[ACS Symposium Series] Inorganic and Organometallic Polymers Volume 360 (Macromolecules Containing Silicon, Phosphorus, and Other Inorganic Elements) || Synthetic Routes to Oligosilazanes and Polysilazanes

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<ul><li><p>Chapter 10 </p><p>Synthetic Routes to Oligosilazanes and Polysilazanes Polysilazane Precursors to Silicon Nitride </p><p>Richard M. Laine 1, Yigal D. Blum, Doris Tse, and Robert Glaser 2 </p><p>Inorganic and Organometallic Chemistry Programs, SRI International, Menlo Park, CA 94025 </p><p>Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, compounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the general synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. The latter part of the chapter focusses on catalyzed dehy-drocoupling reactions as a route to polysilazanes. </p><p>Non-oxide ceramics such as s i l i c o n carbide ( S i C ) , s i l i c o n n i t r i d e (S13N4), and boron n i t r i d e (BN) o f f e r a wide v a r i e t y of unique p h y s i c a l p r o p e r t i e s such as high hardness and high s t r u c t u r a l s t a b i l i t y under environmental extremes, as well as vari e d e l e c t r o n i c and o p t i c a l p r o p e r t i e s . These advantageous p r o p e r t i e s provide the d r i v i n g force f o r intense research e f f o r t s d i r e c t e d toward developing new p r a c t i c a l a p p l i c a t i o n s for these m a t e r i a l s . These e f f o r t s occur despite the considerable expense often associated with t h e i r i n i t i a l preparation and subsequent transformation into f i n i s h e d products. </p><p>The expense in preparation derives from the need f o r high p u r i t y products. The cost in f a b r i c a t i n g three-dimensional shapes of SiC, ^ or BN a r i s e s from the need to s i n t e r powders of these materials at high temperatures (&gt;1500C) and often under high pressure. Coatings of these materials are at present produced by chemical or </p><p>Current address: Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195 </p><p>2On leave from the Department of Chemistry, Ben Gurion University, Beer Sheva, Israel </p><p>0097-6156/88/0360-0124$06.00/0 1988 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Nov</p><p>embe</p><p>r 20</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>010</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>10. LAINE ET AL. Synthetic Routes to Polysilazanes 125 </p><p>p h y s i c a l vapor deposition (CVD or PVD). In each instance, the process is equipment and energy int e n s i v e and, has numerous l i m i t a -t i o n s . I f inexpensive methods for the synthesis and f a b r i c a t i o n of non-oxide ceramics can be developed, then the number of u s e f u l a p p l i c a t i o n s could be enormous. </p><p>One p o t e n t i a l s o l u t i o n to these problems, suggested some 20 years ago by C h a n t r e l l and Popper (1), involves the use of inorganic or organo-m e t a l l i c polymers as precursors to the desired ceramic m a t e r i a l . The concept (2) centers on the use of a t r a c t a b l e ( s o l u b l e , meltable or malleable) inorganic precursor polymer that can be shaped at low temperature (as one shapes organic polymers) i n t o a coating, a f i b e r or as a matrix (binder) for a ceramic powder. Once the f i n a l shape is obtained, the precursor polymer can be p y r o l y t i c a l l y transformed int o the desired ceramic m a t e r i a l . With c a r e f u l c o n t r o l of the py r o l y s i s c o n d i t i o n s , the f i n a l piece w i l l have the appropriate p h y s i c a l and/or e l e c t r o n i c p r o p e r t i e s . </p><p>The commercialization