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Chapter 15 Polymers containing heterocycles in the backbone
641
CHAPTER 15
Polymers Containing Heterocycles in the Backbone 15.1
POLYMERS WITH VARIOUS HETEROCYCLES IN THE MAIN CHAIN
- General aspects
The diversity of polymers containing heterocyclic groups is quite large. These polymers may include macromolecules generated from the polymerization of a unique monomer, such as poly(furfuryl alcohol), CAS# 25212-86-6, with the formula shown below:
/~o~CH~#H2~-~CH# OH or they may include bi- or poly-functional heterocyclic groups together with aromatic and/or aliphatic fragments. The result is that some polymers from this class have a simple and uniform composition, such as poly(thiophene-2,5-diyl). Other polymers may have a more complicated structure. Among these are poly(quinoxalines), poly(benzimidazoles, poly(benzoxaxoles), poly(oxadiazoles), poly(triazines), etc. A number of imides containing a pyrroline-2,5-dione ring (maleimide) also can be considered as macromolecules containing heterocycles. However, these polymers were included in the class of imides and were discussed in Section 13.4. Examples of polymers with heterocycles in their structures (not including maleimide polymers) are shown below:
.........
NH"- ~
~Rb ....
Of ~
poly(benzimidazoles) ......Rb~.,......,~,.. i N
'........l . . s N y
poly(benzoxazoles)
poly(oxadiazoles) Rb. ~...N.
~.~
Rb,....
L~N/N
~RI~ ....
N~ ~ / R b , , ,
-'HN
/~O--~~R b.....
poly(asymm,triazines)
Ra
.~
...N~y..,...R,b
NHf
poly(bis-benzimidazoles)
poly(bis-quinoxalines)
where R a is nought, OH2, O, phenylene, etc. and Rb is phenylene, oxydiphenyl, or more complicated fragments. These polymers are synthesized by different procedures, such as condensations of the type: O.
OH
R a x ~ NH2+ OZRb - - . v
--NH2
Ra
NH
~
O
"~"
+2H20
Polymers containing heterocycles in the backbone
642
which leads to 1,2-dihydroquinoxalin-2-one derivatives. Other polymers are generated by reactions in more steps, such as the derivatives of poly(parabanic acid), which can be obtained by the following chain of reactions: O
O
II
O=CN--Rb--NC=O
+ HCN
o II
---"
~_
o
II
C
~1~
+ O=CN--Rb--NC=O
o
o II
NC--C--HN--Rb
II
L
NC--C--HN---RD--NH--C--CN
II
........R b ~ N / C ~ N - - R I ~ ..... + H20 ........Rb~NJC~N__Rb, .....
~NH--Rb--NC=O
I O~----C--CN
~
o--c
I
I
~
C"~NH
- NH3
I
O--C
I
C~o
Some polymers containing heterocycles can be included in resins following further condensation. This is possible, for example, for poly(furfuryl alcohol), which can condense with formaldehyde, phenol, melamine and urea. Furfuryl alcoholformaldehyde copolymer can be synthesized in the reaction of furfuryl alcohol with formaldehyde in the presence of oxalic acid as catalyst. Depending on their structure, the polymers containing heterocycles have various applications. For example, poly(furfuryl alcohol) is used in composite materials with fillers such as sand and concrete, in copolymers with formaldehyde, etc. Some of the polymers from this group have special properties such as good electrical conductivity (after appropriate doping). Among these polymers are poly(thiophene-2,5-diyl) and particularly polypyrrole, CAS# 109-97-7, (usually in carbon black doped with an organic acid anion). The structure of this polymer is shown below: H
H
....
Ix-
H
I
H
X-
X" = organic sulfonic acid dopant anion
The increase in the size of the units from the polymeric backbone was shown to diminish the flexibility of the macromolecular chain, which has as a result an increase in the melting point. The increase in the melting point of a polymer is paralleled by the increase in its thermal stability. Stiffening the polymeric chain by the inclusion of ring structures, possibly with fused-ring groups, leads to the polymers with higher thermal stability. Typical ring systems that have been introduced into polymer chains (as diyl groups) include benzene, diazine, triazine, triazole, thiazole, oxadiazole, isoindoline-l,3dione, 3-pyrrolino[3,4-f]isoindoline-l,3,5,7-tetraone, etc. Also, multiple bonds in the backbone, as in ladder polymers, increase thermal stability (see Section 3.1). Following these ideas a series of polymers with heterocyclic groups were synthesized, leading to good thermal resistance. A few examples are shown in Table 15.1.1.
Polymers containing heterocycles in the backbone TABLE 15.1.1. Polymers with elevated decomposition temperature containing
heterocycles in the backbone.
643
644
Polymers containing heterocycles in the backbone
TABLE 15.1.1 (continued). Polymers with elevated decomposition temperature containing heterocycles in the backbone.
FIGURE 15.1.1. Variation of weight % loss for poly(thiophene-2, 5-diyl) in a TGA experiment at a heating rate of 10~ C/min.
Polymers containing heterocycles in the backbone
645
The TGA shows that the polymer starts to lose some weight around 400 ~ C, and the decomposition accelerates above 500 o C. Volatile product may include H2S (see flash pyrolysis results) and oxidation products such as SO2. - Homopolymers and alt-copolymers containing heterocyclic groups
The thermal decomposition of the polymers containing heterocyclic groups was studied frequently for practical purposes [1-3]. A number of polymers from this class, having good thermal stability, are used in applications such as preparation of adhesives with high temperature resistance, manufacturing of high performance composites used in the aerospace industry, etc. Other polymers are used in photolithographic industry. Some of the thermal stability studies on polymers containing heterocyclic groups, as reported in literature, are summarized in Table 15.1.2 [4]. TABLE 15.1.2. Summary regarding literature information on thermal decomposition of polymers containing heterocyclic groups [4].
