[ACS Symposium Series] Inorganic and Organometallic Polymers Volume 360 (Macromolecules Containing Silicon, Phosphorus, and Other Inorganic Elements) || Boron Nitride and Its Precursors

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<ul><li><p>Chapter 32 </p><p>Boron Nitride and Its Precursors </p><p>K. J. L. Paciorek 1, W. Krone-Schmidt 1, D. H. Harris 1, R. H. Kratzer 1, and Kenneth J. Wynne 2 </p><p>1Ultrasystems Defense and Space, Inc., Irvine, CA 92714 2Office of Naval Research, Arlington, VA 22217 </p><p>Stepwise condensation of a series of borazines was investigated. The compounds studied were B-tri-chloro-N-triphenylborazine, B-triamino-N-triphenyl-borazine, B-trianilinoborazine, B-tris[di(trimethyl-silyl)amino]-borazine, B-trichloro-N-tris(trimethyl-silyl)borazine, and B-triamino-N-tris(trimethyl-silyl)borazine. The formation of preceramic polymers occurs by ring opening mechanism which leads to singly and doubly joined borazine rings. This process requires the presence of a combined total of six protons on the ring and exocyclic nitrogens. Compounds where this arrangement is absent, B-trichloro-N-triphenylborazine, B-tris[di(trimethylsilyl)amino]borazine, and B-trichloro-N-tris(tri-methylsilyl)borazine, failed to undergo condensation to any significant degree. Materials obtained from the phenyl-substituted borazines gave essentially nonprocessible polymers on pyrolysis; from B-tri-amino-N-tris(trimethylsilyl)borazine, preceramic polymers amenable to fiber manufacture were obtained. Investigation of potential linear precursor to boron-nitrogen polymers resulted in the synthesis of -imido-bis[bis(trimethylsilyl)-aminotrimethylsilylamino]borane (I) and -x-bis-[bis(trimethylsilyl)aminotrimethylsilylamino]borane (II). The crystal structures of the two compounds were essentially identical with the exception of the bridging atoms and their immediate environments. The same applied to mass spectral breakdown patterns wherein the major fragments differed by one amu, with the exception of the ion 332+ which is characteristic to the oxygen-bridged compound. </p><p>R e f r a c t o r i e s such as boron n i t r i d e , s i l i c o n n i t r i d e , s i l i c o n carbide, and boron carbide are of great importance f o r the product i o n or pr o t e c t i o n of systems which can be operated i n very high </p><p>0097-6156/88/0360-0392506.00/0 1988 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>TE</p><p>NN</p><p>ESS</p><p>EE</p><p> KN</p><p>OX</p><p>VIL</p><p>LE</p><p> on </p><p>Dec</p><p>embe</p><p>r 21</p><p>, 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Jan</p><p>uary</p><p> 7, 1</p><p>988 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1988</p><p>-036</p><p>0.ch</p><p>032</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>32. PACIOREK ET AL. Boron Nitride and Its Precursors 393 </p><p>temperature environments. The a p p l i c a b i l i t y of these materials as coatings, f i b e r s , as well as bulk items depends on the a b i l i t y to produce a r e a d i l y p r o c e s s i b l e polymer, which can be formed in t o the desired shape of the f i n a l item p r i o r to exhaustive p y r o l y s i s and transformation into a ceramic. The technology to provide such precursors f o r s i l i c o n carbide, s i l i c o n n i t r i d e , and graphite, i n p a r t i c u l a r the l a t t e r , has been developed and although work i s p r o c e e d i n g on improvements, the premises are r e l a t i v e l y well e s t a b l i s h e d Q ) . The s i t u a t i o n i n the case of boron n i t r i d e i s e n t i r e l y d i f f e r e n t . The extensive work performed i n the s i x t i e s was di r e c t e d at thermally s t a b l e polymers. The products were poorly characterized and the l i t e r a t u r e data are c o n f l i c t i n g (2). More recently, claims have been made to the synthesis of BN preceramic polymers amenable to f i b e r production using the p y r o l y s i s of B-triamino-N-triphenylborazine (3). Unfortunately, t