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Metal complexation of methylthiomethyl-pendant polyimides
Fur. Polym. J. Vol. 29, No. 8, pp. 1047-1051, 1993 Printed in Great Britain. All rights reserved
0014-3057/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd
METAL COMPLEXATION OF M E T H Y L T H I O M E T H Y L - P E N D A N T POLYIMIDES WEN-YEN CHIANG* and WANG-PING MEI Department of Chemical Engineering, Tatung Institute of Technology, 40 Chungshan North Road, 3rd See. Taipei 10451, Taiwan, Republic of China (Received 26 October 1992) Abstract--Some methylthiomethyl (MTM)-pendant polyimides were allowed to react with bivalent ions of Ni, Zn, Cd and Hg. The values for metal ion uptake for these polyimides followed the order Hg(II) > Cd(II) > Zn(II) > Ni(II). The selective chelating behaviour is discussed. Tentative structures for the complexed polyimides are proposed on the basis of i.r., SEM, TGA, inherent viscosity and adsorption capacity analyses. Upon complexation with metal ions, the solubilities of the polyimides decrease while the thermal stabilities remain unchanged. Investigation of the recycling possibility for Hg(II)-complexed PI-42 shows a recyclability of 90%.
INTRODUCTION In recent years, many sulphur-containing polymers have been reported as efficient chelators for metal ions [1-3, 7]. The unique chelating property of the sulphur atom has made these polymers useful in removing harmful trace metal ions [1, 2] or in recovering rare metal ions [3]. In previous papers [4, 5], we reported the syntheses of a series of methylthiomethyl ( M T M ) - p e n d a n t polyimides. The structures of these polyimides are shown in Fig. 1. In this work, these polyimides were allowed to react with metal ions such as Ni(II), Zn(II), CD(II) and Hg(II); their sorption abilities and selectivities towards individual ions were studied. Beside adsorption capacity analysis, many procedures were followed to determine the structures of these complexed polymers, including i.r., T G A , SEM and viscosity studies. Effects of complexation on polymer properties such as solubility and thermal stability were examined. The possibility of recycling, i.e. desorption of the metal ions, was also investigated.
washed with water and methanol and then dried under vacuum. Measurements Inherent viscosities of polymers or complexed polymers in DMF were measured at 30° with an Ubbelohde viscometer. Atomic absorption spectroscopy was performed with a Varian model AA-30 atomic absorption spectrometer. Infrared spectra were obtained on a Jasco FTIR-7000 spectrophotometer. Thermal stabilities of polymers were evaluated by thermogravimetric analysis with a Du Pont 951 thermogravimetric analyser. Scanning electron micrographs were recorded on an Akashi ABT-55 microscope. Recyclability The Hg(lI)-complexed PI-42 was added to a methanolic solution of quinone and stirred for 24 hr, and then the polymer was filtered and washed with methanol. Afterwards, the quinone-absorbed polyimide was treated with saturated aqueous sodium bisulphite and stirred for 30 min. The resultant recycled polyimide was extracted with chloroform, and again subjected to complexation. RESULTS AND DISCUSSION
EXPERIMENTAL PROCEDURES
Adsorption capacity
Materials and reagents The preparations of polyimides PI-I, PI-2, PI-31 and PI-42 have been described [4, 5]. Properties of these polyimides(PIs) are shown in Table 1. Water was distilled and deionized. All other reagents and solvents were of analytical grade and were used without further purification. Complexation Complexation was carried out by adding an aqueous solution of metal chloride (0.03 M, 20 ml) to the PI solution in chloroform (0.02 g/ml, 5 ml) and shaking for 24 hr. The complexed polymer became insoluble and was collected by filtration. The concentration of metal ion in the remaining aqueous layer was determined by atomic absorption, and the adsorption capacity was evaluated from the difference between the initial and the final concentrations in the aqueous phase. The complexed polymer was repeatedly *To whom all correspondence should be addressed.