646
Polymers containing heterocycles in the backbone
TABLE 15.1.2 (continued). Summary regarding literature information on thermal
decomposition of polymers containing heterocyclic groups [4].
where Ar represents an aromatic moiety. Pyrolysis at 650 ~ C of several such polymers generated specific small molecules. For example, for Ar = 1,4-penylene the pyrolysate contained phenol; for Ar = 1,3-phenylene the pyrolysate contained phenol and resorcinol; for Ar = 4,4'-biphenyl the pyrolysate contained 4-phenylphenol and 4,4'bisphenol; for Ar = 2,7-naphthalene the pyrolysate contained 2-naphthol, 2,7dihydroxynapthalene, and naphthalene; for Ar = bis(4-phenylene) ketone the pyrolysate contained phenol and CO2; for Ar = bis(4-phenylene) sulfone the pyrolysate contained phenol and SO2; for Ar = bis(4-phenylene)isopropane the pyrolysate contained phenol, isopropylphenol, and propane; for Ar = bis(4-phenylene)fluorene the pyrolysate contained phenol, fluorene, and phenylfluorene; and for Ar = bis(4-phenylene) sulfide the pyrolysate contained phenol, H2S, and thiophenol [20]. Thermal properties of some other polymers containing heterocycles are reported in literature. For example, the polymers containing triazine networks substituted with perfluoro-n-propyl or perfluoroalkyloxy groups do not decompose easily upon heating. These types of polymers are used as heat resistant oils, and several studies on their thermal decomposition are available in literature [21-23]. The results for a Py-GC/MS analysis of poly(thiophene-2,5-diyl) Br terminated, CAS # 110-02-1, are shown in Figure 15.1.2. Since the polymer is rather resistant to heat, pyrolysis was done at 850 ~ C in He. Other conditions were kept similar to those for other examples previously described (see Table 4.2.2). The heating rate was kept 20 ~ C/ms and THT 10 s. The separation was done on a Carbowax column (60 m, 0.32 mm i.d., 0.32 ~m film thickness) with the GC starting at 40 o C with a ramp of 2 o C/min. up to 240 ~ C and a final oven time of 20 min. The MS was operated in El+ mode. The peak
Polymers containing heterocycles in the backbone
647
identification was done using MS spectral library searches only and is given in Table 15.1.3.
648
Polymers containing heterocycles in the backbone
TABLE 15.1.3 (continued). Compounds identified in the pyrogram of a poly(thiophene2, 5-diyl) Br terminated sample as shown in Figure 15.1.2.
Hydrogen sulfide is the main decomposition product seen from the pyrolysis of poly(thiophene-2,5-diyl). Some 2,2'-bithiophene (17.4% of pyrolysate) and only a small proportion of thiophene are generated (less than 5% of pyrolysate). However, the pyrolysis in He also forms char, which is not volatile and cannot be seen in the pyrogram. The bonds that appear to cleave more easily are the S-C bonds and the bonds between the thiophene units (C-C type). Since the hydrogen content of the polymer is low, the formation of SH2 is associated with the formation of char. The elimination of some carbon and sulfur as CS2 or Sx explains the formation of benzene, thiophene, etc. Mechanical properties of poly(thiophene-2,5-diyl) are not suitable for many practical uses. Polymers with higher flexibility and still good thermal resistance or with other special conductivity properties (after doping) can be obtained from the polymerization of substituted polythiophenes. Some examples are shown below:
poly(3-hexylthiophene-2,5-diyl)
poly(3,4-ethylenedioxythiophene)
poly(3-octyl-2,2'-dithiophene)
Thermal decomposition of poly(3-hexylthiophene-2,5-diyl) is described by its TGA curve shown in Figure 15.1.3. The thermogram was obtained from a sample with Mw = 87,000 in air, by heating between 30 ~ C and 830 ~ C at a rate of 10~ C/min.
Polymers containing heterocycles in the backbone
649
FIGURE 15.1.3. Variation of weight % loss for poly(3-hexylthiophene-2,5-diyl) Mw = 87, 000 in a TGA experiment at a heating rate of 10~ C/min. The hexyl groups represent about 50% of the weight of the polymer. However, the weight loss between 400 ~ C and 500 ~ C is larger than 50%, indicating that the structure of the polymer is affected by heat not only through the elimination of the side chain groups, but also including the backbone modification. The char remaining at temperatures higher than 500 ~ C is oxidized in air with formation of oxides of carbon and sulfur. The results for a Py-GC/MS analysis of a poly(3-hexylthiophene-2,5-diyl) sample, CAS # 104934-50-1, (regioregular) with Mw = 87,000 are shown in Figure 15.1.4. Since the polymer is rather resistant to heat, pyrolysis was done at 850 ~ C in He. Other conditions were kept similar to those for other examples previously described (see Table 4.2.2). The separation was done on a Carbowax column. The peak identification was done using MS spectral library searches only and is given in Table 15.1.4.