of Nicalon (Nippon Carbon Co.), a SiC based ceramic f i b e r f a b r i c a t e d by p y r o l y s i s of f i b e r s of polycarbosilane (3), has sparked i n t e n s i v e e f f o r t s to synthesize polymer precursors of use in the f a b r i c a t i o n of Si^N^ and BN f i b e r s , coatings and binders. In t h i s regard, o l i g o - and pol y s i l a z a n e s have r e c e n t l y been shown to o f f e r considerable p o t e n t i a l as precursors to S13N4. The obje c t i v e of t h i s chapter is to provide an overview of the general methods for synthesizing o l i g o - and pol y s i l a z a n e s and to show how some of these routes have been used to synthesize o l i g o - and poly-s i l a z a n e preceramic polymers. In the l a t t e r part of the chapter, we w i l l emphasize our own work on t r a n s i t i o n metal catalyzed dehydro-coupling r e a c t i o n s . </p><p>General Synthetic Approaches </p><p>H i s t o r i c a l l y , o l i g o - and polysilazane s y n t h e t i c chemistry has passed through three stages of development with quite d i f f e r i n g goals. I n i t i a l e f f o r t s , beginning with the work of Stock and Somieski in 1921, were d i r e c t e d simply towards the preparation and c l a s s i f i c a t i o n of the general properties of polysilazanes (4,5). The commercial success of polysiloxanes or s i l i c o n e s in the f i f t i e s and s i x t i e s prompted studies on the synthesis of pol y s i l a z a n e s as p o t e n t i a l analogs (without much success). As mentioned above, current i n t e r e s t derives from t h e i r use as s i l i c o n n i t r i d e preceramic polymers. </p><p>Reactions ( l ) - ( 5 ) i l l u s t r a t e known methods for forming s i l i c o n -n itrogen bonds of p o t e n t i a l use in the formation of o l i g o - and poly-s i l a z a n e s . The most common method of forming s i l a z a n e s is v i a ammo-n o l y s i s or aminolysis as shown in r e a c t i o n (1) (4,5): </p><p>R 3 S i C l + 2R f 2NH &gt; R 3 S i N R f 2 + R , 2NH 2 + C 1"" ( X) </p><p>Si-N bonds can also be formed by a dehydrocoupling r e a c t i o n catalyzed by a l k a l i (6-11) or t r a n s i t i o n metals (12): </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Nov</p><p>embe</p><p>r 20</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>010</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>126 INORGANIC AND ORGANOMETALLIC POLYMERS </p><p>R 3SiH + R f 2NH c a t a l y s t &gt; R 3 S i N R 2 + ^ ( 2 ) </p><p>In rare instances, one can also obtain displacement of oxygen by nitrogen as i l l u s t r a t e d by react i o n (3) (13). Reaction (3) proceeds despite the f a c t that the Si-0 bond is 25-30 kcal/mole stronger than the Si-N bond (14). </p><p>R 3Si-0H + R'2NH &gt; R 3SiNR f 2 + H 20 (3) </p><p>Verbeek and Winter have used a deamination/condensation r e a c t i o n to form new Si-N bonds as exemplified by re a c t i o n (4) (15,16). </p><p>2R 2Si(NHR f) 2 &gt; (R fNH)R 2SiN(R ,)SiR 2(NHR l) + R'NH2 (4) </p><p>It has been suggested that r e a c t i o n (4) passes through an R 2Si=NR f intermediate, although no su b s t a n t i a t i o n e x i s t s (17,18). </p><p>Van Wazer (19) has described the use of a r e d i s t r i b u t i o n r e a c t i o n , i l l u s t r a t e d in (5), to form l i n e a r oligomers from c y c l i c species: </p><p>2 R 2 S i C l 2 + i-[R 2SiNH] 3-l &gt; ClR 2SiNHSiR 2Cl + C l - [ R 2 S i N H ] 2 - S i R 2 C l (5) </p><p>With the exception of re a c t i o n (3), the u t i l i t y of a l l of the above reactions for the synthesis of o l i g o - and pol y s i l a z a n e s has been examined in varying d e t a i l . In the following paragraphs, we