Catalytic Trimerization of Aromatic Nitriles for Synthesis of Polyimide

11 Catalytic Trimerization of Aromatic Nitriles for

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 5, 2017 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0004.ch011

Synthesis of Polyimide Matrix Resins LI-CHEN HSU NASA, Lewis Research Center, Cleveland, Ohio 44135

Synopsis Aromatic nitriles may be t r i n t e r i z e d a t moderate tempera t u r e and p r e s s u r e w i t h p - t o l u e n e s u l f o n i e acid as catalyst. S t u d i e s were c o n d u c t e d t o establish t h e effect o f t h e r e action t e m p e r a t u r e , p r e s s u r e , t i m e , and catalyst concentration on yield o f t h e trimerized product. Trimerization s t u d i e s were a l s o c o n d u c t e d t o establish t h e effect o f substituting e l e c t r o n d o n a t i n g o r w i t h d r a w i n g groups on benzonitrile. Preliminary results o f u s i n g the catalytic trimerization approach to prepare s-triazine cross-linked p o l y i m i d e / g r a p h i t e fiber c o m p o s i t e s a r e p r e s e n t e d . Introduction H i g h t e m p e r a t u r e r e s i n / f i b e r c o m p o s i t e s have t h e p o t e n t i a l o f m e e t i n g t h e p e r f o r m a n c e r e q u i r e m e n t s f o r many advanced a e r o s p a c e s t r u c t u r e s . The c o m p o s i t e s need t o e x h i b i t r e t e n t i o n o f mechanical p r o p e r t i e s during continuous use a t 316°C (600°F) o r above ( 1 ) . Among t h e h i g h t e m p e r a t u r e r e s i n s , p o l y a m i d e s occupy a p r e e m i n e n t p o s i t i o n . Aromatic polyimides f P I s ) e x h i b i t thermal s t a b i l i t y i n e x c e s s o f 500°C (932 F) as d e t e r m i n e d by t h e r m a l g r a v i = m e t r i c a n a l y s i s ( 2 ) . However, p r o c e s s i n g d i f f i c u l t i e s have l i m i t e d t h e i r use as m a t r i c e s i n r e s i n / f i b e r composites. V a r i o u s a p p r o a c h e s have been u s e d t o s o l v e t h e p r o c e s s a b i l i t y p r o b l e m o f p o l y i m i d e s . L u b o w i t z (3) and B u r n s e t . a l . (4) d e v e l o p e d a new s y s t e m o f p r o c e s s a b l e a d d i t i o n - t y p e (A-type) P I s by e n d - c a p p i n g i m i d e o l i g o mers w i t h n o r b o r n e n y l g r o u p s . A f t e r removal o f the s o l v e n t , the norbornene-terminated imide oligomers are p o l y m e r i z e d t h r o u g h t h e d o u b l e bonds w i t h o u t e v o l u t i o n o f b y p r o d u c t s . S e r a f i n i e t . a l (5) and D e l v i g s e t . a l . (j3) d e v e l o p e d an improved p r o c e s s i n g t e c h n i q u e f o r A - t y p e P I s c a l l e d t h e i n s i t u p o l y m e r i z a t i o n o f monomeric r e a c t a n t s (PMR). T

T

T

145

Deanin; New Industrial Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

NEW

146

INDUSTRIAL

POLYMERS

Although the i n s i t u PMR approach does provide v o i d f r e e A-tyge PI composites with good property retention at 316 C (600 F ) , the a l i c y c l i c r i n g structure derived from the norbornene groups does appear to l i m i t the thermo-oxidative s t a b i l i t y (TOS) of A-type P I s Γ7). To achieve A-type PI?s with improved TOS at 316°C (600 F) or above, our approach was to replace the norbornenyl groups with aromatic n i t r i l e s . T r i m e r i z a t i o n of aromatic n i t r i l e - t e r m i n a t e d imide oligomers should lead to new polyimides containing t r i a r y l - s - t r i a z i n e r i n g s . T r i a r y l - s - t r i a z i n e r i n g i s known to e x h i b i t good thermal s t a b i l i t y (8). The purpose o f the present i n v e s t i g a t i o n was to study the t r i m e r i z a t i o n o f aromatic n i t r i l e s under the conven t i o n a l r e s i n / f i b e r composite f a b r i c a t i o n conditions using p-toluenesulfonic a c i d as a c a t a l y s t . T r i m e r i z a t i o n para meters i n v e s t i g a t e d included r e a c t i o n temperature, pressure, time, and concentration o f c a t a l y s t . The influence o f the nature o f aromatic n i t r i l e s on t r i m e r i z a t i o n was also studied. Also presented are preliminary r e s u l t s on the use of the c a t a l y t i c t r i m e r i z a t i o n o f the n i t r i l e - t e r m i n a t e d imide oligomers to f a b r i c a t e graphite f i b e r r e i n f o r c e d composites. Q


