Aromatic nitriles may be trimerized at moderate temperature and pressure with p-toluenesulfonic acid as catalyst. Studies were conducted to establish the effect of the reaction temperature, pressure, time, and catalyst concentration on yield of the trimerized product. Trimerization studies were also conducted to establish the effect of substituting electron donating or withdrawing groups on benzonitrile. Preliminary results of using the catalytic trimerization approach to prepare s-triazine cross-linked polyimide/graphite fiber composites are presented

Hsu, L. C.

English

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NASA TECHNICAL NASA TM X- 71525
MEMORANDUM
(NASA-TM-X-71525) CATALYTIC TRINIERIZATION N74-19773OF AROMATIC NITRILES FOR SYNTHESIS OF
ur% POLYIHIDE MATRIX RESINS (NASA) 14 p BC
CSCL 07D Unclas
G3/06 34363 )
I-
CATALYTIC TRIMERIZATION OF AROMATIC NITRILES
FOR SYNTHESIS OF POLYIMIDE MATRIX RESINS
by Li-Chen Hsu C>
Lewis Research Center
Cleveland, Ohio 44135
TECHNICAL PAPER proposed for presentation at
Symposium on New Industrial Polymers, Division of Organic
Coatings and Plastics Chemistry, 167th Spring National
Meeting of the American Chemical Society
Los Angeles, California, April 2, 1974
Reproduced by
NATIONAL TECHNICAL
INFORMATION SERVICE
US Department of Commerce
Springfield, VA. 22151
https://ntrs.nasa.gov/search.jsp?R=19740011660 2020-03-23T11:45:34+00:00Z
CATALYTIC TRIMERIZATION OF AROMATIC NITRILES
FOR SYNTHESIS OF POLYIMIDE MATRIX RESINS
by Li-Chen Hsu
Lewis Research Center
SYNOPSIS
Aromatic nitriles may be trimerized at moderate temperature and
pressure with p-toluenesulfonic acid as catalyst. Studies were conducted
to establish the effect of the reaction temperature, pressure, time, and
catalyst concentration on yield of the trimerized product. Trimerization
studies were also conducted to establish the effect of substituting electron
donating or withdrawing groups on benzonitrile. Preliminary results of
using the catalytic trimerization approach to prepare s-triazine cross-linked
polyimide/graphite fiber composites are presented.
INTRODUCTION
High temperature resin/fiber composites have the potential of meeting
the performance requirements for many advanced aerospace structures. The
composites need to exhibit retention of mechanical properties during contin-
uous use at 3160 C (6000 F) or above (1). Among the high temperature resins,
polyimides occupy a preeminent position. Aromatic polyimides (PiPs) exhib-
it thermal stability in excess of 5000 C (9320 F) as determined by thermal
gravimetric analysis (2). However processing difficulties have limited their
use as matrices in resin/fiber composites. Various approaches have been
used to solve the processability problem of polyimides. Lubowitz (3) and
Burns et. al. (4) developed a new system of processable addition-type (A-type)
PI's by end-capping imide oligomers with norbornenyl groups. After removal
of the solvent, the norbornene-terminated imide olfgomers are polymerized
through the double bonds without evolution of byproducts. Serafini et. al.
(5) and Delvigs et. al. (6) developed an improved processing technique for
A-type P's called the in situ polymerization of monomeric reactants (PMR).
2Although the in situ PMR approach does provide void free A-type PI com-
posites with good property retention at 3160 C (6000 F), the alicyclic ring
structure derived from the norbornene groups does appear to limit the
thermo-oxidative stability (TOS) of A-type PI's (7). To achieve A-type PIs
with improved TOS at 3160 C (6000 F) or above, our approach was to re-
place the norbornenyl groups with aromatic nitriles. Trimerization of
aromatic nitrile-terminated imide oligomers should lead to new polyimides
containing triaryl-s-triazine rings. Triaryl-s-triazine ring is known to
exhibit good thermal stability (8).
The purpose of the present investigation was to study the trimerization
of aromatic nitriles under the conventional resin/fiber composite fabrication
conditions using p-toluenesulfonic acid as a catalyst. Trimerization param-
eters investigated included reaction temperature, pressure, time, and
concentration of catalyst. The influence of the nature of aromatic nitriles
on trimerization was also studied. Also presented are preliminary results
on the use of the catalytic trimerization of the nitrile-terminated imide
oligomers to fabricate graphite fiber reinforced composites.