h i s work could not be reproduced (4,5). </p><p>Boron n i t r i d e , i n view of i t s unique p r o p e r t i e s , namely absence of e l e c t r i c a l c o n d u c t i v i t y , oxidation r e s i s t a n c e , o p t i c a l transparency, and high neutron capture c r o s s - s e c t i o n f o r s p e c i a l a p p l i c a t i o n s , o f f e r s advantages over other ceramics. Thus, f o r the past several years the group at Ultrasystems has been a c t i v e l y involved i n i n v e s t i g a t i n g routes leading to preceramic BN polymers using both c y c l i c and l i n e a r s t a r t i n g materials. Some of the find i n g s generated w i l l be discussed. </p><p>Results and Discussion </p><p>Borazine synthesis. The ease of borazine r i n g condensation would be expected to depend on the nature of the substituents. To i n v e s t i -gate these aspects, a s e r i e s of compounds were synthesized, namely B - t r i c h l o r o - N - t r i phenyl b o r a z i n e , B-triamino-N-triphenylborazine, B - t r i a n i l i n o b o r a z i n e , B - t r i s [ d i ( t r i m e t h y l s i l y l ) a m i n o ] b o r a z i n e , B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e , and B-triamino-N-tris-( t r i m e t h y l s i l y l ) b o r a z i n e . </p><p>B - t r i c h l o r o - N - t r i p h e n y l b o r a z i n e , mp 290-292C, was obtained i n 86* y i e l d f o l l o w i n g the procedure of Groszos and S t a f i e j (6). T h i s m a t e r i a l was then transformed i n t o B-triamino-N-triphenyl-borazine i n 67% y i e l d using the method of Toeniskoetter and Hall (1) B - t r i a n i l i n o b o r a z i n e and the novel B - t r i s [ d i ( t r i m e t h y l s i l y l ) -aminojborazine (mp, 131.5-132C.; characterized by GC/MS, molecular ion 558 amu, and elemental a n a l y s i s ) were synthesized i n 76 and 71* y i e l d s , r e s p e c t i v e l y , by i n t e r a c t i o n of a n i l i n e and hexamethyl-d i s i l a z a n e with chloroborazine i n the presence of triethylamine. </p><p>The s y n t h e s i s of B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e was much more complicated than that of the other borazines. The o v e r a l l scheme i s given below: </p><p>BC1 3 + E t 3 N B C l 3 ' N E t 3 CI </p><p>Me QSiN NSiMe Q 3 I I 3 CIB^ </p><p>HN(SiMe.) 2 + </p><p>NEt 3 ^ B C l P y r o l y s i s </p><p>I SiMe </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>TE</p><p>NN</p><p>ESS</p><p>EE</p><p> KN</p><p>OX</p><p>VIL</p><p>LE</p><p> on </p><p>Dec</p><p>embe</p><p>r 21</p><p>, 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Jan</p><p>uary</p><p> 7, 1</p><p>988 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1988</p><p>-036</p><p>0.ch</p><p>032</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>394 INORGANIC AND ORGANOMETALLIC POLYMERS </p><p>Boron t r i c h l o r i d e - t r i e t h y l a m i n e adduct, mp 89-91C, was obtained i n 80* y i e l d f o l l o w i n g e s s e n t i a l l y the procedure of Ohashi, et a l . (8). The product was washed w i t h methanol, but not c r y s t a l -l i z e d from i t . I t was found that c r y s t a l l i z a t i o n from d i l u t e e t h a n o l r e p o r t e d by G e r r a r d , L a p p e r t , and Pearce (9) caused extensive degradation . B i s ( t r i m e t h y l s i l y l ) a m i n o d i c h l o r o b o r a n e was prepared f o l l o w i n g the procedure of Wells and C o l l i n s (10). </p><p>The p y r o l y s i s of b i s ( t r i m e t h y l s i l y l )aminodichloroborane to B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e , contrary to l i t e r a t u r e (11^, d i d not take place i n b o i l i n g xylene. Temperatures above 150 C were necessary f o r t r i m e t h y l c h l o r o s i l a n e e l i m i n a t i o n . The highest y i e l d of the r e l a t i v e l y pure product was around 20*. The transformation of the B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l J b o r a z i n e to B - t r i a m i n o - N - t r i s ( t r i m e t h y l s i