T

Experimental Procedure M a t e r i a l s . A l l of the aromatic n i t r i l e s except pcyanophthalanil used i n t h i s study were purchased from commercial sources and used as received. The p-eyanophthal a n i l was synthesized by a method s i m i l a r to that used f o r synthesizing N-phthalyl-L-^-phenylalanine (9) except that p-aminobenzonitrile was used instead of L-phenylalanine. C a t a l y t i c T r i m e r i z a t i o n . About 0.01 mole of the aromatic n i t r i l e togethter with 0.5 to 5.0 mole percent of the p - t o l u e n e s u l f o n i c a c i d (PTSA) c a t a l y s t was introduced into a M-S-milliliter s t a i n l e s s s t e e l pressure v e s s e l . The v e s s e l was flushed with nitrogen gas and the i n i t i a l £ pressure i n the v e s s e l was v a r i e d from 0 to 2.76 MN/m (0 to M-00 psi) . The v e s s e l was then heated to temperatures i n the range of 100 to 316°C. The selected temperature was maintained f o r 24 to 90 hours. The PTSA c a t a l y s t and unreacted n i t r i l e were then removed from the product by washing with water followed by d i s t i l l a t i o n under reduced pressure. The product was then r e c r y s t a l l i z e d from xylene or g l a c i a l a c e t i c a c i d . M e l t i n g p o i n t and i n f r a r e d spectrum were determined f o r i d e n t i f i c a t i o n purposes. Results and Discussion T r i m e r i z a t i o n Study.

Bengelsdorf (10) reported that



11.

HSU

Trimerization of Aromatic Nitriles

147

aromatic n i t r i l e s could be t r i m e r i z e d i n the absence o f c a t a l y s t s at temperatures i n the range o f 350° to 500°C and at pressures which ranged from 3.55 χ 10 to 5.06 χ 10 MN/m (35,000 to 50,000 atmospheres). Cairns e t . a l . (11) used various alcohols as c a t a l y s t s and were able to e f f e c t t r i m e r i z a t i o n o f aromatic n i t r i l e s at 60° to 150°C and 0.3 χ 10 MN/m (above 3000 atmospheres). Kunz e t . a l . (12) employed c h l o r o s u l f o n i c a c i d to t r i m e r i z e aromatic n i t r i l e s at temperatures i n the range o f -10° to 30°C and at atmos pheric pressure. These l a t t e r workers used an excess o f c h l o r o s u l f o n i c a c i d which apparently served as both the s o l vent and c a t a l y s t . Because o f the high pressures or the nature and quantity o f c a t a l y s t employed none o f the methods described above are s u i t a b l e f o r the synthesis o f high temperature r e s i s t a n t s - t r i a z i n e c r o s s - l i n k e d polyimide matrix r e s i n s f o r f i b e r r e i n f o r c e d composites. The aromatic n i t r i l e and c a t a l y s t selected f o r t h i s study were b e n z o n i t r i l e and PTSA, r e s p e c t i v e l y . Studies were conducted to e s t a b l i s h the e f f e c t o f r e a c t i o n condi tions on y i e l d o f t r i m e r i z e d product. Figure 1 shows the e f f e c t o f varying the r e a c t i o n temperature on y i e l d between 100 and 290 C at a constant PTSA c a t a l y s t concentration o | 5 mole percent, pressure i n the range o f M-.14- to 5.17 MN/m (600 to 700 p s i ) , and f o r a constant r e a c t i o n time o f 66 hours. I t can be seen from the f i g u r e that there was no yieJLd at 100 C and the y i e l d nearly doubled on going from 232 to 290 C. Because o f p r a c t i c a l processing considera tions f o r the f a b r i c a t i o n o f f i b e r r e i n f o r c e d composites, higher temperatures were not i n v e s t i g a t e d . The e f f | c t o f reaction pressures i n the range o f 0.2 to 5.17 MN/m (30 to 750 psi) on y i e l d are shown i n f i g u r e 2. The data shown i n t h i s f i g u r e were obtained f o r reactions conducted with a PTSA concentration o f 5 mole percent at 232°C f o r 66 hours. The f i g u r e shows that the use o f higher pressures r e s u l t e d i n higher y i e l d s . Hereto, p r a c t i c a l processing considera t i o n l i m i t e d the highest pressure studied to 5.17 MN/m (750 p s i ) . Figure 3 shows the e f f e c t o f r e a c t i o n time on y i e l d f o r reactions conducted with 5 mole percent PTSA at 232°C (M-50°F) and 5.17 MN/m (750 p s i ) . I t can be seen i n the f i g u r e that the y i e l d upon i n c r e a s i n g r e a c t i o n time from 2M- to 66 hours underwent s l i g h t l y more than a two f o l d increase. Figure 4 which shows e f f e c t o f c a t a l y s t concen-p t r a t i o n on y i e l d shows that at 232°C (450°F) and 5.17 MN/m (750 p s i ) and 66 hours the y i e l d increased from 5% to 17% f o r a ten f o l d increase i n c a t a l y s t concentration. The r e s u l t s o f these t r i m e r i z a t i o n parameter studies i n d i c a t e d that u s e f u l l e v e l s o f t r i m e r i z e d product (cross l i n k s ) could be a n t i c i p a t e d from the use o f t h i s c a t a l y t i c t r i m e r i z a t i o n approach i n f a b r i c a t i n g r e s i n / f i b e r composites. Two a d d i t i o n a l points need to be made with respect to the