EXPERIMENTAL PROCEDURE
Materials
All of the aromatic nitriles except p-cyanophthalanil used in this study
were purchased from commercial sources and used as received. The
p-cyanophthalanil was synthesized by a method similar to that used for
synthesizing N-phthalyl-L--phenylalanine (9) except that p-amninobensoni-
trile was used instead of L-phenylalanine.
Catalytic Trimerization
About 0. 01 mole of the aromatic nitrile together with 0. 5 to 5.0 mole
percent of the p-toluenesulfonic acid (PTSA)'catalystwas introduced into a
45-milliliter stainless steel pressure vessel. The vessel was flushed with
nitrogen gas and the initial pressure in the vessel was varied from 0 to 2. 76
MN/m 2 (0 to 400 psi). The vessel was then heated to temperatures in the
range of 100 to 3160 C. The selected temperature was maintained for 24 to
90 hours. The PTSA catalyst and unreacted nitrile were then removed from
3the product by washing with water followed by distillation under reduced
pressure. The product was then recrystallized from xylene or glacial
acetic acid. Melting point and infrared spectrum were determined for
identification purposes.
RESULTS AND DISCUSSION
Trimerization Study
Bengelsdorf (10) reported that aromatic nitriles could be trimerized
in the absence of catalysts at temperatures in the range of 3500 to 5000 C
and at pressures which ranged from 3. 55x10 3 to 5. 06x10 3 MN/m2 (35, 000 to
50, 000 atmospheres). Cairns et al. (11) used various alcohols as catalysts
and were able to effect trimerization of aromatic nitriles at 600 to 1500 C
and 0. 3x103 MN/m2 (above 3000 atmospheres). Kunz et al. (-12) eimployed
chlorosulfonic acid to trimerize aromatic nitriles at temperatures in the
range of -100 to 30 0 C and at atmospheric pressure. These latter workers
used an excess of chlorosulfonic acid which apparently served as both the
solvent and catalyst. Because of the high pressures or the nature and quan-
tity of catalyst employed none of the methods described above are suitable
for the synthesis of high temperature resistant s-triazine cross-linked polyi-
mide matrix resins for fiber reinforced composites.
The aromatic nitrile and catalyst selected for this study were benzoni-
trile and PTSA, respectively. Studies were conducted to establish the
effect of reaction conditions on yield of trimerized product. Figure 1
shows the effect of varying the reaction temperature on yield between 1000
and 2900 C at a constant PTSA catalyst concentration of 5 mole percent,
pressure in the range of 4. 14 to 5. 17 MN/m2 (600 to 750 psi), and:for a con-
stant reaction time of 66 hours. It can be seen from the figure that there
4was no yield at 1000 C and the yield nearly doubled on going from 2320 to
2900 C. Because of practical processing considerations for the fabrication
of fiber reinforced composites, higher temperatures were not investigated.
The effect of reaction pressures in the range of 0.2 to 5.17 MN/m 2 (30 to
750 psi) on yield are shown in figure 2. The data shown in this figure were
obtained for reactions 'conducted with a PTSA concentration of 5 mole per-
cent at 2320 C for 66 hours. The figure shows that the use of higher pres-
sures resulted in higher yields. Hereto, practical processing consideration
limited the highest pressure studied to 5. 17 MN/m 2 (750 psi). Figure 3
shows the effect of reaction time on yield for reactions conducted with
5 mole percent PTSA at 2320 C (4500 F) and 5. 17 MN/m 2 (750 psi). It can
be seen in the figure that the yield upon increasing reaction time from 24 to
66 hours underwent slightly more than a two fold increase. Figure 4 which
shows effect of catalyst concentration on yield shows that at 2320 C (4500 F)
and 5. 17 MN/m 2 (750 psi) and 66 hours the yield increased from 5% to 17%
for a ten fold increase in catalyst concentration.
The results of these trimerization parameter studies indicated that
useful levels of trimerized product (cross-links) could be anticipated from
the use of this catalytic trimerization approach in fabricating resin/fiber
composites. Two additional points need to be made with respect to the yield
of trimerized product. First, the yield of trimerized product (cross-links)
in an actual resin/fiber composite might be increased by post-curing at
elevated temperatures. And secondly, extensive cross-linking may not be
necessary for improved composite properties and indeed may be deleterious
to certain composite mechanical properties.