FIGURE 15.1.4. Pyrogram from a Py-GC/MS analysis of a poly(3-hexyithiophene-2,5diyl) sample, regioregular, Mw = 87, 000. Pyrolysis done on O.4 mg material at 850 ~ C in He, with the separation on a Carbowax type column.
650
Polymers containing heterocycles in the backbone
TABLE 15.1.4. Compounds identified in the pyrogram of a poly(3-hexylthiophene-2,5diyl), Mw = 87, 000 sample as shown in Figure 15.1.4.
As seen from the pyrogram and from the pyrolysate composition shown in Table 15.1.4, the main components are generated from the hexyl side chain. Some other pyrolysis
Polymers containing heterocycles in the backbone
651
products are similar to those for poly(thiophene-2,5-diyl). Char is also formed in the pyrolysis of this polymer. Among the heterocyclic compounds that were studied by Py-GC/MS are also a number of polybenzimidazopyrrolones [24]. The synthesis of these compounds has been achieved based on the following reactions:
Polymers containing heterocycles in the backbone
652
The results shown in Table 15.1.5 indicate that a considerable proportion of bond cleavage occurs at the extender connection, and also that the extender influences the thermal stability of the polymer. Regarding the decomposition of the heterocyclic moiety, the ether bond between the two isoindolino[2,1-a]benzimidazol-11-one units and the C(O)-N bond from the pyrrolydin-2-one ring are likely to cleave more easily than other parts of the molecule. In a different study [25] a series of poly[phenyl-(asymm, triazines)] were analyzed by PyGC/MS. The following types of polymers were synthesized:
AF =
The polymers were resistant to heat, and the decomposition was minimal below 6000 C. For this reason, the pyrolysis was done at four temperatures, 650 ~ C, 750 o C, 850 ~ C, and 950 ~ C, followed by GC/MS analysis. For some compounds such as benzene, the yield in the pyrolysate increased as temperature increased, but for other compounds, either the yield had a maximum in the range or even slightly decreased. Among the compounds determined in the pyrolysate were some common for all polymers such as benzene, benzonitrile, benzylamine, 1,4-benzenedicarbonitrile, and phenanthridine. Other compounds in the pyrolysate were specific for the aromatic extender Ar. Among these were 1H-benzotriazole, 1-phenylphthalazine, 9-anthracenecarbonitrile, o~aminobenzeneacetonitrile, 4-phenoxybenzonitrile, 4,4'-oxydibenzonitrile, 1-hydroxy-4phenoxybenzonitrile, 4-benzylphenylcarbonitrile, 4-phenoxy-4'-phenoxybenzonitrile etc. The degradation mechanism of these polymers starts with a random homolytic cleavage of the N-N and C-N bonds in the triazine ring. This leads to the formation of benzonitrile radicals and p-dicyanobenzene radicals that further propagate the decomposition. The termination occurs with various recombinations and rearrangements.
- Copolymers containing heterocyclic groups
Several copolymers with heterocycles studied using pyrolytic techniques include poly[pyrrole-graft-(2-N-pyrrolyl)ethylvinylether] [26] and several polypyrrole/ pol~etrahydrofuran graft copolymers including pyrrole ended-pol~etrahydrofuran, p-toluene sulfonate doped polypyrrole, and p-toluene sulfonate doped polypyrrole-copol~etrahydrofuran [27]. The data showed that pyrrole ended-pol~etrahydrofuran degrades via mixed random cleavage and unzipping mechanism followed by hydrogen transfer reactions. Cleavage of the pyrrole ring and a lack of high mass fragments are noticed for the doped polymers.
Polymers containing heterocycles in the backbone
653
References 15.1
1. R. Sanchez. C. Hernandez, J. Jalovszky, G. Czira, Eur. Polym. J., 30 (1994) 37. 2. H. J. O'Neill, J. Putscher, R. E. Dynako, C. I. Boquist, J. Gas Chromatogr., 1 (1963) 28. 3. T. Kojima, H. Takaku, Y. Urata, K. Gotoh, J. Appl. Polym. Sci., 48 (1993) 1395. 4. J. Liggat, in J. Brandrup, E. H. Immergut, E. A. Grulke, eds., PolymerHandbook, J. Wiley, New York, 2000. 5. N. A. Lapina, V. S. Ostrovskii, I. V. Kamenskii, Vysokomol. Soedin., Ser. A, 11 (1969) 2073. 6. A. T. Radcliffe, T. J. Lens, Kunststoffe, 63 (1973) 854. 7. A. A. Berlin, V. V. Yarkina, A. P. Firsov, Polym. Sci. USSR, 10 (1968) 2219. 8. R. Kiebooms, A. Aleshin, K. Hutchison, F. Wudl, J. Phys. Chem. B, 101 (1997) 11037. 9. P. Selsbo, I. Ericsson, Polym. Degrad. Stab., 51 (1996)83. 10. N. R. Lemer, J. Polym. Sci., Chem. Ed., 15 (1977) 1145. 11. G. Camino, M. P. Luda, L. Costa, M. Guaita, Macromol. Chem. Phys., 197 (1996)41. 12. D. A. Chatfield, I. N. Einhom, J. Polym. Sci., Chem. Ed., 19 (1981) 601. 13. J. R Luongo, H. Schonhom, J. Polym. Sci., Chem. Ed., 13 (1975) 1363. 14. A. A. Caraculacu, G. Caraculacu, Makromol. Chem., 185 (1984) 1079. 15. J. L. Cotter, H. Dickinson, G. J. Knight, W. W. Wright, J. Appl. Polym. Sci., 15 (1971) 317. 