Table 2 presents the adsorption capacities for the transition metal ions of M T M - p e n d a n t polyimides. The adsorption capacities of PI-31 for Hg(II) and Cd(II) are much higher than those for the Ni(II) or Zn(II) ions. The selectivity in metal ion uptake of the polymers can be well explained by Pearson's principle [6], i.e. hard acids prefer to bind hard bases and soft acids prefer to bind soft bases. Since the pendant sulphur-containing M T M groups are soft bases and the Hg(II) and Cd(II) ions are classified as soft acids, chelation of the M T M groups with Hg(II) or Cd(II) ions is much more efficient than that with the harder Ni(II) and Zn(II) ions. The binding ability of M T M groups for Ni(II) and Zn(II) ions, with outer electronic configurations d 7 and d t° respectively, also parallels the softness of the ions, which increases with increasing availability of d electrons of the metal ions. The lowest amount of Ni(II) incorporation and hence
1047
WEN-YEN CHIANG and WANG-PING MEI
1048
H3
0
o
In
PI-I
3
c
u j n
PI-2
r (~I2SCH3
I
,
CH2
[
L CH2ScH3 %csH2c
cH
L
o Jl-
PI-31
[ CH2SCH 3 H3CSH2C
L G'I2SCH3
"I ,"
H3CSH2'C 0
-
~ J x L (]'[2SCH3 H3CSH2~ PI-42
O
~
-v -x
Fig. 1. Structures of the polyimides. the lowest complexation density accord with the observation that the PI-42 film was swollen a n d became mud-like after it was dipped in the solution of NiCI 2 (0.5 M) in 1 : 1 H 2 0 / T H F for 24 hr, whereas the dimensions o f the PI-42 film remained u n c h a n g e d after it was treated with 0.5 M solutions o f o t h e r metal chlorides in 1 : 1 H 2 0 / T H F for 24 hr. F r o m the application s t a n d p o i n t , the low selectivity for Ni a n d Z n ions can be used to separate the individual metal ions from their mixtures, such as N i - C d , N i - H g , Z n ~ : : d a n d Z n - H g ion mixtures.
Possible structure of the complex The scanning electron m i c r o g r a p h s o f the uncomplexed a n d Hg(II)-complexed PI-42 films are given in Fig. 2. The irregular pin holes on the uncomplexed PI-42 film surface result from rapid e v a p o r a t i o n o f solvent. After the PI-42 film was dipped in 0.5 M HgC12 solution in 1:1 H 2 0 / T H F for 24 hr, the pin holes became larger a n d more regular in size. Obviously, this change is caused by the release o f the strain between polymer chains. Since the strain can be
Table I. Properties of the polyimides Polymer ~i.h* Tensile strengtht Tg:~ code (dl/g) (MPa) CC) PI- 1 0.39 75 272 PI-2 0.36 81 296 PI-31 0.30 67 296 PI-42 0.35 70 337 *Measured at a concentration of 0.5 g/dl in DMF at 30". "['Averageof six specimens. ~/Measur~l by DSC at a heating rate of 10°/rain under N2. Table 2. Adsorption capacities of the polyimides for the bivalent ions of Ni, Zn, Cd and Hg Concentration
Polymer code PI-! PI-2 PI-31 PI-42
of MTM group (mmol/g) 1.84 3.31 3.31 4.50
Ion intake mmol/g (mmol/mmoI-MTM group)
Ni(II) --0.20 (0.06) --
Zn(ll) --0.80 (0.24) --
Cd(II) --2.25 (0.68) --
Hg(ll) 0.65 (0.35) 2.42 (0.73) 2.37 (0.72) 2.67 (0.59)
Metal complexation of MTM-pendant polyimides
Fig. 2. SEM of PI-42 and Hg(II) complexed PI-42.
1049
induced either from intra- or inter-polymer metal complexation, and since the inter-polymer complexation will increase while the intra-polymer complexation will decrease the viscosity of polymer solution, the viscosity of polymers before and after complexation were measured. It was found that, after complex formation with Hg(II), the inherent viscosities of PI- 1 and PI-42 dropped drastically from 0.39 and 0.35 to 0.23 and 0.21 respectively. This result means complexation proceeds in an intra-polymer manner. Furthermore, according to Table 2, the fact that two MTM groups in PI-2, PI-31 or PI-42 can absorb more than one Hg(II) ion suggests the mercury ion uses for its coordination not only MTM groups but also other ligands. This inference is in accordance with the results of thermogravimetry. As shown in Fig. 3, after the Hg(II)-complexed PIs have been heated to 150 ° for 5 min, the thermogravimetric analyses show mass losses in the range 200-300 °, which are not observed for the uncomplexed PIs. Since the initial decomposition temperatures of the pendant methylthiomethyl group and the imide ring occur at 300 and 500 ° respectively, the mass losses in the range 200-300 ° are considered to be due to the evaporation of sorbed small molecules, probably water. Examining the i.r. spectra of PI-42 and Hg(II)-complexed PI-42 shown in Fig. 4, we find that the two spectra are similar, except that the spectrum of the complex shows a broad O---H absorption peak at 3400 cm-~, indicating the presence of water molecules. So it is very possible that water molecules also are involved in coordination, and serve as ligands to saturate the coordination number required for the metal ion.