NEW


148

Figure 1. Effect of reaction ternperature on trimerization of benzonitrile (PTSA 5 moles %, 600750 psi, 66hr)

Figure 3. Effect of reaction tiie on trimerization of benzonitrile (PTSA 5 mole %, 450°F, 750 psi)

INDUSTRIAL

POLYMERS

Figure 2. Effect of reaction pressure on trimerization of benzonitrih (PTSA 5 mole % 450°F 66 hr)

' $ f °* ^f trimerization of benzonitrih (PTSA, 750 psi, 66 hr) 4

E

e

catal


™°v 450°F,

11.

HSU

149



y i e l d o f t r i m e r i z e d product. F i r s t , the y i e l d of t r i m e r i z e d product (cross-links) i n an a c t u a l r e s i n / f i b e r composite might be increased by p o s t - c u r i n g at elevated temperatures. And secondly, extensive c r o s s - l i n k i n g may not be necessary f o r improved composite p r o p e r t i e s and indeed may be delet e r i o u s to c e r t a i n composite mechanical p r o p e r t i e s . Nature o f Aromatic N i t r i l e s . The experimental r e s u l t s on the influence of r i n g substituents on the ease of trimeri z i n g b e n z o n i t r i l e s are summarized i n Table I . I t can be seen that the b e n z o n i t r i l e s bearing electron withdrawing r i n g substituents such as carboxyl and n i t r o groups are more susceptible to t r i m e r i z a t i o n than those bearing electron donating substituents such as methyl and methoxy groups. The lower y i e l d o f t r i m e r i z e d product from the o-nitrobenzon i t r i l e compared to p - n i t r o b e n z o n i t r i l e can be accounted f o r by s t e r i c e f f e c t s . The very high y i e l d of t r i m e r i z e d product from the p-cyanobenzoic a c i d might have r e s u l t e d from a r e a c t i o n i n which the p-cyanobenzoic a c i d i t s e l f served as a c o - c a t a l y s t . For the synthesis o f processable polyimides, our res u l t s suggested the use of 4-cyanophthalic anhydride as the end-capping reagent. The 4-eyanophthalic anhydride or i t s esters might be p r e f e r r a b l e because the electron withdrawing carbonyl groups would be d i r e c t l y attached to the aromatic r i n g containing the n i t r i l e to be t r i m e r i z e d . However, because of the commercial a v a i l a b i l i t y of p-aminob e n z o n i t r i l e , i t was s e l e c t e d as the end-capping reagent f o r preliminary s t u d i e s . T r i m e r i z a t i o n o f p-Cyanophthalanil. p-Cyanophthaianil was synthesized as the model compound to study the e f f e c t i v e ness of PTSA i n promoting t r i m e r i z a t i o n o f a chemical structure which would be found i n the polyimide precursors. p-Cyanophthalanil was prepared from commercially a v a i l a b l e p-aminobenzonitrile and p h t h a l i c anhydridg. The whije c r y s t a l l i n e powder has a melting p o i n t o f 189 C ( l i t . 187 C, r e f . 16). I t s i n f r a r e d spectrum showed a n i t r i l e band at 2240 cm" , imide bands at 1795, 1755, 1735, and 1380 c n r , and phenyl r i n g bands at 1610 and 1520 cm" respectively ( f i g . 5(a)). C a t a l y t i c t r i m e r i z a t i o n o f p-cyanophthalanil with 5 mole percent p - t o l u e n e s u l f o n i c a c i d at 250-300 C and 4.97 to 5.52 MN/m (720-800 p s i ) f o r 90 hours gave a 97% y i e l d product, with a m.p. > 3 4 0 ° C . The i n f r a r e d spectrum showed the disappearance of the n i t r i l e band at 2240 cm" and the broadening of the s - t r i a z i n e bands at 1520 and 1380 cm" ( f i g . 5 ( b ) ) . Further i d e n t i f i c a t i o n o f the formation o f the s - t r i a z i n e was done by r e f l u x i n g the t r i m e r i z e d product with a 10% NaOH s o l u t i o n f o r 4 hours. The i n f r a r e d spectrum of 1