l y l Jborazine proceeded r e a d i l y using l i q u i d ammonia. </p><p>B o r a z i n e P y r o l y s i s Studies. In a s i m p l i s t i c manner, one can v i s u a l i z e the formation of boron n i t r i d e v i a stepwise e l i m i n a t i o n s between r i n g s , i . e . , </p><p>R I </p><p>X MONOMER PRECERAMIC POLYMER </p><p>\ </p><p>BORON NITRIDE CROSSLINKED POLYMER </p><p>g i v i n g i n i t i a l l y the " l i n e a r " preceramic polymer, then the l i g h t l y c r o s s l i n k e d s t r u c t u r e B, followed by a p a r t i a l B-N system, C, and f i n a l l y pure boron n i t r i d e . The p y r o l y s i s techniques u t i l i z e d were f u l l y described p r e v i o u s l y (4J and thus w i l l not be r e i t e r a t e d here. The p o t e n t i a l borazine candidates can be broadly d i v i d e d i n t o two c l a s s e s of m a t e r i a l s , one where the s u b s t i t u e n t on the r i n g boron i s n i t r o g e n - f r e e , e.g., a halogen, and the second where the su b s t i t u e n t i s an amino moiety. With respect to the s u i t a b i l i t y of any given borazine, the important aspects are the ease of formation of the l e a v i n g molecule and i t s v o l a t i l i t y . In t h i s respect, B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e would seem an i d e a l candidate. Unfortunately, p y r o l y s i s at ,^60 C l i b e r a t e d only 6.1* of the expected t r i m e t h y l c h l o r o s i l a n e ; 75* of the s t a r t i n g m a t e r i a l was recovered. Heating at 360C f o r 19 h r e s u l t e d i n 56.0* y i e l d of t r i m e t h y l c h l o r o s i l a n e and a gl a s s y residue. Further heating at </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>TE</p><p>NN</p><p>ESS</p><p>EE</p><p> KN</p><p>OX</p><p>VIL</p><p>LE</p><p> on </p><p>Dec</p><p>embe</p><p>r 21</p><p>, 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Jan</p><p>uary</p><p> 7, 1</p><p>988 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1988</p><p>-036</p><p>0.ch</p><p>032</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>32. PACIOREK ET AL. Boron Nitride and Its Precursors 395 </p><p>360C f o r 72 h afforded only an a d d i t i o n a l 7.3% of the expected t r i m e t h y l c h l o r o s i l a n e . This r e s u l t shows c l e a r l y that t h i s type of borazine does not lead to a d e s i r a b l e preceramic polymer, inasmuch as at 260 C the condensation process proceeds only to a l i m i t e d degree, with the s t a r t i n g m a t e r i a l being l a r g e l y recovered and the p o l y m e r i c p r o d u c t formed b e i n g both i n s o l u b l e and i n f u s i b l e . B - t r i c h l o r o - N - t r i p h e n y l b o r a z i n e would not be expected to undergo condensation r e a d i l y inasmuch as the formation of chlorobenzene i s not a f a v o r e d p r o c e s s . The experimental data supported t h i s p o s t u l a t i o n ; e s s e n t i a l l y no degradation occurred at 300C. </p><p>B-triamino-N-triphenylborazine, and i n p a r t i c u l a r i t s isomer, B - t r i a n i l i n o b o r a z i n e , the l a t t e r e i t h e r alone or i n co - r e a c t i o n w i t h B-triamino-N-triphenylborazine, gave f u s i b l e , </p></li><li><p>396 INORGANIC AND ORGANOMETALLIC POLYMERS </p><p>( M e 3 S i ) 2 N </p><p>HN I </p><p>(Me S i ) NB &gt; </p><p>H I </p><p>N ( S i M e 3 ) 2 </p><p>I HN </p><p>BN(SiMeJ I NH </p><p>I N(SiMe 3), </p><p> 2 </p><p>This apparently i s not the case. On prolonged exposure i n ammonia at 250-260c some l i b e r a t i o n of hexamethyldisilazane took place, but only to a very low degree. </p><p>The behavior of B - t r i a m i n o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e was i n good agreement with the r i n g opening mechanism. This compound was much more r e a c t i v e than i t s phenyl analogues; thus, the pure monomer could not be i s o l a t e d . The product obtained from the i n t e r a c t i o n of ammonia and B - t r i c h l o r o - N - t r i s ( t r