Nature of Aromatic Nitriles
The experimental results on the influence of ring substituents on the
ease of trimerizing benzonitriles are summarized in Table I. It can be
seen that the benzonitriles bearing electron withdrawing ring substituents
such as carboxyl and nitro groups are more susceptible to trimerization
than those bearing electron donating substituents such as methyl and methoxy
groups. The lower yield of trimerized product from the o-nitrobenzonitrile
compared to p-nitrobenzonitrile can be accounted for by steric effects. The
very high yield of trimerized product from the p-cyanobenzoic acid might
5have resulted from a reaction in which the p-cyanobenzoic acid itself served
as a co-catalyst.
For the synthesis of processable polyimides, our results suggested the
use of 4-cyanophthalic anhydride as the end-capping reagent. The 4-cyanoph-
thalic anhydride or its esters might be preferrable because the electron with-
drawing carbonyl groups would be directly attached to the aromatic ring con-
taining the nitrile to be trimerized. However, because of the commercial
availability of p-aminobenzonitrile, it was selected as the end-capping reagent
for preliminary studies.
Trimerization of p-Cyanophthalanil
p-Cyanophthalanil was synthesized as the model compound to study the
effectiveness of PTSA in promoting trimerization of a chemical structure
which would be found in the polyimide precursors.
p-Cyanophthalanil was prepared from commercially available p-amino-
benzonitrile and phthalic anhydride. The white crystalline powder has a
melting point of 1890 C (lit. 1870 C, ref. 16'). Its infrared spectrum showed
-1 -
a nitrile band at 2240 cm- , imide bands at 1795, 1755, 1735, and 1380 cm -1
and phenyl ring bands at 1610 and 1520 cm -1 respectively (fig. 5(a)).
Catalytic trimerization of p-cyanophthalanil with 5 mole percent p-toluene-
sulfonic acid at 250-3000 C and 4. 97 to 5. 52 MN/m2 (720-800 psi) for 90 hours
gave a 97% yield product, with a m. p. > 3400 C. The infrared spectrum
-1
showed the disappearance of the nitrile band at 2240 cm-1 and the broadening
of the s-triazine bands at 1520 and 1380 cm -1 (fig. 5(b)). Further identifi-
cation of the formation of the s-triazine was done by refluxing the trimerized
product with a10% NaOH solution for 4 hours. The infrared spectrum,
of the insoluable hydrolysis product showed that those imide bands at 1795,
1755, 1735, and 1380 cm -1 had nearly disappeared (or greatly weakened).
The characteristic s-triazine band at 1520 cm - 1 was not effected (fig. 5(c)).
6Trimerization of Terephthalonitrile
Since terephthalonitrile has two nitrile groups and the nitrile group
itself is also electron withdrawing, catalytic trimerization of terephthal-
onitrile should proceed readily and result in a polymeric product expected
to exhibit good thermal stability. The experimental results confirmed this
prediction: Catalytic trimerization of terephthalonitrile with 5 mole percent
of PTSA catalyst at 2320 C and 5. 17 MN/m2 (750 psi) for 48 hours gave a
product (99. 5%'yield) with a melting point > 3400 (6440 F). The infrared
spectrum of terephthalonitrile showed a very strong nitrile band at 2230 cm
and a sharp aromatic ring band at 1500 cm -1 (fig. 6(a)). The infrared spectrum
of the trimerized product showed strong and broad characteristic s-triazine
ring-bands at 1525 and 1370 cm -1 with a residual nitrile band of medium
strength at 2230 cm -1 (fig. 6(b)). Thermal gravimetric analysis (fig. 7)
showed that the weight losses of terephthalonitrile polymer were about 7%
after heating to 3160 C (6000 F) and 18% after heating to 5380 C (10000 F)
respectively. Anderson and Holovka (17) reported weight losses of about
25% after heating to 3160 (6000 F) and 75% after heating to 5380 C (10000 F)
respectively from the terephthalonitrile polymer which they obtained by
treating terephthalontrile with chlorosulfonic acid at 00 C. This greater
thermal oxidative stability of the trimerized product using PTSA as the cata-
lyst may be due to having achieved a higher cross-link density during reaction
at 2320 C (4500 F) and 5.17 MN/m2 (750 psi).
Polyimide/Graphite Fiber Reinforced Composite
Since aromatic nitrile-terminated imide oligomers are similar in
nature as p-cyanophthalanil and possess the same' functionality as' terephthal-
onitrile, they should be able to trimerize and form the s-triazine ring con-
taining polymers under the similar reaction conditions.