attempt to examine the pertinent studies in each area e s p e c i a l l y as i t re l a t e s to the synthesis of preceramic polymers. </p><p>Polymerization by Ammonolysis and Aminolysis </p><p>The s i m p l i c i t y of the ammonolysis/aminolysis of d i h a l o h a l o s i l a n e s , reactions (6) and (7), made them the o r i g i n a l method of choice f o r the synthesis of o l i g o - and po l y s i l a z a n e s , e s p e c i a l l y because of the analogy to the h y d r o l y t i c synthesis of po l y s i l o x a n e s . The ammono-l y s i s of d i h a l o s i l a n e s , r e a c t i o n (6), has been found to be extremely s e n s i t i v e to s t e r i c f a c t o r s (6). </p><p>R 1 R S i C l 2 4- 3xNH3 &gt; -[R 1RSiNH] x~ + 2xNH 4 +Cl" (6) </p><p>For example, with R 1 - R - H, re a c t i o n (6) gives only o l i g o s i l a z a n e s ; with R 1 = CH 3 and R = H, both c y c l i c and o l i g o s i l a z a n e s are formed; and with R 1 &gt; CH 3 and R = H then one obtains mostly c y c l o t r i - and c y c l o t e t r a s i l a z a n e s as the products. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Nov</p><p>embe</p><p>r 20</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>010</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>10. LAINE ET AL. Synthetic Routes to Polysilazanes 127 </p><p>Aminolysis of d i h a l o s i l a n e s , r e a c t i o n (7), is also extremely s e n s i t i v e to the s t e r i c bulk of the amine (20). For R 1 = CH3, </p><p>x R 1 R S i C l 2 + 3xR'NH2 &gt; -[R 1RSiNR t ] - + 2xR ,NH 3 +Cl~ (7) </p><p>R CH3 and Rf &gt; Et, the products are predominantly the c y c l o t r i - and c y c l o t e t r a s i l a z a n e s and/or simple d i s i l a z a n e s , e.g., R^RSi(NHR ?) 2. For reactions where R 1 = H or CH3, R = H and R1 = CH3, the products can be mostly l i n e a r o l i g o s i l a z a n e s depending on conditions (see below) (21-23). The preference for c y c l o t r i - and c y c l o t e t r a s i l a zanes, in these r e a c t i o n s , is not s u r p r i s i n g given that the hydrol y s i s of d i h a l o s i l a n e s also leads to c y c l o t r i - and c y c l o t e t r a -s i l o x a n e s . </p><p>The search f o r s i l i c o n n i t r i d e precursors has renewed i n t e r e s t in ammonolysis as a route to p o l y s i l a z a n e s . In our own work, we have followed up on e a r l y studies by Seyferth and Wiseman (20,21) on the preparation of -[H 2SiNMe] x- from MeNH2 and H 2 S i C l 2 . We f i n d that the molecular weight of th i s oligomer is g r e a t l y a f f e c t e d by the re a c t i o n c o n d i t i o n s . O r i g i n a l l y , Seyferth and Wiseman were able to produce mixtures of oligomers of -[H 2SiNMe] x- and the cyclotetramer (22). Removal of the cyclotetramer by d i s t i l l a t i o n gave a polymer with Mn - 600 Daltons (x = 10). We have now succeeded in preparing quant i t i e s of -[H 2SiNMe] x- with Mn 1000-1200 (x = 18-20) with l e s s than 10% cyclomers (23). The r e s u l t i n g o l i g o s i l a z a n e can be used d i r e c t l y as a precursor f o r t r a n s i t i o n metal catalyzed dehydrocoupling polymerization (see below). </p><p>A r a i et a l . (24) have devised a novel approach to ammonolysis wherein the s i l y l h a l i d e is modified by complexation to py r i d i n e (py) p r i o r to ammonolysis: </p><p>R H S i C l 2 + 2py &gt; RHSiCl 2 2py (8) </p><p>RHSiCl 22py + NH3 &gt; NH4C1 (or py.HCl) + -[RHSiNH] x~ (9) R = Me or H </p><p>The o l i g o s i l a z a n e s formed v i a t h i s approach give higher molecular weights than obtainable by d i r e c t ammonolysis. When R = H, the product is a polydispersed polymer with a weight average