16. K. Erdmann, W. Czerwinski, B. C. Gerstein, M. Pruski, J. Polym. Sci., Polym. Phys. Ed., 32 (1994) 1799. 17. D. A Scola, J. H. Vontell, CHEMTECH, 19 (1989) 112. 18. N. Qian, Q. Zha, S. C. Moldoveanu (unpublished results). 19. H. -J. Dussel, A. Recca, J. Kolb, D. O. Hummel, J. K. Stile, J. Anal. Appl. Pyrol., 3 (1982) 307. 20. M. Blazso, E, Jakab, J. Anal. Appl. Pyrol., 11 (1987) 245. 21. V. A. Ponomarenko, V. N. Shilgayev, A. G. Kechina, A. A. Yarosh, S. P. Krukovskii, Polym. Sci. USSR, 16 (1974) 637. 22. L. A. Wall, S. Straus, J. Res. Natl. Bur,. Std., 65A (1961) 227. 23. R. H. Mobbs, F. Heatley, C. Price, C. Booth, Prog. Rubber Plast. Technol., 11 (1995) 94. 24. Z. Jiang, X. Jin, X. Gao, W. Zhou, F. Lu, Y. Luo, J. Anal. Appl. Pyrol., 33 (1995) 231. 25. Z. Jiang, Y. Luo, X. Jin, F. Lu, J. Anal. Appl. Pyrol., 26 (1993) 145. 26. T. Uyar, L. Toppare, J. Hacaloglu. J. Anal. Appl. Pyrol., 68 (2003) 15. 27. C. Ozdilek, L. Toppare, Y. Yagci, J. Hacaloglu, J. Anal. Appl. Pyrol., 64 (2002) 363.
15.2
LADDER TYPE POLYMERS WITH HETEROCYCLIC STRUCTURE
- General aspects
Ladder polymers have their backbone made from an interrupted series of condensed rings. In such polymers, very frequently the rings contain heteroatoms, and the polymer can be considered as part of the class of polymers with heterocycles in the main chain. One of the first synthesized polymers from this class was obtained from the oxidation and heating of polyacrylonitrile [1, 2]. Multifunctional condensations also can lead to ladder polymers. For example, a poly(phenoxazine) is formed from the reaction of a substituted quinone and a diaminodihydroxybenzene as shown below:
CH3OOI IOOOH: H
H2N"
O
~ ~
NH2 'OH
. H20 y - CH3COOH
Polymers from various other groups have a ladder type backbone. Most of these polymers have good thermal resistance since they frequently contain aromatic bonds
Polymers containing heterocycles in the backbone
654
and more than one bond must be cleaved in order to fragment the macromolecule. However, the reported decomposition temperature may vary from study to study, and it depends on the heating conditions and the choice for the temperature value at which a decomposition is considered as beginning. References 15.2
1. T. H. Ko, J. Appl. Polym. Sci., 47 (1993) 707. 2. Y. H. Bang, S. Lee, H. H. Cho, J. Appl. Polym. Sci., 68 (1998) 2205.
15.3
THERMOSETTING MELAMINE RESINS
- General aspects
Similar to urea (see Section 13.2), melamine can react with formaldehyde forming a thermosetting resin. Melamine is a trimer of cyanamide and contains three amino groups that offer six possible points of reaction. Since melamine can be considered a triamine of 1,3,5-triazine, this group of resins can be included in the class of polymers with heterocycles in their structure. The first stage of the reaction of melamine with formaldehyde can be written as follows:
NH2
NH-CH2OH
NH-CH2OH
NH-CH2OH
'~ ] 1 + HCH=O = I I H2N//"~N~NH2 HOCH2-HN/~N ~/~'NH_CH20H H2N/~N I~NH-CH2OH H2N/~N I~NH 2 Further elimination of water between the methylol groups and amino groups or among themselves generates a thermosetting resin. Similarly to phenol formaldehyde or urea formaldehyde resins, the condensation reaction for melamine is typically carried out in two stages, the resulting material from the first phase having a large number of methylol groups and being still water soluble. In the second stage, the water elimination leads to a crosslinked material. The two-stage process is necessary for handling the resin during the manufacturing of the desired objects. Both triazine- and urethane-linked networks are highly resistant to paraffin and silicone oils. Triazine-linked networks are resistant to a wide range of organic liquids, whereas urethane-linked networks are attacked by polar organic solvents. Both types are easily attacked by fluorinated species [1]. The Py-GC/MS result for a sample of poly(melamine-co-formaldehyde) methylated, with the molar ratio melamine/formaldehyde/methoxy 1/5.8/5.0, is given in Figure 15.3.1. The polymer has CAS# 68002-20-0 and Mn = 511. The pyrolysis was done at 6000 C in He at a heating rate of 20 ~ C/ms. The separation was done on a Carbowax column in conditions described in Table 4.2.2. The peak identification for the chromatogram was done using MS spectral library searches only, and it is given in Table 15.3.1.
Polymers containing heterocycles in the backbone
655
Results from Table 15.3.1 indicate that among the main decomposition products of these polymers are methanol, CO2, formaldehyde, and dimethoxymethane. A few nitrogenous compounds also are detected. Some of the decomposition products may be too polar to be detected in the experimental conditions used for the analysis.