1

1

2

1

1



M. P. C

5.0 6.0 14.0 37.8 52.3 75.0

Percent yield

14

15

13

13

13

> 340 (278-9 ) .13, > 340 (217 and 224") 232-235 (232 ) > 340 > 340 (> 360 ) > 340 (374-5 )

M. P. °C (Value in Lit. )

Trimerized Product

* Reaction conducted at 232 C, and 600 to 750 psi., with 5 mole percent of PTSA catalyst for 48 hours.

p-Tolunitrile 26-28 Anisonitrile 55-56 Benzonitrile -14 o-Nitrobenzonitrile 102-106 p-Nitrobenzonitrile 146-149 p-Cyanobenzoic acid 220-222

Aromatic Nitrile

SUBSTITUTED BENZONITRILES

TABLE I. - CATALYTIC TRIMERIZATION* OF


11.

HSU


151

the insoluable h y d r o l y s i s product showed that those imide bands at 1795, 1755, 1735, and 1380 cm" had nearly d i s appeared (or greatly weakened). The c h a r a c t e r i s t i c s - t r i azine band at 1520 cm" was not affected ( f i g . 5 ( c ) ) . 1


1

Trimerization of T e r e p h t h a l o n i t r i l e . Since terephthalon i t r i l e has two n i t r i l e groups and the n i t r i l e group i t s e l f i s also electron withdrawing, c a t a l y t i c t r i m e r i z a t i o n of t e r e p h t h a l o n i t r i l e should proceed r e a d i l y and r e s u l t i n a polymeric product expected to e x h i b i t good thermal s t a b i l i t y . The experimental r e s u l t s confirmed t h i s p r e d i c t i o n : Catal y t i c t r i m e r i z a t i o n of t e r e p h t h a l o n i t r i l e with 5 mole percent of PTSA c a t a l y s t at 232°C and 5.17 MN/m (750 psi) f o r 48 hours gave a product (99.5% y i e l d ) with a melting point >340°C (644 F ) . The i n f r a r e d spectrum of terephthalon i t r i l e showed a very strong n i t r i l e band at 2230 cm" and a sharp aromatic r i n g band at 1500 cm" ( f i g . 6(a)). The i n f r a r e d spectrum of the trimerized product showed strong and broad c h a r a c t e r i s t i c s - t r i a z i n e r i n g bands at 1525 and 1370 cm" with a r e s i d u a l n i t r i l e band of medium strength at 2230 cm" ( f i g . 6(b)). Thermal gravimetric analysis ( f i g . 7) showed that the weight losses of t e r e p h t h a l o n i t r i l e polymer were about 7% a f t e r heating to 316 C (600 F) and 18% a f t e r heating to 538°C (1000°F) r e s p e c t i v e l y . Anderson and Holovka (17) reported weight losses of about 25% a f t e r heatigg to 316°C (600°F) and 75% a f t e r heating to 538°C (1000 F) r e s p e c t i v e l y from the t e r e p h t h a l o n i t r i l e polymer which they obtained by t r e a t i n g t e r e p h t h a l o n t r i l e with c h l o r o s u l f o n i c a c i d at 0°C. T h i s greater thermal oxidative s t a b i l i t y of the trimerized product using PTSA as the catal y s t may be due to having achieved a higher cross-linkp density during reaction at 232°C (450 F) and 5.17 MN/m (750 psi) . 1

