Makromol. Chem., Rapid Commun. 7, 743 - 746 (1986) 743

Preparation and properties of polyamides containing heterocyclic units

Prema Subramaniam, Mahalingam Srinivasan *

Department of Chemistry, Indian Institute of Technology, Madras-600 036, India

(Date of receipt: June 26, 1986)

Introduction

Completely aromatic polyamides are known to be remarkably stable substances'**). A wide variety of polyamides containing aromatic and heterocyclic units have been synthesized in the past decade and it has been recognised that the thermal stability of the polymers increases markedly with the incorporation of double-strand hetero- c y c l e ~ ~ ~ ~ ) . In view of this stabilizing effect new heteroaromatic polyamides were prepared in place of polyamides with open-chain diphenyl ether and biphenyl linkages and their properties were evaluated. Sat0 et a1.j) have already described some poly- amides containing phenoxaphosphine rings.

Experimental part

Materials: 2,8-Dichloroformyl-lO-phenylphenoxaphosphine-lO-oxide (l), m. p. 220 "C, was prepared according to the procedure of Sato et al.5). 2,s-Diaminodibenzothiophene (2a), m. p. 199 "C, 2,8-diaminodibenzothiophene-5,5-dioxide (2b), m. p. 317 "C, 2,8-diaminophenoxathiin (Zc), m. p. 173 "C, 2,8-diaminophenoxathiin-l0,10-dioxide (2d), m. p. 247 "C and 2,7-diamino- thianthrene (Ze), m. p. 188 "C were also prepared according to literature procedures6-*).

Condensation polymerization: The polycondensations were carried out in N-methyl-2-pyrro- lidone (NMP) in the presence of lithium chloride. The preparation of 3a is given as an example.

Polyamide 3a from 1 and 2a: 0,214 g (1 mmol) of 2a was dissolved in 10 ml of NMP containing 4 wt.-% of lithium chloride, and cooled to 0 - 5 "C in an ice/water bath; 0,416 g (1 mmol) of 1 was then added to this solution in one portion, while maintaining a nitrogen atmosphere. The mixture was stirred at 0 "C for 30 min and then the cooling bath was allowed to warm up to room temperature. After 6 h, the solution was poured into excess of water, allowed to stand for 1 h, and the precipitated polymer was filtered, washed with 10% sodium hydrogen carbonate solution, then with dilute hydrochloric acid solution and finally with water and hot acetone. The polymer was dried to give a light-brown powder.

Yield: 0,49 g (89%); qinh : 1,05 dl/g at 30 "C in N,N-dimethylacetamide (DMA). IR (KBr): 3320 (>NH) and 1660 cm-' (>C=O). ' H NMR (perdeuterated dimethylsulfoxide (DMSO-d6)): 6 = 6,5 - 8,2 (m; ArI4).

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744 P. Subramaniam, M. Srinivasan

Results and discussion

The polycondensation was carried out by the low temperature solution polymeri- zation technique in N-methyl-2-pyrrolidone (NMP) which acts as solvent and acid ac- ceptor. Four weight per cent of lithium chloride was added to prevent precipitation of the polymer formed. The results are summarized in Tab. 1.

3

Zs, 3s b C d

I

All of the polymerizations proceeded in a homogeneous fashion and gave quantita- tive yields of polyamides having inherent viscosities in the range of 0,85 - 1,05 dl/g. Polyamide 3b has the lowest inherent viscosity. This may be attributed t o the lower basicity of diamine 2b which possesses an electron-drawing sulfone group. The polymers were characterized by IR and 'H NMR spectroscopy as well as elemental analysis (see Exptl. part).