References 15.3
1. R. H. Mobbs, F. Heatley, C. Price, C. Booth, Prog. Rubber Plast. Technol., 11 (1995) 94.
CHAPTER 15
Polymers Containing Heterocycles in the Backbone 15.1
POLYMERS WITH VARIOUS HETEROCYCLES IN THE MAIN CHAIN
- General aspects
The diversity of polymers containing heterocyclic groups is quite large. These polymers may include macromolecules generated from the polymerization of a unique monomer, such as poly(furfuryl alcohol), CAS# 25212-86-6, with the formula shown below:
/~o~CH~#H2~-~CH# OH or they may include bi- or poly-functional heterocyclic groups together with aromatic and/or aliphatic fragments. The result is that some polymers from this class have a simple and uniform composition, such as poly(thiophene-2,5-diyl). Other polymers may have a more complicated structure. Among these are poly(quinoxalines), poly(benzimidazoles, poly(benzoxaxoles), poly(oxadiazoles), poly(triazines), etc. A number of imides containing a pyrroline-2,5-dione ring (maleimide) also can be considered as macromolecules containing heterocycles. However, these polymers were included in the class of imides and were discussed in Section 13.4. Examples of polymers with heterocycles in their structures (not including maleimide polymers) are shown below:
.........
NH"- ~
~Rb ....
Of ~
poly(benzimidazoles) ......Rb~.,......,~,.. i N
'........l . . s N y
poly(benzoxazoles)
poly(oxadiazoles) Rb. ~...N.
~.~
Rb,....
L~N/N
~RI~ ....
N~ ~ / R b , , ,
-'HN
/~O--~~R b.....
poly(asymm,triazines)
Ra
.~
...N~y..,...R,b
NHf
poly(bis-benzimidazoles)
poly(bis-quinoxalines)
where R a is nought, OH2, O, phenylene, etc. and Rb is phenylene, oxydiphenyl, or more complicated fragments. These polymers are synthesized by different procedures, such as condensations of the type: O.
OH
R a x ~ NH2+ OZRb - - . v
--NH2
Ra
NH
~
O
"~"
+2H20
Polymers containing heterocycles in the backbone
642
which leads to 1,2-dihydroquinoxalin-2-one derivatives. Other polymers are generated by reactions in more steps, such as the derivatives of poly(parabanic acid), which can be obtained by the following chain of reactions: O
O
II
O=CN--Rb--NC=O
+ HCN
o II
---"
~_
o
II
C
~1~
+ O=CN--Rb--NC=O
o
o II
NC--C--HN--Rb
II
L
NC--C--HN---RD--NH--C--CN
II
........R b ~ N / C ~ N - - R I ~ ..... + H20 ........Rb~NJC~N__Rb, .....
~NH--Rb--NC=O
I O~----C--CN
~
o--c
I
I
~
C"~NH
- NH3
I
O--C
I
C~o
Some polymers containing heterocycles can be included in resins following further condensation. This is possible, for example, for poly(furfuryl alcohol), which can condense with formaldehyde, phenol, melamine and urea. Furfuryl alcoholformaldehyde copolymer can be synthesized in the reaction of furfuryl alcohol with formaldehyde in the presence of oxalic acid as catalyst. Depending on their structure, the polymers containing heterocycles have various applications. For example, poly(furfuryl alcohol) is used in composite materials with fillers such as sand and concrete, in copolymers with formaldehyde, etc. Some of the polymers from this group have special properties such as good electrical conductivity (after appropriate doping). Among these polymers are poly(thiophene-2,5-diyl) and particularly polypyrrole, CAS# 109-97-7, (usually in carbon black doped with an organic acid anion). The structure of this polymer is shown below: H
H
....
Ix-
H
I
H
X-
X" = organic sulfonic acid dopant anion
The increase in the size of the units from the polymeric backbone was shown to diminish the flexibility of the macromolecular chain, which has as a result an increase in the melting point. The increase in the melting point of a polymer is paralleled by the increase in its thermal stability. Stiffening the polymeric chain by the inclusion of ring structures, possibly with fused-ring groups, leads to the polymers with higher thermal stability. Typical ring systems that have been introduced into polymer chains (as diyl groups) include benzene, diazine, triazine, triazole, thiazole, oxadiazole, isoindoline-l,3dione, 3-pyrrolino[3,4-f]isoindoline-l,3,5,7-tetraone, etc. Also, multiple bonds in the backbone, as in ladder polymers, increase thermal stability (see Section 3.1). Following these ideas a series of polymers with heterocyclic groups were synthesized, leading to good thermal resistance. A few examples are shown in Table 15.1.1.
Polymers containing heterocycles in the backbone TABLE 15.1.1. Polymers with elevated decomposition temperature containing
heterocycles in the backbone.
643
644
Polymers containing heterocycles in the backbone
TABLE 15.1.1 (continued). Polymers with elevated decomposition temperature containing heterocycles in the backbone.
FIGURE 15.1.1. Variation of weight % loss for poly(thiophene-2, 5-diyl) in a TGA experiment at a heating rate of 10~ C/min.
Polymers containing heterocycles in the backbone
645
The TGA shows that the polymer starts to lose some weight around 400 ~ C, and the decomposition accelerates above 500 o C. Volatile product may include H2S (see flash pyrolysis results) and oxidation products such as SO2. - Homopolymers and alt-copolymers containing heterocyclic groups
The thermal decomposition of the polymers containing heterocyclic groups was studied frequently for practical purposes [1-3]. A number of polymers from this class, having good thermal stability, are used in applications such as preparation of adhesives with high temperature resistance, manufacturing of high performance composites used in the aerospace industry, etc. Other polymers are used in photolithographic industry. Some of the thermal stability studies on polymers containing heterocyclic groups, as reported in literature, are summarized in Table 15.1.2 [4]. TABLE 15.1.2. Summary regarding literature information on thermal decomposition of polymers containing heterocyclic groups [4].