110
90
v
'G
70 PI-I A H8(II) complexed PI-1
•
• PI-2
O Hg(II) complexed PI-2 50 ¸
• PI-31
0 Hg(II) complexed PI-31 • PI-42
v H8(II ) complexed PI-42
30
I
200
i
!
400
.
Temperature
0
6 0 ('C)
Fig. 3. TG curves of PIs and Hg(II) complexed PIs.
!
800
1000
1050
WEN-YENCHIANGand WANG-PINGMEI
(a)
(b)
I 4600
I
I
I
I
I
I
2000
1500
1000
I
400
em -I
Fig. 4. Infrared spectra of PI-42 and Hg(II) comptexed PI-42.
Thermal stabilities and solubilities of the complexes Figure 3 presents the thermograms of the Hg(II)complexed and uncomplexed PIs. It is found in contrast with other reports [7] that, for our polymers, complexation does not bring any improvement in thermal stability. Again, this is indirect evidence for intra-polymer crosslinking because, if there were interchain crosslinking, a strengthened matrix and consequently a higher thermal stability would have been observed. However, when the solutions of the title PIs in chloroform were reacted with the Hg(II) ion in DMF, yellow precipitate was formed immediately. The decrease in solubility is probably due to alteration in polymer polarity and the intra-polymer crosslinking.
Recyclability The recyclability of the complexed PI was investigated by utilizing Pearson's principle. In the case of Hg(II)-complexed PI-42, another soft acid, viz. quinone, was employed to replace the chelated Hg(lI) ion. Except for its strong tendency to be bound to sulphur atoms, another reason for choosing quinone
as the recycling agent is that it can be easily reduced to hydroquinone, which is considerably harder and would probably be released from the polymer matrix readily. If so, a complete recycling system can be made. Indeed, when I g of PI-42 was complexed, recycled and subjected to recomplexation, 2.40 mmol of Hg(II) ion were absorbed. This result means that 90% recyclability is achieved. CONCLUSION
The selective complexation behaviour of the title polyimides for metal ions is discussed. The existence of an intra-polymer complexed structure is demonstrated by SEM and viscosity studies. Investigating the sorption ability oftbe PIs as well as the i.r. spectra and TG curves of the complexes leads to the conclusion that water molecules which are not directly bound to the polymers are also utilized as coordinate ligands of the metal ions. On complexation, solubility decreases while thermal stability is not improved. Applying the quinone-hydroquinone recycling process to Hg(II)-complexed PI-42, we achieved 90% recyclability.
Metal complexation of MTM-pendant polyimides
Acknowledgements--The authors express their gratitude to Dr T. S. Lin, President of Tatung Institute of Technology, for his encouragement and support. Thanks are also due to the National Science Council of the Republic of China for financial support under Contract No. NSC79-0405-E036-02 and NSC80-0405-E036-03. REFERENCES
1. M. M. Jones, H. D. Coble and T. H. Pratt. J. lnorg. Nucl. Chem. 37, 2409 (1975). 2. B. Mathew and V. N. R. Pillai. Polym. Int. 28, 201 (1992).
1051
3. Y. Xu and Y. Yang. Wuhan Daxue Xuebao, Ziran Kexueban 1, 63 (1991); Chem. Abstr. 116 (1992) 84763a. 4. W. Y. Chiang and W. P. Mei. Tetrahedron Lett. (in press). 5. W. Y. Chiang and W. P. Mei. J. Polym. Sci., Part A: Polym. Chem. (in press). 6. J. E. Huheey. Inorganic Chemistry: Principles of Structure and Reactivity, 2nd Edn, p. 278. Harper & Row, New York (1978). 7. Y. Yagci, S. Denizligil, N. Bicak and T. Atay. Angew. Makromolek. Chem. 195, 89 (1992).
