Macromol. Chem. Phys. 199,1421-1426 (1998) 142 1

Sodium sulfonate-functionalized poly(ether ether ketone)s

Feng Wang*, Tianlu Chen, Jiping Xu

Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China

(Received: November 10, 1997; revised: December 3, 1997)

SUMMARY A new monomer, sodium 5,5'-carbonylbis(2-fluorobenzenesulfonate) (l), was synthesized by sulfonation of 4,4'-difluorobenzophenone (2) with fuming sulfuric acid. Poly(ether ether ketone)s containing sodium sulfonate groups were synthesized directly via aromatic nucleophilic substitution from the sodium sulfonate-functionalized monomer 1 and Bisphenol A (3) in the presence of potassium carbonate in dimethyl sulfoxide. The polycondensation proceeds without any side reactions. The differential scanning calorimetry measurement indicated that the polymers are amorphous and the glass transition temperatures increase with the content of sodium sulfonate groups in the polymer chain. The degree of substitution with sodium sulfo- nate groups has strong influence on their thermal stability and solubility.

Introduction Poly(ary1ene ether ketone)s (PAKs) are widely regarded as high performance engineering thermoplastics due to their good solvent resistance, high thermo-oxidative sta- bility and good mechanical properties. In the last decade, several PAKs have been developed and commercially used in electronic, aircraft, and aerospace ind~striesl-~). In order to improve their solubility and thermal stabilities, a number of PAKs with different backbones were synthe- sizedM). In the last few years, considerable attention has been devoted to their functionali~ation~-~).

Functionalized PAKs could be obtained by introducing pendant groups into the polymer main chain. Wang et. al. have reported the bromomethyl modification reaction of methyl-substituted PAKs, synthesized using methylhy- droquinone as one of the comonomers. Bromomethyl poly(ary1 ether ether ketone) was then converted into the corresponding bromomethyl derivatives and further trans- formed to aldehyde and carboxylic acid via substitution and oxidation reactions7). Sulfonation is another versatile route to polymer modification that is especially suitable to PAKs. Several PAKs were sulfonated by different sul- fonating agents, including concentrated sulfuric acid". 'I),

pure or complexed sulfur trioxide", 13), chlorosulfonic acidI4), and methanesulfonic acidsulfuric acid15). Recently, Kerres et. al. reported a new method to prepare good permselectivity, good hydrolytic stability poly(ether ether sulfone) containing sodium sulfonate groups via metalation-sulfination-oxidation'@. However, postfunctio- nalization of a polymer is often difficult to control and can sometimes lead to side reactions. The sulfonation of PEEK was believed to induce degradation and crosslink- ing via the condensation of sulfuric acid group and a phe- nyl proton to form an intermolecular sulfone link'2-'4. l').

An alternative approach of obtaining a functionalized polymer involves the use of functionalized monomers.

This method is more advantageous than postmodification method, since the degree of function can be controlled easily by changing the ratio of the monomers and some side reactions could be avoided. Ritter et. al.") have synthesized carboxylic acid PAKs using 4,4-bis(4-hydro- xypheny1)pentanoic acid as a bisphenol component and condensated the carboxylic acid group with several amines. The amino-functionalized PAKs have also been successfully synthesized from isomeric amino-functiona- lized monomers and Bisphenol A'9).

In this paper, we report the synthesis and characteriza- tion of a functionalized monomer and the synthesis of PAKs containing the sodium sulfonate groups via copoly- condensation of 5,5'-carbonylbis(2-fluorobenzenesulfo- nate), 4,4'-difluorobenzophenone, and Bisphenol A. Solu- bilities and thermal properties of the polymers were also determined and discussed in terms of the content of func- tionalized monomer in the polymer chain.

Results and discussion

Monomer synthesis A new monomer, sodium 5,5'-carbonylbis(2-fluorobenze- nesulfonate) (1) was synthesized by sulfonating 4,4'- difluorobenzophenone (2) with fuming sulfui-ic acid (50%S03), followed by neutralization with NaOH and NaC1, according to Scheme 1.