646
Polymers containing heterocycles in the backbone
TABLE 15.1.2 (continued). Summary regarding literature information on thermal
decomposition of polymers containing heterocyclic groups [4].
where Ar represents an aromatic moiety. Pyrolysis at 650 ~ C of several such polymers generated specific small molecules. For example, for Ar = 1,4-penylene the pyrolysate contained phenol; for Ar = 1,3-phenylene the pyrolysate contained phenol and resorcinol; for Ar = 4,4'-biphenyl the pyrolysate contained 4-phenylphenol and 4,4'bisphenol; for Ar = 2,7-naphthalene the pyrolysate contained 2-naphthol, 2,7dihydroxynapthalene, and naphthalene; for Ar = bis(4-phenylene) ketone the pyrolysate contained phenol and CO2; for Ar = bis(4-phenylene) sulfone the pyrolysate contained phenol and SO2; for Ar = bis(4-phenylene)isopropane the pyrolysate contained phenol, isopropylphenol, and propane; for Ar = bis(4-phenylene)fluorene the pyrolysate contained phenol, fluorene, and phenylfluorene; and for Ar = bis(4-phenylene) sulfide the pyrolysate contained phenol, H2S, and thiophenol [20]. Thermal properties of some other polymers containing heterocycles are reported in literature. For example, the polymers containing triazine networks substituted with perfluoro-n-propyl or perfluoroalkyloxy groups do not decompose easily upon heating. These types of polymers are used as heat resistant oils, and several studies on their thermal decomposition are available in literature [21-23]. The results for a Py-GC/MS analysis of poly(thiophene-2,5-diyl) Br terminated, CAS # 110-02-1, are shown in Figure 15.1.2. Since the polymer is rather resistant to heat, pyrolysis was done at 850 ~ C in He. Other conditions were kept similar to those for other examples previously described (see Table 4.2.2). The heating rate was kept 20 ~ C/ms and THT 10 s. The separation was done on a Carbowax column (60 m, 0.32 mm i.d., 0.32 ~m film thickness) with the GC starting at 40 o C with a ramp of 2 o C/min. up to 240 ~ C and a final oven time of 20 min. The MS was operated in El+ mode. The peak
Polymers containing heterocycles in the backbone
647
identification was done using MS spectral library searches only and is given in Table 15.1.3.
648
Polymers containing heterocycles in the backbone
TABLE 15.1.3 (continued). Compounds identified in the pyrogram of a poly(thiophene2, 5-diyl) Br terminated sample as shown in Figure 15.1.2.
Hydrogen sulfide is the main decomposition product seen from the pyrolysis of poly(thiophene-2,5-diyl). Some 2,2'-bithiophene (17.4% of pyrolysate) and only a small proportion of thiophene are generated (less than 5% of pyrolysate). However, the pyrolysis in He also forms char, which is not volatile and cannot be seen in the pyrogram. The bonds that appear to cleave more easily are the S-C bonds and the bonds between the thiophene units (C-C type). Since the hydrogen content of the polymer is low, the formation of SH2 is associated with the formation of char. The elimination of some carbon and sulfur as CS2 or Sx explains the formation of benzene, thiophene, etc. Mechanical properties of poly(thiophene-2,5-diyl) are not suitable for many practical uses. Polymers with higher flexibility and still good thermal resistance or with other special conductivity properties (after doping) can be obtained from the polymerization of substituted polythiophenes. Some examples are shown below:
poly(3-hexylthiophene-2,5-diyl)
poly(3,4-ethylenedioxythiophene)
poly(3-octyl-2,2'-dithiophene)
Thermal decomposition of poly(3-hexylthiophene-2,5-diyl) is described by its TGA curve shown in Figure 15.1.3. The thermogram was obtained from a sample with Mw = 87,000 in air, by heating between 30 ~ C and 830 ~ C at a rate of 10~ C/min.
Polymers containing heterocycles in the backbone
649
FIGURE 15.1.3. Variation of weight % loss for poly(3-hexylthiophene-2,5-diyl) Mw = 87, 000 in a TGA experiment at a heating rate of 10~ C/min. The hexyl groups represent about 50% of the weight of the polymer. However, the weight loss between 400 ~ C and 500 ~ C is larger than 50%, indicating that the structure of the polymer is affected by heat not only through the elimination of the side chain groups, but also including the backbone modification. The char remaining at temperatures higher than 500 ~ C is oxidized in air with formation of oxides of carbon and sulfur. The results for a Py-GC/MS analysis of a poly(3-hexylthiophene-2,5-diyl) sample, CAS # 104934-50-1, (regioregular) with Mw = 87,000 are shown in Figure 15.1.4. Since the polymer is rather resistant to heat, pyrolysis was done at 850 ~ C in He. Other conditions were kept similar to those for other examples previously described (see Table 4.2.2). The separation was done on a Carbowax column. The peak identification was done using MS spectral library searches only and is given in Table 15.1.4.
