond. Possibly a condensation temperature of 250 "C does suffice if a longer reaction time is applied.

This hypothesis was checked with the syntheses of poly(benzobisthiazo1e)s 9 and 10. These copolymers with aliphatic substituents were designed to improve meltability and solubility in organic solvents. The 2,5-substituted terephthaloyl chlorides required for the preparation of 9 and 10 were either described in the literature [bis(dodecyloxy)] 13)

or in the experimental part of this work [bis(3-phenylpropoxy)]. The lower chemical and thermal stability of aliphatic substituents requires condensation temperatures below 350 "C or, even better, below 300 "C. The polycondensations conducted at 250 "C for 3,O h yielded copolymers with satisfactory inherent viscosities and satisfactory elemental analyses (Tab. 2). However, the IR spectra show the presence of an NH or OH stretching vibration around 3350 cm-' (Fig. 4(B)). A fraction of these

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New polymer syntheses, 45 2023

Fig. 4. IR spectra (KBr pellets) of (A): poly(benzobisthiazo1e) 8 prepared at 250 OC, and (B): poly(benzobis- thiazole) 9 prepared at 25OoC/3 h

LdOO 3000 2600' 1600 li00 800 LbO Wavenumber in cm-'

copolymers was then heated for 2 h to 300 "C but the IR spectra did not change. These results suggest that cyclization was complete at 250 "C, whereas a small fraction of side chains was cleaved leaving a pendent OH group.

Comparison of poly(benzobisthiazo1e)s 8-10 with each other and with the unsubstituted polymer 1 revealed the expected progress in solubility. Polymers 8-10 are soluble in dichloromethane, chloroform or I ,2-dichloroethane containing 20 vo1.-Yo or more trifluoroacetic acid. Such solvent mixtures are volatile and allow dissolution of numerous commercial polymers. Thus, the improved solubility allows the preparation of various polymer blends from a cosolvent. None the less, it is to be mentioned that none of the substituted poly(benzobisthiazo1e)s was soluble in an aprotic organic solvent (including DMF + 5 wt-% LiCl).

DSC measurements conducted with a heating rate of 20°C as well as WAXS measurements proved the amorphous character of all three polymers (8-10). In the case of 8 no phase transition was detectable in the DSC traces. The DSC traces of 9 and 10 displayed a weak, broad endotherm around 410-440°C (followed by rapid decomposition). Microscopic observation with crossed polarizers revealed for 9 and 10 a melting process around 430-440 "C, when the samples were heated under nitrogen. Yet, even under nitrogen, rapid thermal degradation prevented observation of the melt above 450 "C, and the typical texture of a nematic mesophase was never detectable. When heated in air, so that mechanical pressure could be applied, degradation was even more rapid and the melting process was not clearly observable. In the case of polymer 8 the viscous melt did not flow up to temperatures around 500OC, regardless whether oxygen was present or not. These results agree with the properties reported for substituted poly(benzobisoxazo1e)s lo). The lower thermostability of copolymers 9 and 10 compared to 8 is demonstrated by thermogravimetrical analyses conducted in air at 360°C (Fig. 5).

Conclusions

The present study on synthesis and characterization of poly(benzobisthiazo1e)s demonstrates that the silyl method is again suited to prepare substituted poly(benzobis-

2024 H. R. Kricheldorf, J. Engelhardt

10

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Fig. 5. Thermogravimetric analyses conducted at 360 "C in air; (A): poly(benzobisthiazo1e) 8; (B): poly(benzobisthiazo1e) 10

0 1 2 Time in h

thiazo1e)s with improved meltability and solubility. However, the results discussed above need to be improved in two directions. First, a pure dilated 2,s-diamino-I ,Cben- zenedithiol(4) is desirable as starting material. Second, terephthalic acids with longer aromatic side chains are required to improve meltability and solubility further. Aliphatic side chains are more effective for this purpose, but their chemical and thermal stability is not satisfactory. Flexible aromatic side chains with three or more aromatic rings seem to be advantageous. Work in this direction is in progress and will be reported in a future publication.

Experimental part

Materials: 2,5-Diamino-l,4-benzenethiol dihydrochloride was a gift of Bayer AG. It was used without further purification. (4-Cumy1phenaxy)terephthaloyl chloride 12) and 2,5-bis(dodecyl- oxy)terephthaloyl chloride 13) were prepared as described in the literature. Marlotherm-S, a mixture of isomeric dibenzylbenzenes, was a gift of HIils AG (Marl, FRG). Diethyl2,5-dihydroxy- terephthalate was purchased from Riedel de Haen (3016 Seelze, FRG) and used without purification.