1

1

Polyimide/Graphite F i b e r Reinforced Composite. Since aromatic n i t r i l e - t e r m i n a t e d imide oligomers are s i m i l a r i n nature as p-cyanophthalanil and possess the same functiona l i t y as t e r e p h t h a l o n i t r i l e , they should be able to t r i m e r i z e and form the s - t r i a z i n e r i n g containing polymers under the s i m i l a r reaction conditions. Preliminary work f o r the synthesis of n i t r i l e terminated polyimides was c a r r i e d out by using p-aminobenzonitrile, 4,4'-methylenedianiline, 3,3 ,4,4 -benzophenonetetracarb o x y l i c dianhydride and methanol with 2.5 mole percent of PTSA c a t a l y s t . The stoichiometry of the monomeric reactants was adjusted to y i e l d an i n s i t u prepolymer having an average formulated molecular weight of 1500. Composite f a b r i c a t i o n and t e s t i n g were performed e s s e n t i a l l y according to the method used i n reference 5,. The r e s u l t s from some p r e l i m i nary composite f a b r i c a t i o n and characterization studies T

f



2000

1500

1000 2500 2000 1500

1000

c

N-C /

J

100 200 300 400 500 600 700 800 900 1000 TEMPERATURE» °C cs-6886i

M. P. 231° C

M.P. > 3 4 0 ° C

V

Ν Ν « i

2

C)

Figure 7. TGA thermograms (10°C/min N ) of terephthalonitrile (O) and its trimerized product

100 0

80

60

40

20

0

C=N

Figure 6. (a) IR spectrum of terephthalonitrile; (b) IR spectrum of trimerized product of therephthalonitrile.

2500

2000

(b)

1500 1

Figure 5. (a) IR spectrum of p-cyanophthalanil (PCPLAL); (b) IR spectrum of trimerized product of PCPLAL; (c) IR spectrum of PCPLAL trimerized product after refluxing with 10% NaOH for 4 hr.

FREQUENCY, C M '

2000

1500

(b) IR SPECTRUM OF TRIMERIZED PROD UCT OF PCPLAL

Λ·1 FREQUENCY, CM"

1500

(a) IR SPECTRUM OF Ρ CYANOPHTHALANIL (PCPLAL).

2000



a

8 370 10 130

temper ature

Room

4620 4520

No post cure 7500 6800

Post cure (16 hr 600° F)

600° F

Interlaminar shear strength, psi

Resin/fiber ~40/60 by weight.

1 2

men

Speci

155 000 130 200

temper ature

Room

a

148 200 145 700

No post cure

167 500 157 500

Post cure (16 hr 600° F)

600° F

Flexural strength, psi

CROSS-LINKED PI/HMS FIBER COMPOSITES

TABLE Π. - MECHANICAL PROPERTIES OF TRIARYL-S-TRIAZINE



154

NEW

INDUSTRIAL

POLYMERS

i n d i c a t e that the t r i m e r i z a t i o n technique by employing PTSA c a t a l y s t provides high performance composites. Table I I shows the interlaminar shear strength and f l e x u r a l strength o f the t r i a r y l - s - t r i a z i n e cross-linkgd PI/HMS graphite composites at room temperature and 316 C with and without post curing. The data c l e a r l y i n d i c a t e that the HMS graphite f i b e r r e i n f o r c e d composite prepared from a n i t r i l e terminated PI exhibited very good r e t e n t i o n o f f l e x u r a l strength during short time exposure i n a i r at 316 C. More important, the data also i n d i c a t e that both the interlaminar shear strength and f l e x u r a l strength o f the composites improved a f t e r a 16 hour post cure a t 316 C. Apparently t h i s r e s u l t e d from an increase i n t r i a r y l - s t r i a z i n e r i n g s during post cure a t 316 C. Q