FIGURE 15.1.4. Pyrogram from a Py-GC/MS analysis of a poly(3-hexyithiophene-2,5diyl) sample, regioregular, Mw = 87, 000. Pyrolysis done on O.4 mg material at 850 ~ C in He, with the separation on a Carbowax type column.
650
Polymers containing heterocycles in the backbone
TABLE 15.1.4. Compounds identified in the pyrogram of a poly(3-hexylthiophene-2,5diyl), Mw = 87, 000 sample as shown in Figure 15.1.4.
As seen from the pyrogram and from the pyrolysate composition shown in Table 15.1.4, the main components are generated from the hexyl side chain. Some other pyrolysis
Polymers containing heterocycles in the backbone
651
products are similar to those for poly(thiophene-2,5-diyl). Char is also formed in the pyrolysis of this polymer. Among the heterocyclic compounds that were studied by Py-GC/MS are also a number of polybenzimidazopyrrolones [24]. The synthesis of these compounds has been achieved based on the following reactions:
Polymers containing heterocycles in the backbone
652
The results shown in Table 15.1.5 indicate that a considerable proportion of bond cleavage occurs at the extender connection, and also that the extender influences the thermal stability of the polymer. Regarding the decomposition of the heterocyclic moiety, the ether bond between the two isoindolino[2,1-a]benzimidazol-11-one units and the C(O)-N bond from the pyrrolydin-2-one ring are likely to cleave more easily than other parts of the molecule. In a different study [25] a series of poly[phenyl-(asymm, triazines)] were analyzed by PyGC/MS. The following types of polymers were synthesized:
AF =
The polymers were resistant to heat, and the decomposition was minimal below 6000 C. For this reason, the pyrolysis was done at four temperatures, 650 ~ C, 750 o C, 850 ~ C, and 950 ~ C, followed by GC/MS analysis. For some compounds such as benzene, the yield in the pyrolysate increased as temperature increased, but for other compounds, either the yield had a maximum in the range or even slightly decreased. Among the compounds determined in the pyrolysate were some common for all polymers such as benzene, benzonitrile, benzylamine, 1,4-benzenedicarbonitrile, and phenanthridine. Other compounds in the pyrolysate were specific for the aromatic extender Ar. Among these were 1H-benzotriazole, 1-phenylphthalazine, 9-anthracenecarbonitrile, o~aminobenzeneacetonitrile, 4-phenoxybenzonitrile, 4,4'-oxydibenzonitrile, 1-hydroxy-4phenoxybenzonitrile, 4-benzylphenylcarbonitrile, 4-phenoxy-4'-phenoxybenzonitrile etc. The degradation mechanism of these polymers starts with a random homolytic cleavage of the N-N and C-N bonds in the triazine ring. This leads to the formation of benzonitrile radicals and p-dicyanobenzene radicals that further propagate the decomposition. The termination occurs with various recombinations and rearrangements.
- Copolymers containing heterocyclic groups
Several copolymers with heterocycles studied using pyrolytic techniques include poly[pyrrole-graft-(2-N-pyrrolyl)ethylvinylether] [26] and several polypyrrole/ pol~etrahydrofuran graft copolymers including pyrrole ended-pol~etrahydrofuran, p-toluene sulfonate doped polypyrrole, and p-toluene sulfonate doped polypyrrole-copol~etrahydrofuran [27]. The data showed that pyrrole ended-pol~etrahydrofuran degrades via mixed random cleavage and unzipping mechanism followed by hydrogen transfer reactions. Cleavage of the pyrrole ring and a lack of high mass fragments are noticed for the doped polymers.
Polymers containing heterocycles in the backbone
653
References 15.1
1. R. Sanchez. C. Hernandez, J. Jalovszky, G. Czira, Eur. Polym. J., 30 (1994) 37. 2. H. J. O'Neill, J. Putscher, R. E. Dynako, C. I. Boquist, J. Gas Chromatogr., 1 (1963) 28. 3. T. Kojima, H. Takaku, Y. Urata, K. Gotoh, J. Appl. Polym. Sci., 48 (1993) 1395. 4. J. Liggat, in J. Brandrup, E. H. Immergut, E. A. Grulke, eds., PolymerHandbook, J. Wiley, New York, 2000. 5. N. A. Lapina, V. S. Ostrovskii, I. V. Kamenskii, Vysokomol. Soedin., Ser. A, 11 (1969) 2073. 6. A. T. Radcliffe, T. J. Lens, Kunststoffe, 63 (1973) 854. 7. A. A. Berlin, V. V. Yarkina, A. P. Firsov, Polym. Sci. USSR, 10 (1968) 2219. 8. R. Kiebooms, A. Aleshin, K. Hutchison, F. Wudl, J. Phys. Chem. B, 101 (1997) 11037. 9. P. Selsbo, I. Ericsson, Polym. Degrad. Stab., 51 (1996)83. 10. N. R. Lemer, J. Polym. Sci., Chem. Ed., 15 (1977) 1145. 11. G. Camino, M. P. Luda, L. Costa, M. Guaita, Macromol. Chem. Phys., 197 (1996)41. 12. D. A. Chatfield, I. N. Einhom, J. Polym. Sci., Chem. Ed., 19 (1981) 601. 13. J. R Luongo, H. Schonhom, J. Polym. Sci., Chem. Ed., 13 (1975) 1363. 14. A. A. Caraculacu, G. Caraculacu, Makromol. Chem., 185 (1984) 1079. 15. J. L. Cotter, H. Dickinson, G. J. Knight, W. W. Wright, J. Appl. Polym. Sci., 15 (1971) 317. 