Preliminary work for the synthesis of nitrile terminated polyimides was
carried out by using p-aminobenzonitrile, 4, 4' -methylenedianiline,, 3, 31,
4,4' -benzophenonetetracarboxylic dianhydride and methanol with 2.5 mole
percent of PTSA catalyst. The stoichiometry of the monomeric reactants
was adjusted to yield an in situ prepolymer having an average formulated
7molecular weight of 1500. Composite fabrication and testing were performed
essentially according to the method used in reference 5. The results from
some preliminary composite fabrication and characterization studies indi-
cate that the trimerization technique by employing PTSA catalyst provides
high performance composites.
Table II shows the interlaminar shear strength and flexural strength of
the triaryl-s-triazine cross-linked PI/HMS graphite composites at room tem-
perature and 3160 C with and without post curing. The data clearly indicate
that the HMS graphite fiber reinforced composite prepared from a nitrite ter-
minated PI exhibited very good retention of flexural strength during short time
exposure in air at 3160 C. More inportant, the data also indicate that both
the interlaminar shear strength and flexural strength of the composites im-
proved after a 16 hour post cure at 3160 C. Apparently this resulted from
an increase in triaryl-s-triazine rings during post cure at 3160 C.
CONCLUSIONS
The results of this investigation lead to the following conclusions:
1. Aromatic nitriles can be trimerized in the temperature range of
2000 to 3160 C (3920 to 6000 F) and pressure range of 1. 38 to 5. 52
MN/m 2 (200 to 800 psi) with p-toluenesulfonic acid as catalyst.
2. Benzonitriles bearing electron withdrawing ring substituents were
found more susceptible to trimerization than those bearing electron do-
nating ring substituents.
3. Polyimide matrix resins containing s-triazine cross-links can be
easily prepared using the aromatic nitrile end-capping approach and tri-
merization with a p-toluenesulfonic acid catalyst.
8REFERENCES
1. T. T. Serafini in "Aerospace Structural Materials," National Aero-
nautics Space Administration Special Publication No. 227,
Washington, D.C., 1970, p. 207.
2. C.E. Sroog, J. Polymer Sci., C, 16, 1191 (1967).
3. H.R. Lubowitz, E.R. Wilson, W.P. Kendrick, and E.A. Burns,
French Patent 1,572,798 (1969); H.R. Lubowitz, U.S. Patent 3,528,950
(1970); Chem. Abstr., 72, 56085k (1970).
4. E.A. Burns, R.J. Jones, R.W. Vaughan, and W.P. Kendrick, "Ther-
mally Stable Laminating Resins," Report TRW-11926-6013-RO-00;
NASA CR-72633, TRW System Group, Redondo Beach, Calif., 1970.
5. T. T. Serafini, P. Delvigs, and G. R. Lightsey, "Thermally Stable
Polyimides from Solutions of Monomeric Reactants," Report TN-D-6611,
National Aeronautics and Space Administration, Washington, D.C.,
1972; J. Appl. Polymer Sci., 16, 905 (1972).
6. P. Delvigs, T.T. Serafini, G.R. Lightsey, "Addition-Type Polyimides
from Solutions of Monomeric Reactants," Report TN-D-6877,
National Aeronautics and Space Administration, Washingyon, D.C.,
1972; Materials Review for '72: Proceedings of the National
Symposium and Exhibition, Science of Advanced Materials and Pro-
cess Engineering Series, Vol. 17, 1972, p. III-B-7-1.
7. R.J. Jones, R.W. Vaughn, and E.A. Burns, "Thermally Stable Lam-
inating Resins," Report TRW-16402 -6012 -RO -00; NASA CR-72984,
TRW System Group, Redondo Beach, Calif., 1972.
8. E.S. Blake, W.C. Hammann, J.W. Edwards, T.E. Reichard, and
M.R. Ort, J. Chem. Eng. Data, 16, 87, (1961).
9. A.K. Bose, "Organic Synthesis," Collect. Vol. 5, Wiley, New York,
N.Y., 1973, p. 973.
10. I.S. Bengelsdorf, J. Amer. Chem. Soc., 80, 1442 (1958).
11. T.L.S. Cairns, A.W. Larchar, and B.C. McKusick, U.S. Patent
2,503,999 (1950); Chem. Abstr., 44, 6445c (1950).
912. M.A. Kunz, K. Koeferle, and E. Berthold, U.S. Patent 1,989,042
(1935); Chem. Abstr., 29, 1834 (1935).
13. E. M. Smolin and L. Rapoport, "s-Triazines and Derivatives," Inter-
science, New York, 1967, p. 172.
14. 14. J.E. Mahan and S.D. Turk, U.S. Patent2,598,811 (1952); Chem.