Sulfonation is an aromatic electrophilic reaction. The choice of sulfonating agents and the chemical structure of the product depends on the substituents at the phenyl ring. Electron-donating substituents will favor the reac- tion whereas electron-withdrawing groups will not. Com- pound 2, a common monomer for the synthesis of poly- (ether ether ketone)s, has 2 electron-withdrawing groups at each phenyl ring. Therefore, a powerful sulfonation

0 1998, Huthig & Wepf Verlag, Zug CCC 1022-1352/98/$18.00

1422 F. Wang, T. Chen, J. Xu

polymer 4a 4b 4c 4d 4e

mk 0 5/95 10190 20/80 ?on0

Scheme 1:

4f

40160

SO 3N a /

N a0 3s' (I)

Scheme 2:

K2C03/DMSO, 170 c I 4 CO2, Hzo, KF)

agent, fuming sulfuric acid (50% SO3) and a relative high temperature (lOO"C), were chosen in the sulfonation reaction. On the basis of the electronic theory of orienta- tion in electrophilic aromatic substitution, fluoro is an o,p-orienting group, whereas carbonyl is a m-orienting group. So the product of the sulfonation reaction is expected to be compound 1, i.e., the substitution occurred preferentially on the position that is ortho to fluoro and metu to the carbonyl group. The chemical structure was also confirmed by elemental analysis, IR, and 'H NMR. The IR spectrum of 1 had the C--U absorption at 1 661 cm-' and showed a strong absorption at 1032 cm-',

assigned to symmetric O=S=O stretching of the sulfo- nate group. The results of elemental analysis and 'H NMR spectroscopy were all well assigned (see Experi- mental part).

Polymer (4a-4f) synthesis Due to the poor solubility of monomer 1 in sulfolane, dimethyl sulfoxide (DMSO) was used as reaction solvent. As shown in Scheme 2, poly(ether ether ketone)s (4a- 4f) were obtained by condensation of various amounts of 4,4'-difluorobenzophenone (2), sodium 5,5'-carbonyl-

Sodium sulfonate-functionalized poly(ether ether ketone)s 1423

Tab. 1. Synthesis of poly(ether ether ketone)s

VRV a) _- No. Amount of 1 Amount of 2 Amount of BPA Amount of Yield KzCOsing in% dL/g

4a - - 2.1821 10.00 2.2829 10.00 1.52 99 1.40

in g inmmol in g in mmol in g in mmol

4b 0.2111 0.500 2.0729 9.50 2.2829 10.00 1.52 99 1.27 4c 0.4233 1 .00 1.9638 9.00 2.2829 10.00 1.52 98 1.26 4d 0.8446 2.00 1.7456 8.00 2.2829 10.00 1.52 99 1.03 4e 1.2669 3.00 1 S274 7.00 2.2829 10.00 1.52 96 0.87 4f 1.6892 4.00 1.3092 6.00 2.2829 10.00 1.52 95 0.85

a) RV: Reduced viscosity, measured at a concentration of 0.5 g/dL in DMF at 25 * 0.1 "C.

s 9 s: <

.v

s

t I I I I I 1 1 2000 1800 i 600 1400 1200 1000 800 600

Wavenumben

Fig. 1. IR spectra of 4a, 4c and 4f

bis(2-fluorobenzenesulfonate) (l), and Bisphenol A (3) via nucleophilic aromatic substitution using toluene to remove the water formed during the polycondensation. The polymerization proceeded smoothly and homoge- neously. The polymerization results and the analytical data are summarized in Tab. 1. The yields of the polymers were quantitative and the reduced viscosities of the sodium sulfonate-functionalized poly(ether ether ketone)s were above 0.85 d u g , measured in 0.5 g/dL N,N- dimethylformamide (DMF) solution, indicating a high molecular weight of the polycondensates. The reduced viscosity of 4a is 1.4 d u g , whereas the reduced viscosity of 4f is only 0.85 dL/g under the same polycondensation conditions. This difference suggests that the reactivity of monomer 1 is less than that of monomer 2. Two major factors may compete with each other to affect the reactiv- ity of monomer 1, as compared with monomer 2. First,

the electron-withdrawing group, -SOz-, favoring the nucleophilic polycondensation due to the increasing den- sity of positive charge of the carbon atom connecting with the fluorine atom. Second, the steric hindrance of sodium sulfonate group reduces the reactivity. Compared with the postsulfonation method, there was no sulfuric acid group formed in the polymerization process, thus the crosslinking caused by sulfuric acid and phenyl proton could be avoided. This result was also proven by IR and elemental analysis.