0014-3057/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd
METAL COMPLEXATION OF M E T H Y L T H I O M E T H Y L - P E N D A N T POLYIMIDES WEN-YEN CHIANG* and WANG-PING MEI Department of Chemical Engineering, Tatung Institute of Technology, 40 Chungshan North Road, 3rd See. Taipei 10451, Taiwan, Republic of China (Received 26 October 1992) Abstract--Some methylthiomethyl (MTM)-pendant polyimides were allowed to react with bivalent ions of Ni, Zn, Cd and Hg. The values for metal ion uptake for these polyimides followed the order Hg(II) > Cd(II) > Zn(II) > Ni(II). The selective chelating behaviour is discussed. Tentative structures for the complexed polyimides are proposed on the basis of i.r., SEM, TGA, inherent viscosity and adsorption capacity analyses. Upon complexation with metal ions, the solubilities of the polyimides decrease while the thermal stabilities remain unchanged. Investigation of the recycling possibility for Hg(II)-complexed PI-42 shows a recyclability of 90%.
INTRODUCTION In recent years, many sulphur-containing polymers have been reported as efficient chelators for metal ions [1-3, 7]. The unique chelating property of the sulphur atom has made these polymers useful in removing harmful trace metal ions [1, 2] or in recovering rare metal ions [3]. In previous papers [4, 5], we reported the syntheses of a series of methylthiomethyl ( M T M ) - p e n d a n t polyimides. The structures of these polyimides are shown in Fig. 1. In this work, these polyimides were allowed to react with metal ions such as Ni(II), Zn(II), CD(II) and Hg(II); their sorption abilities and selectivities towards individual ions were studied. Beside adsorption capacity analysis, many procedures were followed to determine the structures of these complexed polymers, including i.r., T G A , SEM and viscosity studies. Effects of complexation on polymer properties such as solubility and thermal stability were examined. The possibility of recycling, i.e. desorption of the metal ions, was also investigated.
washed with water and methanol and then dried under vacuum. Measurements Inherent viscosities of polymers or complexed polymers in DMF were measured at 30° with an Ubbelohde viscometer. Atomic absorption spectroscopy was performed with a Varian model AA-30 atomic absorption spectrometer. Infrared spectra were obtained on a Jasco FTIR-7000 spectrophotometer. Thermal stabilities of polymers were evaluated by thermogravimetric analysis with a Du Pont 951 thermogravimetric analyser. Scanning electron micrographs were recorded on an Akashi ABT-55 microscope. Recyclability The Hg(lI)-complexed PI-42 was added to a methanolic solution of quinone and stirred for 24 hr, and then the polymer was filtered and washed with methanol. Afterwards, the quinone-absorbed polyimide was treated with saturated aqueous sodium bisulphite and stirred for 30 min. The resultant recycled polyimide was extracted with chloroform, and again subjected to complexation. RESULTS AND DISCUSSION
EXPERIMENTAL PROCEDURES
Adsorption capacity
Materials and reagents The preparations of polyimides PI-I, PI-2, PI-31 and PI-42 have been described [4, 5]. Properties of these polyimides(PIs) are shown in Table 1. Water was distilled and deionized. All other reagents and solvents were of analytical grade and were used without further purification. Complexation Complexation was carried out by adding an aqueous solution of metal chloride (0.03 M, 20 ml) to the PI solution in chloroform (0.02 g/ml, 5 ml) and shaking for 24 hr. The complexed polymer became insoluble and was collected by filtration. The concentration of metal ion in the remaining aqueous layer was determined by atomic absorption, and the adsorption capacity was evaluated from the difference between the initial and the final concentrations in the aqueous phase. The complexed polymer was repeatedly *To whom all correspondence should be addressed.