molecular weight (M^j) of 1300 D. The polymer is not stable and c r o s s l i n k s to form an i n t r a c t a b l e product in a short time in the absence of solvent. P y r o l y s i s of t h i s material gives an 80% ceramic y i e l d of s i l i c o n n i t r i d e mixed with s i l i c o n . S i l i c o n is found even when p y r o l y s i s is conducted under an ammonia atmosphere. The t e n t a t i v e l y proposed oligomer structure is (25): </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Nov</p><p>embe</p><p>r 20</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>010</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>128 INORGANIC AND ORGANOMETALLIC POLYMERS </p><p>S i H 3 </p><p>S i H 2 </p><p>iH 2 /N \ c m N </p><p>H 2 s r -S I H 2 S.IH2 </p><p>J. 1 ^ A S I H 2 </p><p>H H </p><p>Of i n t e r e s t is the f a c t that IR and NMR evidence point to the presence of SiH3 groups in the o l i g o s i l a z a n e . The SiH3 groups must a r i s e as a consequence of a r e d i s t r i b u t i o n process, although further study remains to v e r i f y these r e s u l t s . When R = Me, the r e s u l t i n g polymer, -[MeHSiNH] x-, can have molecular weights of 1700 D. Pyrol y s i s of t h i s m a t e r i a l gives high ceramic y i e l d s . The composition of the ceramic product was not reported. </p><p>Ring Opening Polymerization </p><p>Base and ac i d catalyzed r i n g opening polymerization of c y c l o t r i -s i l o x a n e , -[Me2SiO]3- [e.g. rea c t i o n (10)], is a well-known method of generating high molecular weight polysiloxanes. </p><p>R0- + L [ M e 2 S i O ] 3 - l &gt; RO-[Me 2SiO] 3 - (10) R0- = alkoxide or hydroxide </p><p>A number of groups have attempted to develop the analogous r e a c t i o n for c y c l o t r i - and c y c l o t e t r a s i l a z a n e s . Ring opening polymerization represents an a l t e r n a t i v e to d i r e c t ammonolysis even though the cyclomer precursors are normally made by ammonolysis or aminolysis. </p><p>Thus, Andrianov et a l . (26) attempted to catalyze polymerization of a number of a l k y l and a l k y l / a r y l c y c l o s i l a z a n e s using c a t a l y t i c amounts of KOH or other strong bases at temperatures of up to 300C. In general, the reactions proceed with e v o l u t i o n of NH3, hydrocarbons and the formation of i n t r a c t a b l e , c r o s s l i n k e d , b r i t t l e products even at low temperatures. Contrary to what is observed with c y c l o t r i -s i l o x a n e s , no evidence was found f o r the formation of l i n e a r polys i l a z a n e s . Copolymerization of mixtures of c y c l o s i l a z a n e s and cyc l o s i l o x a n e s gave somewhat more t r a c t a b l e polymers with l e s s e v o l u t i o n of hydrocarbons or ammonia, however very l i t t l e was done to c h a r a c t e r i z e the r e s u l t i n g m a t e r i a l s . </p><p>An explanation f o r the lack of formation of l i n e a r p o l y s i l a z a n e s in base catalyzed r i n g opening is suggested by one of Fink's p u b l i c a t i o n s ( 7 ) . Fink observes that o l i g o s i l a z a n e amides w i l l undergo fragmentation to simple amides under the conditions employed by Andrianov: </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>CSF</p><p> LIB</p><p> CK</p><p>M R</p><p>SCS </p><p>MG</p><p>MT</p><p> on </p><p>Nov</p><p>embe</p><p>r 20</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>010</p><p>In Inorganic and Organometallic Polymers; Zeldin, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. </p></li><li><p>10. LAINE ET AL. Synthetic Routes to Polysilazanes 129 </p><p>R3SiN(Li)SiR 2N(SiR 3)2 -&gt; L i N ( S i R 3 ) 2 + 0 . 5 ( R 3 S i N...</p></li></ul>