I,4-Bis(trimethy~i~lamino)-2,5-bis(trimethylsilylthio)benzene (4): 2,5-Diamino-l,4-benzene- thiol .2HC1(0,5 mol) was suspended in 3,O 1 of dry toluene. Chlorotrimethylsilane (2,l mol) was added at once and triethylamine (3,2 mol) was added dropwise with stirring and reflux. After ca. 2 h of reflux, the reaction mixture was cooled with ice and filtered under an atmosphere of dry nitrogen. The filtrate was concentrated i. vac., the residue was diluted with 100 ml of dry toluene, 20 ml of hexamethyldisilazane and 20 ml of chlorotrimethylsilane, and refluxed for 30 min. Afterwards, the toluene solution was filtered and concentrated again. The residual viscous red oil was distilled i.vac. bar) over a short-way apparatus at a bath temperature of 180-230OC. When approx. 2/3 of the product had distilled in the form of a red oil, the further distillate gradually began to crystallize, and a yellow solid could be isolated as a second fraction. However, even repeated distillation of this second fraction did not allow a perfect separation of red oil and yellow product. The last fraction of the second distillation (yield ca. 21%) was subjected to DSC and 'H NMR measurements, m.p.:109"C (DSC).

'H NMR (CDCl, with int. tetramethylsilane (TMS)): 6 = 0,27 (s, 18H), 0,28 (s, MH), 6,79 ppm (s, 2H). 2,6-Bis(4-chlorophenyl)benzo [1,2-rl;4,5-~]bisthiazole (6): In a glove box tetrasilyl compound 4

(25 mmol) and 4-chlorobenzoyl chloride (50 mmol) were weighed into a 100-ml round-bottomed flask containing 30 ml Marlotherm-S. The temperature was raised to 300 "C in steps of 50 "C, and all volatile reaction products were removed with a slow stream of nitrogen. After 1 3 h at 300 "C, the reaction mixture was cooled, diluted with acetone and filtered. The crystalline product was

New polymer syntheses, 45 2025

washed several times with acetone and dried i. vac. Yield: 64'70, m. p. 358 OC (DSC, heating rate: 20 "C/min).

'H NMR (CDCI,/trifluoroacetic acid, vol. ratio 4: 1, with TMS): 6 = 7,75 (d; 4H), 8,12 (d; 4H); 9,09 ppm (s; 2H).

Polycondensation: In a glove box tetrasilyl compound 4 (20 mmol) and substituted tereph- thaloyl chloride (or a 1 : 1 mixture of substituted terephthaloyl chlorides) (20 mmol) were weighed into a 100-ml round-bottomed flask containing 40 ml Marlotherm-S. The temperature was gradually raised to 300 "C and maintained for 3 h. After cooling, the reaction mixture was diluted with acetone, the precipitated product isolated by filtration and washed with acetone. Afterwards, the product was extracted with 1,4-dioxane, reprecipitated from CH2C12 /CH,SO,H (vol. ratio 4 : 1) into methanol and dried at 120 "C i. vac. 2,5-Bis(3-phenylpropoxy)terephthaloyl chloride: Diethyl2,5-dihydroxyterephthalate (0,6 mol)

was suspended in 600 ml of dry dimethylformamide (DMF). Potassium tert-butoxide and 3-phenylpropyl bromid were added at ca. 60 "C in three portions with stirring. After complete addition, the reaction mixture was stirred at 80 "C for 1 h and poured onto 21 of water. The oily product was extracted with two 700-ml portions of ethyl acetate, the combined extracts were washed four times with water and dried over sodium sulfate. The ethyl acetate was partially removed i. vac., and the product was crystallized by portionwise addition of ligroin under cooling with ice. Yield 74%; m.p. 72-74 "C.

C382406 (490,601 Calc. C 73,30 H 6,97 Found C 73,11 H 7,Ol

The diethyl ester thus obtained was saponified in refluxing 2-propanol with 4 M sodium hydroxide. The resulting diacid was recrystallized from l,rl-dioxane/water. Yield 97%; m. p. 177-179OC.

C26H2606 (434,491 Calc. C 71,87 H 6,03 Found C71,80 H 6,19

The free diacid (0,4 mol) was refluxed with 400 ml of freshly distilled thionyl chlorid with stirring until a clear solution was obtained. The excess of thionyl chloride was then removed i. vac. and the resulting acid chloride was recrystallized from chloroform/ligroin. Yield 92%; m. p. 103- 105 "C.

C2,H,Cl,Oz (471,40) Calc. C66,25 H5,13 C1 15,O4 Found C 66,19 H 5,24 C1 14,79

Measurements: Inherent viscosities were measured with an automated Ubbelohde viscometer thermostatted at 20 "C. The differential scanning calorimetry (DSC) measurements were conducted with a Perkin-Elmer DSC-4 at a heating (cooling) rate of 20 "C/min. The 'H NMR spectra were obtained on a Bruker AC-100 in 5 mm 0. d. sample tubes. The wide-angle X-ray scattering (WAXS) powder patterns were recorded with a Siemens D-500 powder diffractometer by means of Ni-filtered CuK,-radiation at 20 "C. The IR spectra were measured with KBr pellets on a Nicolet SXB 20 FT-spectrometer.

We thank the 'Bundesrninisterium far Forschung und Echnik' for financial support and Dr. R. Pakull (Bayer AG, Krefeld) for the TG measurements.

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