Table 2 presents the adsorption capacities for the transition metal ions of M T M - p e n d a n t polyimides. The adsorption capacities of PI-31 for Hg(II) and Cd(II) are much higher than those for the Ni(II) or Zn(II) ions. The selectivity in metal ion uptake of the polymers can be well explained by Pearson's principle [6], i.e. hard acids prefer to bind hard bases and soft acids prefer to bind soft bases. Since the pendant sulphur-containing M T M groups are soft bases and the Hg(II) and Cd(II) ions are classified as soft acids, chelation of the M T M groups with Hg(II) or Cd(II) ions is much more efficient than that with the harder Ni(II) and Zn(II) ions. The binding ability of M T M groups for Ni(II) and Zn(II) ions, with outer electronic configurations d 7 and d t° respectively, also parallels the softness of the ions, which increases with increasing availability of d electrons of the metal ions. The lowest amount of Ni(II) incorporation and hence
1047
WEN-YEN CHIANG and WANG-PING MEI
1048
H3
0
o
In
PI-I
3
c
u j n
PI-2
r (~I2SCH3
I
,
CH2
[
L CH2ScH3 %csH2c
cH
L
o Jl-
PI-31
[ CH2SCH 3 H3CSH2C
L G'I2SCH3
"I ,"
H3CSH2'C 0
-
~ J x L (]'[2SCH3 H3CSH2~ PI-42
O
~
-v -x
Fig. 1. Structures of the polyimides. the lowest complexation density accord with the observation that the PI-42 film was swollen a n d became mud-like after it was dipped in the solution of NiCI 2 (0.5 M) in 1 : 1 H 2 0 / T H F for 24 hr, whereas the dimensions o f the PI-42 film remained u n c h a n g e d after it was treated with 0.5 M solutions o f o t h e r metal chlorides in 1 : 1 H 2 0 / T H F for 24 hr. F r o m the application s t a n d p o i n t , the low selectivity for Ni a n d Z n ions can be used to separate the individual metal ions from their mixtures, such as N i - C d , N i - H g , Z n ~ : : d a n d Z n - H g ion mixtures.
Possible structure of the complex The scanning electron m i c r o g r a p h s o f the uncomplexed a n d Hg(II)-complexed PI-42 films are given in Fig. 2. The irregular pin holes on the uncomplexed PI-42 film surface result from rapid e v a p o r a t i o n o f solvent. After the PI-42 film was dipped in 0.5 M HgC12 solution in 1:1 H 2 0 / T H F for 24 hr, the pin holes became larger a n d more regular in size. Obviously, this change is caused by the release o f the strain between polymer chains. Since the strain can be
Table I. Properties of the polyimides Polymer ~i.h* Tensile strengtht Tg:~ code (dl/g) (MPa) CC) PI- 1 0.39 75 272 PI-2 0.36 81 296 PI-31 0.30 67 296 PI-42 0.35 70 337 *Measured at a concentration of 0.5 g/dl in DMF at 30". "['Averageof six specimens. ~/Measur~l by DSC at a heating rate of 10°/rain under N2. Table 2. Adsorption capacities of the polyimides for the bivalent ions of Ni, Zn, Cd and Hg Concentration
Polymer code PI-! PI-2 PI-31 PI-42
of MTM group (mmol/g) 1.84 3.31 3.31 4.50
Ion intake mmol/g (mmol/mmoI-MTM group)
Ni(II) --0.20 (0.06) --
Zn(ll) --0.80 (0.24) --
Cd(II) --2.25 (0.68) --
Hg(ll) 0.65 (0.35) 2.42 (0.73) 2.37 (0.72) 2.67 (0.59)
Metal complexation of MTM-pendant polyimides
Fig. 2. SEM of PI-42 and Hg(II) complexed PI-42.
1049
induced either from intra- or inter-polymer metal complexation, and since the inter-polymer complexation will increase while the intra-polymer complexation will decrease the viscosity of polymer solution, the viscosity of polymers before and after complexation were measured. It was found that, after complex formation with Hg(II), the inherent viscosities of PI- 1 and PI-42 dropped drastically from 0.39 and 0.35 to 0.23 and 0.21 respectively. This result means complexation proceeds in an intra-polymer manner. Furthermore, according to Table 2, the fact that two MTM groups in PI-2, PI-31 or PI-42 can absorb more than one Hg(II) ion suggests the mercury ion uses for its coordination not only MTM groups but also other ligands. This inference is in accordance with the results of thermogravimetry. As shown in Fig. 3, after the Hg(II)-complexed PIs have been heated to 150 ° for 5 min, the thermogravimetric analyses show mass losses in the range 200-300 °, which are not observed for the uncomplexed PIs. Since the initial decomposition temperatures of the pendant methylthiomethyl group and the imide ring occur at 300 and 500 ° respectively, the mass losses in the range 200-300 ° are considered to be due to the evaporation of sorbed small molecules, probably water. Examining the i.r. spectra of PI-42 and Hg(II)-complexed PI-42 shown in Fig. 4, we find that the two spectra are similar, except that the spectrum of the complex shows a broad O---H absorption peak at 3400 cm-~, indicating the presence of water molecules. So it is very possible that water molecules also are involved in coordination, and serve as ligands to saturate the coordination number required for the metal ion.