Abstr., 46, 10212h (1952).
15. G. Martins Filho, J. Goncalves da Costa, F. Soares de Azevedo, and
J.O. Falcao de Morals, Anais. Acad. Brasil, Cienc., 35, 185 (1963).
16. M. T. Bogert and L. E. Wise, J. Amer. Chem. Soc., 34, 693 (1912).
17. D. R. Anderson and J. M. Holovka, J. Polymer Sci., A-i, 4, 1689 (1966).
10
TABLE I. - CATALYTIC TRIMERIZATION* OF
SUBSTITUTED BENZONITRILES
Aromatic Nitrile M. P. 0 C Trimerized Product
Percent yield M. P. OC (Value in Lit.)
13p-Tolunitrile 26-28 5.0 > 340 (278-913)
Anisonitrile 55-56 6.0 > 340 (217 and 224 )
Benzonitrile -14 14.0 232-235 (23213)
o-Nitrobenzonitrile 102-106 37. 8 > 34014
p-Nitrobenzonitrile 146-149 52.3 > 340 (> 3601)
p-Cyanobenzoic acid 220-222 75.0 > 340 (374-515)
* Reaction conducted at 2320 C, and 600 to 750 psi., with 5 mole percent
of PTSA catalyst for 48 hours.
TABLE II. - MECHANICAL PROPERTIES OF TRIARYL-S-TRIAZINE
CROSS-LINKED PI/HMS FIBER COMPOSITESa
Speci- Interlaminar shear strength, psi Flexural strength, psi
men
Room 6000 F Room 6000 F
temper- temper-
ature No post Post cure ature No post Post cure
cure (16 hr 6000 F) cure (16 hr 6000 F)
1 8 370 4620 7500 155 000 148 200 167 500
2 10 130 4520 6800 130 200 145 700 157 500
aResin/fiber 
~40/60 by weight.
28- 17-
24- 16
20- 15
16 - / 14 -
12 - 13
S 8- 12-
4 11
L I L I 1 111 I
200 300 400 500 600 0 100 200 300 400 500 600 700 800
cs-68s39 REACTION TEMP, OF cs-68853 REACTION PRESSURE, PSI
Figure 1. - Effect of reaction temperature on Figure 2 - Effect of reation pressure on trimerization of
trimerization of benzonitrile (PTSA 5 moles benzonitrile (PTSA 5 mole 4500 F, 66 hr.
%, 600-750 psi, 66 hr.)
18- 18
16-
6 14
o 12
8-
10 -
6-
8 I I 4 I I
24 48 72 0 1 2 3 4 5Cs-688s4 REACTION TIME, HR cs-68s40 CONCENTRATION OF CATALYST, MOLE %
Figure 3. - Effect of reaction time on trimer- Figure 4. - Effect of catalgst concentration on trimerization of
ization of benzonitrile (PTSA 5 mole %, benzonitrile (PTSA, 450 F, 750 psi, 66 hr).
4500 F, 750 psi).
100 -
100 -
80 - N N 60N N
X N X C
-CEN -CEN N
CN
40 20 CC
-C-N A
20- , -CEN,,o I I I
2500 2000 1500 1000 2500 2000 1500 1000
0 - (a) IR SPECTRUM OF (b) IR SPECTRUM OF TRIMER-
2000 1500 2000 1500 TEREPHTHALONITRILE. IZED PRODUCT OF
FREQUENCY, CM- 1  THEREPHTHALONITRILE.
(a) IR SPECTRUM OF P- (b) IR SPECTRUM OF
CYANOPHTHALANIL TRIMERIZED PROD- Figure 6.
(PCPLAL). UCT OF PCPLAL
100
80- C-N
CEN
60 - C C N
40 -N=C N N N=C
N=C CC N
20 2 N N-C-
C=N
" 0-
20D 100 MP 340. P. > 40  C
FREQUENCY, CM-1 2
(c) IR SPECTRUM OF 40
PCPLAL TRIMERIZED -
PRODUCT AFTER 60 - C\
> NaOH FOR 4 HR. 80- °"°C -N.z REFLUXING WITH 10% 
_
NaH FOR 4HR. M. P. 2310 C
Figure5. 0 100 200 300 400 500 600 700 800 900 1000
TEMPERATURE, OC cs-68861
Figure 7. - TGA thermograms (100 C/min, N2) of terephthalonitrile (o)
and its trimerized product (e).


Catalytic trimerization of aromatic nitriles for synthesis of polyimide matrix resins

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