Tab. 1. Results of condensation polymerization

Polymer Yield Vinh a) Temp. in "C at weight loss ofb) T,, c, in q o d1.g-I > in "C A

2% 10% 30% 50%

3a 89 1,05 450 484 (478)d) 584 672 500 3b 86 0,85 470 505 (478)d) 576 636 515 3c 93 0,98 442 475 (457)d) 543 625 495 3d 88 0 9 458 498 (457)d) 540 603 536 3e 91 0,92 430 460 520 595 473

a) Concentration = 0,2 g/dl in DMA at 30°C. b, Determined by thermogravimetry: Heating rate: 15 "C/min. ') Temperature of maximum exothermal peak observed in differential thermal analysis. d, From ref.s): temperature at 10% weight loss for corresponding polymers containing open-

chain biphenyl and diphenyl ether linkages. Heating rate: 10 "C/min.

The X-ray diffractograms of all the polymers indicate that they are amorphous in nature. However, 3a and 3c, containing dibenzothiophene and phenoxathiin moieties, show some low degree of crystallinity.

Preparation and properties of polyamides containing heterocyclic units 145

This observation is again confirmed by density measurements which indicate that polyamides 3 b and 3d, containing dibenzothiophene-5,s-dioxide and phenoxathiin- 10,lO-dioxide moieties, have a lower density than polyamides 3a and 3c, containing dibenzothiophene and phenoxathiin moieties (Tab. 2).

Tab. 2. Density and solubilities of the products of condensation polymerization

Polymer Densitya) Solubilityb) in Percentage of in g/cm3 r h solubilityc)

DMF DMSO m-cresol conc. H2S04 in NMP DMA TEA

3a 1,29 + + + + + + + 11 - + + 12 3b 1,12 + +

3c 1,20 + + + + + + + 20 3d 1,14 + + - + + 14 3c 1.02 + + + + + + + + 25

Determined pycnometrically in hexane at 20 "C. A 10% solution was taken as a measure of solubility. (+ +): soluble at room temperature; (+): soluble on heating; ( -): insoluble. DMF: N,N-dimethylformamide; DMSO: dimethyl sulfoxide; DMA: N,N-dimethylacetamide; TFA: trifluoroacetic acid. A weighed amount of polymer was taken in a known volume of NMP and warmed at 100 "C for 5 min. The solution was cooled to room temperature and the weight of remaining solute determined.

The solubility characteristics of the polyamides are shown in Tab. 2. Polyamides 3a, 3c, and 3e, containing dibenzothiophene, phenoxathiin and thianthrene moieties, dissolve easily at room temperature in polar aprotic solvents like N,N-dimethylform- amide, DMA, NMP and DMSO, whereas polyamides 3b and 3d, containing dioxo- heterocycles, dissolve completely only on heating. This is in contrast to the general rule that solubility increases with decreasing ~rystallinity~.~). A quantitative solubility test in NMP indicated that solution of the order of 25% could be obtained for poly- amide 3e. This increased solubility could be attributed to the folded structure') of the thianthrene molecule which prevents a close packing of the polymer chains, thus making solvation easier. The lower density value of 3e also supports the foregoing statement (Tab. 2). All of the polymers dissolve easily in conc. H2S04 and trifluoro- acetic acid. It is worth mentioning that polyamides prepared from 1 and open-chain diamines such as benzidine and diaminodiphenyl ether were found to be insoluble in DMF, DMA and DMSO') .

The thermal stabilities of the polymers were determined by TG and DTA in static air and the data are summarized in Tab. 1. All of the polymers are stable up to 425 "C and a 10% weight loss is observed between 460 and 505°C. Above 5OO"C, all the polyamides undergo a gradual weight loss which is accompanied by a broad exotherm in DTA, indicating that the polymers decompose slowly above this temperature. The absence of any endothermic or exothermic peak below this temperature indicates that no second-order glass transition or other phase transitions such as melting or crystall-

746 P. Subramaniam, M. Srinivasan

ization take place below this temperature. All of the polyamides exhibit superior thermal stability as compared with the polyamides containing open-chain biphenyl as well as diphenyl ether linkages (Tab.

The authors express their gratitude to DAE, India for financial assistance.

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