Conclusions The r e s u l t s o f t h i s i n v e s t i g a t i o n lead to the f o l l o w i n g conclusions: 1. Aromatic n i t r i l e s can be t r i m e r i z e d i n the temperature range o f 200° to 316°C (392° to 600 F) and pressure range o f 1.38 to 5.52 MN/m (200 to 800 p s i ) with p-toluen e s u l f o n i c a c i d as c a t a l y s t . 2. B e n z o n i t r i l e s bearing e l e c t r o n withdrawing r i n g substituents were found more susceptible to t r i m e r i z a t i o n than those bearing e l e c t r o n donating r i n g substituents. 3. Polyimide matrix r e s i n s containing s - t r i a z i n e crossl i n k s can be e a s i l y prepared u s i n g the aromatic n i t r i l e endcapping approach and t r i m e r i z a t i o n with a p-toluenesulfonic acid catalyst.


11.

HSU


155

Literature Cited 1.


2. 3.

4.

5.

6.

7.

8.

9. 10. 11.

12. 13. 14. 15.

16. 17.

S e r a f i n i , T. T., "Aerospace S t r u c t u r a l M a t e r i a l s " , National Aeronautics Space Administration S p e c i a l P u b l i c a t i o n No. 227 (1970), p. 207. Sroog, C. E., J . Polymer S c i . , C, (1967), (16), p.1191. Lubowitz, H. R.; Wilson, E. R.; Kendrick, W. P.; and Burns, Ε. Α., French Patent 1,572,798 (1969); Lubowitz, H. R., U.S. Patent 3,528,950 (1970); Chem. Abstr., (72), 56085k. Burns, E. A.; Jones, R. J . ; Vaughan, R. W.; and Kendrick, W. P., "Thermally Stable Laminating Resins"(1970), Report TRW-11926-6013-RO-00; NASA CR-72633. S e r a f i n i , T. T.; Delvigs, P.; and Lightsey, G. R., "Theimally Stable Polyimides from Solutions o f Mono meric Reactants"(1972), TN D-6611 NASA, J.Appl.Polymer S c i . , (16), p. 905. Delvigs, P.; S e r a f i n i , T. T.; and Lightsey, G. R., "Addition-Type Polyimides from Solutions of Monomeric Reactants", NASA TN D-6877 (1972); M a t e r i a l s Review f o r '72: Proceedings of the National Symposium and Exhi b i t i o n , Science of Advanced M a t e r i a l s and Process Engineering S e r i e s , (17) p. III-B-7-1. Jones, R. J . ; Vaughn, R. W.; and Burns, Ε. Α., "Ther mally Stable Laminating Resins"(1972), TRW Report 16402-6012-RO-00, NASA CR-72984. Blake, E. S.; Hammann, W. C.; Edwards, J . W.; Reichard, T. E.; and Ort, M. R., (1961) J. Chem. Eng. Data (16), p. 87. Bose, Α. Κ., "Organic Synthesis", (1973), C o l l e c t . (Vol.5), Wiley, NY, p. 973. Bengelsdorf, I . S., (1958), J. Amer. Chem. Soc., (80) p. 1442. Cairns, T. L. S.; Larchar, A. W.; and McKusick, B. C., U.S. Patent 2,503,999(1950); Chem. Abstr., (44), 6445c. Kunz, M. A.; K o e f e r l e , K.; and Berthold, E., U.S.Patent 1,989,042(1935); Chem. Abstr. (29), 1834. Smolin, Ε. M.; and Rapoport, L., "s-Triazines and D e r i v a t i v e s " , (1967)Interscience, NY, p. 172. Mahan, J. E.; and Turk, S. D., U.S. Patent 2,598,811 (1952); Chem. Abstr. (46), 10212h. F i l h o , G. Martins; da Costa, J. Goncalves; de Azevedo, F. Soares; and de Morais, J. O. Falcao, Anais. Acad. B r a s i l , Cienc. (35), 185 (1963). Bogert, M. T.; and Wise, L. E., J. Amer. Chem. Soc., (34) 693(1912). Anderson, D. R.; and Holovka, J. M., J . Polymer Sci., A-1, (4), 1689 (1966).


Catalytic Trimerization of Aromatic Nitriles for Synthesis of Polyimide

Recommend Documents