16. K. Erdmann, W. Czerwinski, B. C. Gerstein, M. Pruski, J. Polym. Sci., Polym. Phys. Ed., 32 (1994) 1799. 17. D. A Scola, J. H. Vontell, CHEMTECH, 19 (1989) 112. 18. N. Qian, Q. Zha, S. C. Moldoveanu (unpublished results). 19. H. -J. Dussel, A. Recca, J. Kolb, D. O. Hummel, J. K. Stile, J. Anal. Appl. Pyrol., 3 (1982) 307. 20. M. Blazso, E, Jakab, J. Anal. Appl. Pyrol., 11 (1987) 245. 21. V. A. Ponomarenko, V. N. Shilgayev, A. G. Kechina, A. A. Yarosh, S. P. Krukovskii, Polym. Sci. USSR, 16 (1974) 637. 22. L. A. Wall, S. Straus, J. Res. Natl. Bur,. Std., 65A (1961) 227. 23. R. H. Mobbs, F. Heatley, C. Price, C. Booth, Prog. Rubber Plast. Technol., 11 (1995) 94. 24. Z. Jiang, X. Jin, X. Gao, W. Zhou, F. Lu, Y. Luo, J. Anal. Appl. Pyrol., 33 (1995) 231. 25. Z. Jiang, Y. Luo, X. Jin, F. Lu, J. Anal. Appl. Pyrol., 26 (1993) 145. 26. T. Uyar, L. Toppare, J. Hacaloglu. J. Anal. Appl. Pyrol., 68 (2003) 15. 27. C. Ozdilek, L. Toppare, Y. Yagci, J. Hacaloglu, J. Anal. Appl. Pyrol., 64 (2002) 363.
15.2
LADDER TYPE POLYMERS WITH HETEROCYCLIC STRUCTURE
- General aspects
Ladder polymers have their backbone made from an interrupted series of condensed rings. In such polymers, very frequently the rings contain heteroatoms, and the polymer can be considered as part of the class of polymers with heterocycles in the main chain. One of the first synthesized polymers from this class was obtained from the oxidation and heating of polyacrylonitrile [1, 2]. Multifunctional condensations also can lead to ladder polymers. For example, a poly(phenoxazine) is formed from the reaction of a substituted quinone and a diaminodihydroxybenzene as shown below:
CH3OOI IOOOH: H
H2N"
O
~ ~
NH2 'OH
. H20 y - CH3COOH
Polymers from various other groups have a ladder type backbone. Most of these polymers have good thermal resistance since they frequently contain aromatic bonds
Polymers containing heterocycles in the backbone
654
and more than one bond must be cleaved in order to fragment the macromolecule. However, the reported decomposition temperature may vary from study to study, and it depends on the heating conditions and the choice for the temperature value at which a decomposition is considered as beginning. References 15.2
1. T. H. Ko, J. Appl. Polym. Sci., 47 (1993) 707. 2. Y. H. Bang, S. Lee, H. H. Cho, J. Appl. Polym. Sci., 68 (1998) 2205.
15.3
THERMOSETTING MELAMINE RESINS
- General aspects
Similar to urea (see Section 13.2), melamine can react with formaldehyde forming a thermosetting resin. Melamine is a trimer of cyanamide and contains three amino groups that offer six possible points of reaction. Since melamine can be considered a triamine of 1,3,5-triazine, this group of resins can be included in the class of polymers with heterocycles in their structure. The first stage of the reaction of melamine with formaldehyde can be written as follows:
NH2
NH-CH2OH
NH-CH2OH
NH-CH2OH
'~ ] 1 + HCH=O = I I H2N//"~N~NH2 HOCH2-HN/~N ~/~'NH_CH20H H2N/~N I~NH-CH2OH H2N/~N I~NH 2 Further elimination of water between the methylol groups and amino groups or among themselves generates a thermosetting resin. Similarly to phenol formaldehyde or urea formaldehyde resins, the condensation reaction for melamine is typically carried out in two stages, the resulting material from the first phase having a large number of methylol groups and being still water soluble. In the second stage, the water elimination leads to a crosslinked material. The two-stage process is necessary for handling the resin during the manufacturing of the desired objects. Both triazine- and urethane-linked networks are highly resistant to paraffin and silicone oils. Triazine-linked networks are resistant to a wide range of organic liquids, whereas urethane-linked networks are attacked by polar organic solvents. Both types are easily attacked by fluorinated species [1]. The Py-GC/MS result for a sample of poly(melamine-co-formaldehyde) methylated, with the molar ratio melamine/formaldehyde/methoxy 1/5.8/5.0, is given in Figure 15.3.1. The polymer has CAS# 68002-20-0 and Mn = 511. The pyrolysis was done at 6000 C in He at a heating rate of 20 ~ C/ms. The separation was done on a Carbowax column in conditions described in Table 4.2.2. The peak identification for the chromatogram was done using MS spectral library searches only, and it is given in Table 15.3.1.
Polymers containing heterocycles in the backbone
655
Results from Table 15.3.1 indicate that among the main decomposition products of these polymers are methanol, CO2, formaldehyde, and dimethoxymethane. A few nitrogenous compounds also are detected. Some of the decomposition products may be too polar to be detected in the experimental conditions used for the analysis.
References 15.3
1. R. H. Mobbs, F. Heatley, C. Price, C. Booth, Prog. Rubber Plast. Technol., 11 (1995) 94.