110
90
v
'G
70 PI-I A H8(II) complexed PI-1
•
• PI-2
O Hg(II) complexed PI-2 50 ¸
• PI-31
0 Hg(II) complexed PI-31 • PI-42
v H8(II ) complexed PI-42
30
I
200
i
!
400
.
Temperature
0
6 0 ('C)
Fig. 3. TG curves of PIs and Hg(II) complexed PIs.
!
800
1000
1050
WEN-YENCHIANGand WANG-PINGMEI
(a)
(b)
I 4600
I
I
I
I
I
I
2000
1500
1000
I
400
em -I
Fig. 4. Infrared spectra of PI-42 and Hg(II) comptexed PI-42.
Thermal stabilities and solubilities of the complexes Figure 3 presents the thermograms of the Hg(II)complexed and uncomplexed PIs. It is found in contrast with other reports [7] that, for our polymers, complexation does not bring any improvement in thermal stability. Again, this is indirect evidence for intra-polymer crosslinking because, if there were interchain crosslinking, a strengthened matrix and consequently a higher thermal stability would have been observed. However, when the solutions of the title PIs in chloroform were reacted with the Hg(II) ion in DMF, yellow precipitate was formed immediately. The decrease in solubility is probably due to alteration in polymer polarity and the intra-polymer crosslinking.
Recyclability The recyclability of the complexed PI was investigated by utilizing Pearson's principle. In the case of Hg(II)-complexed PI-42, another soft acid, viz. quinone, was employed to replace the chelated Hg(lI) ion. Except for its strong tendency to be bound to sulphur atoms, another reason for choosing quinone
as the recycling agent is that it can be easily reduced to hydroquinone, which is considerably harder and would probably be released from the polymer matrix readily. If so, a complete recycling system can be made. Indeed, when I g of PI-42 was complexed, recycled and subjected to recomplexation, 2.40 mmol of Hg(II) ion were absorbed. This result means that 90% recyclability is achieved. CONCLUSION
The selective complexation behaviour of the title polyimides for metal ions is discussed. The existence of an intra-polymer complexed structure is demonstrated by SEM and viscosity studies. Investigating the sorption ability oftbe PIs as well as the i.r. spectra and TG curves of the complexes leads to the conclusion that water molecules which are not directly bound to the polymers are also utilized as coordinate ligands of the metal ions. On complexation, solubility decreases while thermal stability is not improved. Applying the quinone-hydroquinone recycling process to Hg(II)-complexed PI-42, we achieved 90% recyclability.
Metal complexation of MTM-pendant polyimides
Acknowledgements--The authors express their gratitude to Dr T. S. Lin, President of Tatung Institute of Technology, for his encouragement and support. Thanks are also due to the National Science Council of the Republic of China for financial support under Contract No. NSC79-0405-E036-02 and NSC80-0405-E036-03. REFERENCES
1. M. M. Jones, H. D. Coble and T. H. Pratt. J. lnorg. Nucl. Chem. 37, 2409 (1975). 2. B. Mathew and V. N. R. Pillai. Polym. Int. 28, 201 (1992).
1051
3. Y. Xu and Y. Yang. Wuhan Daxue Xuebao, Ziran Kexueban 1, 63 (1991); Chem. Abstr. 116 (1992) 84763a. 4. W. Y. Chiang and W. P. Mei. Tetrahedron Lett. (in press). 5. W. Y. Chiang and W. P. Mei. J. Polym. Sci., Part A: Polym. Chem. (in press). 6. J. E. Huheey. Inorganic Chemistry: Principles of Structure and Reactivity, 2nd Edn, p. 278. Harper & Row, New York (1978). 7. Y. Yagci, S. Denizligil, N. Bicak and T. Atay. Angew. Makromolek. Chem. 195, 89 (1992).