IR absorption bands are well known for their marked specificity to individual chemical functionalities. Although the assignment of absorption bands for specific modes of molecular vibration in the polymer is not straightforward, due to some limitations, such as sam- pling history, IR sampling techniques, phase and mor- phology*'), IR has been used to analyze characteristic

1424 F. Wang, T. Chen, J. Xu

bands corresponding to the sulfonate group in the poly- mers, as reported for sulfonated polystyrenes and their blends21- 22), ionic polyesters23) and poly(pheny1ene sulfide sulfuric acid)24). In our study, 4a, 4c and 4f were chosen for the IR characterization, representing non-substituted, low and highly substituted polymers. As shown in Fig. 1, all IR spectra have an absorption band at 1652 cm-I, due to the C=O stretching and an absorption band of aro- matic C% stretching at 1591 cm-'. The introduction of sodium sulfonate was confirmed by a characteristic peak at 1032 cm-' (4c, 4f) assigned to the O=S=O symmetric stretching vibration. There is no O=S=O symmetric stretching mode in the non-substituted polymer 4a, such a mode is found as a weak peak for the low substituted polymer 4c and a stronger absorption peak was found for the highly substituted polymer 4f. No peaks attributed to the sulfone group were observed for 4c and 4f. These results clearly indicate that the sodium sulfonate groups were successfully incorporated into the polymer chain without any detectable crosslinking. The degree of substi- tution with sodium sulfonate groups increases with an increase in the concentration of monomer 1 in the feed. Additionally, the content of sodium sulfonate groups could be controlled easily by changing the ratio of mono- mers 1 and 2, provided the amount of monomer 3 is fixed.

Elemental analysis was also used to check if a proper amount of monomer 1 was incorporated into the polymer. As shown in Tab. 2, the experimental data are in good agreement with the calculated values, indicating the suc- cessful introduction of sodium sulfonate into the poly- mers.

Tab. 2. Analytical data for poly(ether ether ketone)s

No. Elemental analysis

C H S

calc. found calc. found calc. found

4a 82.73 82.65 5.41 5.43 - - 4b 80.72 80.64 5.26 5.36 0.77 0.96 4c 78.81 78.78 5.11 5.11 1.50 1.75 4d 75.25 75.30 4.83 4.86 2.87 2.70 4e 72.00 72.34 4.58 4.60 4.12 4.19 4f 69.01 68.89 4.35 4.23 5.26 5.31

Polymer characterization

Solubility The solubility characteristics of 4a-4f in selected sol- vents are summarized in Tab. 3. Poly(ether ether ketone) 4a based on Bisphenol A is quite soluble in a wide range of solvents such as DMF, DMSO, NMP, chloroform and tetrahydrofuran. The solubility of poly(ether ether sul-

Tab. 3. Solubility of Poly(ether ether ketone)s")

Solvent Poly(ether ether ketone)s

4a 4b 4c 4d 4e 4f

N,N-Dimethylacetamide N,N-Dimethylforrnamide N-Methyl-2-pyrrolidone Dimethyl sulfoxide I, 1,2-TrichIoroethane Chloroform Tetrahydrofuran Water Methanol Acetone

+ + + + + + + -

- +

+ + + + + +

sw - +

+ + + +

sw sw sw -

sw

+ + + +

sw sw -

~~

+ + + + + + + + - -

- - - sw - - - -

a) Solubility: (+) soluble at room temperature, (SW) swollen, (-) insoluble.

fone)s could be altered by introducing a moderate content of sodium ~ul fona te~~) . In the case of poly(ether ether ketone)s 4b-4f, similar phenomena were also observed. The solubility of 4b-4f decreased in some solvents, such as THF and chloroform with the increase of the content of ionic groups in the polymer chain. This may be attribu- ted to the fact that the ion bond is stronger in highly sub- stituted than in low substituted polymers.

Thermal properties of polymers Tab. 4 lists the data the thermal behavior of 4a-4f. None of the polymers shows any crystalline behavior in the DSC diagrams, indicating an amorphous structure of the materials. Obviously, the sodium sulfonate substituted polymers 4 b-4f possess higher glass transition tempera- tures (T,) than the corresponding unsubstituted polymer 4 a. Moreover, Tg increases with increasing content of the repeating unit of monomer 1 in the polymer chain. Simi- lar phenomena were also observed for carboxylic acid substituted PEEKS''). It is accepted that substituted poly- ethers usually have higher Tg than the corresponding non- substituted polyethers, due to the hindrance effect of sub- stituted groups in the polymer chain. For example, methyl-substituted poly(ether ether su1fone)s or ketones have higher Tg than unsubstituted polymersz6). The inter- molecular interactions, such as hydrogen bond and iono- mer effect are other factors to increase the Tg of poly- mers** lo). For sodium sulfonate-functionalized poly(ether ether ketone)s, the introduction of sodium sulfonate increase both the intermolecular interaction through polar ionic sites and the hindrance of the chain rotation, leading to the increase in T, values. In particular for 4f, no glass transition temperature was observed in a temperature range of 1O0-35O0C, indicating that the strong intermo- lecular interaction led to a stiffer polymer segment and the glass transition should take place at a temperature

Sodium sulfonate-functionalized poly(ether ether ketone)s 1425

higher than 350 "C. The absence of any exothermic transi- tion proves that the sodium sulfonate substituted PEEKS do not form sulfone groups by crosslinking reactions between sodium sulfonate groups and phenyl protons dur- ing the heating process.