i m e t h y l s i l y l )-borazine co n s i s t e d of a 1:1 mixture of the monomer and a s i n g l y -bridged dimer, x=l. </p><p>Me 3Si-H2N-</p><p>NH </p><p>N-I </p><p>I NH, SiMe, </p><p>I 3 </p><p>2 M e 3 S i </p><p>NH 2 SiMe n </p><p>NH n </p><p>Both the monomer and dimer were composed most l i k e l y of isomers w i t h some p r o t o n s r e s i d i n g on r i n g nitrogens and with the NHSiMe^ moiety r e p l a c i n g the amino group on some of the boron r i n g atoms i n accordance w i t h the Scheme 1. This assumption i s supported by the l i q u i d n a t u r e of the product and i t s i n f r a r e d spectrum which e x h i b i t e d two broad bands centered at 3430 and 3530 cm" . The i n f r a r e d spectrum of pure B - t r i a m i n o - N - t r i s ( t r i m e t h y l s i l y l ) b o r a z i n e would be expected to be s i m i l a r to that of B - t r i a m i n o - N - t r i s ( t r i -phenyl)borazine where three sharp bands at 3430, 3505, and 3530 were observed (4). In the l a t t e r case, once the condensation process was i n i t i a t e d , the sharp bands disappeared {4). </p><p>To remove v o l a t i l e i m p u r i t i e s , the m a t e r i a l was heated i n vacuo a t 135 C This treatment r e s u l t e d i n f u r t h e r condensation. Thus, the d i s t i l l a t i o n residue was composed of a mixture of doubly-bridged dimers and tetramers, = 1 and 2 as determined from the molecular weight, boron and nitrogen analyses and the v o l a t i l e condensibles produced. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>TE</p><p>NN</p><p>ESS</p><p>EE</p><p> KN</p><p>OX</p><p>VIL</p><p>LE</p><p> on </p><p>Dec</p><p>embe</p><p>r 21</p><p>, 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Jan</p><p>uary</p><p> 7, 1</p><p>988 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1988</p><p>-036</p><p>0.ch</p><p>032</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>32. PACIOREK ET AL. Boron Nitride and Its Precursors 397 </p><p> I </p><p> I </p><p> I </p><p> I </p><p> I </p><p> &gt;H) 1 ^ -</p><p>^N^ / N v I I </p><p> I </p><p> I I </p><p> I I </p><p> I I </p><p>/ ^ / I &lt; /^ \ </p><p>/ </p><p>I </p><p>Scheme . </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>TE</p><p>NN</p><p>ESS</p><p>EE</p><p> KN</p><p>OX</p><p>VIL</p><p>LE</p><p> on </p><p>Dec</p><p>embe</p><p>r 21</p><p>, 201</p><p>4 | h</p><p>ttp://</p><p>pubs</p><p>.acs</p><p>.org</p><p> P</p><p>ublic</p><p>atio</p><p>n D</p><p>ate:</p><p> Jan</p><p>uary</p><p> 7, 1</p><p>988 </p><p>| doi</p><p>: 10.</p><p>1021</p><p>/bk-</p><p>1988</p><p>-036</p><p>0.ch</p><p>032</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>398 INORGANIC AND ORGANOMETALLIC POLYMERS </p><p>H2N-Me 3Si-f-N </p><p>SiMe Q I 3 </p><p>- I I </p><p>*B" I NH 0 </p><p>NH 2 </p><p>J N-I I </p><p>I SiMe </p><p>SlMe 3 NH 0 </p><p>The c o n d e n s a t i o n p r o c e s s a p p a r e n t l y i n v o l v e s e l i m i n a t i o n of t r i m e t h y l s i l y l a m i n e , ( C H 3 ) S i N H ^ f w h i c h , b e i n g u n s t a b l e , d i s p r o p o r t i o n a t e s i n t o hexamethyldisnazane and ammonia. In the condensable v o l a t i l e s c o l l e c t e d , these m a t e r i a l s were present i n a 1:1 r a t i o supporting the postulated process. Further thermolysis at 200C (52 h ) , f o l l o w e d by 4 h at 250-260C, r e s u l t e d i n a mixture of doubly-bridged tetramers and octamers, x=2 and 4. At t h i s stage, 53* of the p o t e n t i a l l e a v i n g groups were l i b e r a t e d . From the tetramer/octamer mixture, f i b e r s with 10-20 diameter could be melt drawn as shown i n Figures 1 and 2. Gradual heat treatment i n ammonia, from 65-950C, r e s u l t e d i n white, pure boron n i t r i d e f i b e r s . The f i b e r s e x h i b i t e d no weight l o s s under thermogravimetric c o n d i t i o n s , both i n a i r and i n nit r o g e n up to 1000C (12). </p><p>P o t e n t i a l Non-Cyclic Precursors of Pre...</p></li></ul>

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