All the polymers were stable up to 300°C both in nitro- gen and air atmosphere, indicating good thermal stability of the materials. The amount of sodium sulfonate groups introduced into the polymers remarkably affected their thermal stability. It is obvious that the 10% weight loss temperatures decrease as the content of ionic groups therein increases. This could be explained by the thermal degradation of sodium sulfonate at a relative low tem- perature, as noted for other sulfonated polymer^'^.^^).

Conclusion Sodium sulfonate-functionalized poly(ether ether ketone)s with high molecular weight were synthesized from the activated difluoro monomer substituted with sodium sulfonate and Bisphenol A without any subse- quent modification. Side reactions such as crosslinking and degradation were not observed in the polymerization process. In addition, the content of functional groups in the polymer chain was easier to control than the polymer sulfonation method. The functionalized polymers were found to posses higher Tg than the corresponding unsub- stituted polymers and good thermal stability. Work on water vapor and gas permeation behaviors, and ion permselectivities through membranes prepared from these functionalized polymers is currently under investi- gation and will be reported in the near future.

Experimental part

Measurements

'H NMR spectra were recorded using a Brucker 400 instru- ment. IR spectra were recorded on a Bio-Rad FTS7 spectro- meter. Differential scanning calorimetry (DSC) measurements and thermogravimetric analysis were con- ducted on a Perkin Elmer 7 Thermal Analysis System at a heating rate of 10"C/min. The DSC curves obtained during the second heating scan were taken into consideration.

Materials

4,4'-Difluorobenzophenone was purchased from Aldrich Chemical Co. and used as received. Dimethyl sulfoxide (DMSO), NJV-dimethylformamide (DMF) and toluene was purified by distillation and stored over 4 A molecular sieves. Potassium carbonate was dried at 180°C for 10 h prior to use. Other reagents and solvents were obtained commercially and used without further purification.

Synthesis of sodium 5,5'-carbonylbis(2-jluorobenzene- sulfonate) (1) 4,4'-Difluorobenzophenone (2) (10.9 g, 50 mmol) was dis- solved in 20 ml fuming sulfuric acid (5O%SO3). The red solution was stirred at 100°C for 12 h, then cooled to room temperature and poured into 120 ml ice water. NaOH (20 g) was added to neutralize the excess fuming sulfuric acid. The mixture was cooled again to room temperature, and addition of NaCl (20 g) resulted the precipitation of white solids which were filtered and dried. Recrystallization from an ethanovwater (1 : 1) mixture yielded white needles. The solid was filtered and dried in a vacuum oven at 120°C for 24 h. Yield: 18.4 g (87%).

/ L N a0 3s

IR (KBr): 1661 (C=O), 1598 (C=C), 1210, 1032 (S-).

H 8.0 (ddd, 8.8 Hz, 4.8Hz, 2.4Hz), e-H 8.0 (dd. 6.8 Hz, 2.4 Hz).

'H NMR (DMSO-&400 MHz): 6 = b-H 7.3 (t, 8.8 Hz), C-

CI3H6O7S2FZNa2 Calc. C 36.97 H 1.43 S 15.18 Found C 36.85 H 1.31 S 15.48

Synthesis of poly(ether ether ketone)s (4a-4f) Into a 100 ml three-necked round bottom flask, equipped with a Dean-Stark trap, and a nitrogen inlet, was added 2,2'- bis(4-hydroxyphenyl)propane, 4,4'-difluorobenzophenone (2), sodium 5,5'-carbonylbis(2-fluorobenzenesulfonate) (1) and potassium carbonate. DMSO and toluene were used to wash any residues stuck to the wall of the flask. The reaction mixture was stirred and refluxed at 150°C for 4 h to remove water by azeotropic distillation. Then the temper' dure was raised to 170°C. After 2 h, appropriate amount of DMSO was added to dilute the viscous solution. The temperature was kept at 170°C for another 4 h. Upon cooling the reaction mixture was diluted with DMF, filtered, and finally precipi- tated into 500 ml methanol. The crude product (4a-4e) was then washed with boiling water and methanol to remove resi- dual inorganic salts and solvents, filtered, and dried in a vacuum oven at 80°C for 5 days. Polymer 4f was purified by dialysis, using a cellulose acetate dialysis tube (SPEC- TRUM) with a molecular weight cut off value of 5000.

Acknowledgement: This work is supported by the National Science Foundation of China (NSFC).

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