Recent Developments in Borazine-Based Polymers - American

Chapter 27

Recent Developments in Borazine-Based Polymers 1

2

R. T. Paine and Larry G. Sneddon 1

Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323

Downloaded by MONASH UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch027

2

Although they have tremendous applications potential, inorganic polymers have received relatively little attention compared to organic polymers. Recent demands for entirely new hybrid materials and advanced ceramics, however, are driving efforts to develop new inorganic polymer systems. This paper summarizes some historical background along with recent results from our two laboratories on the formation of boron-nitrogen polymers based upon borazine monomer chemistry that will be useful in the realization of advanced materials goals. A wide spectrum of organic polymer chemistry has been developed over the last fifty years, and the synthesis, characterization and processing of organic polymers has been one of the most thoroughly studied topics in modern chemistry. Surprisingly, in spite of their abundance in the periodic table and their flexibility in structure and reactivity, the polymer chemistry of main group elements has not enjoyed parallel, active development. This fact was first recognized in the 1950's and 60's, and serious attempts were made at that time to synthesize selected inorganic polymers, including polyphosphazenes, poly silicones, polysilicates, polyborates, and polysulfurirnines. Some initial successes were realized, and even some commercial development, e.g., polysilicones and polyphosphazenes, occurred, but, in general, interest in these polymers declined due to synthetic roadblocks, property shortcomings and synthesis and processing economics. In the 1980's and 90's the advent of new technological needs in materials science has reactivated interest in inorganic polymers, and a number of exciting developments have occurred. In particular, it was found that controlled hydrolysis of tetraethyl orthosilicate (TEOS) leads initially to soluble silicate polymers that form processible silicate gels. This observation spawned the now rich field of sol-gel chemistry that encompasses not only silicate polymers, but also oxo polymers of a variety of metals. In a different arena, Verbeek and Yajima reported the synthesis of processible polycarbosilanes that have found use as precursors for the nonoxide compositions SiC and SiC/Si3N4. These findings and pressing practical needs for more useful forms of existing and new advanced high-temperature materials are encouraging additional studies of inorganic polymers and inorganic/organic hybrid polymers. 1

2

3

4

5-10

0097-6156/94/0572-0358$08.00/0 © 1994 American Chemical Society In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

27.

Recent Developments in Borazine-Based Polymers

PAINE & SNEDDON

Boron nitride is one of several commercially significant high temperature ceramics that is easily obtained in powder formfrompyrolysis of simple inexpensive reagents. Unfortunately, it is difficult or impossible to obtain B N in some desired forms (e.g., fibers, coatings)frompowders. Requirements for more processible boron nitride have encouraged several research groups to seek new approaches to obtain these forms, and boron-nitrogen containing polymers offer an attractive vehicle.


11

One appropriately constituted molecular species upon which polymeric boronnitride precursors could be based is the ring-compound borazine, B 3 N 3 H 6 (1). The compound is easily made and has both the correct boron/nitrogen ratio and a 6membered ring structure analogous to the solid state structure of h-BN. Furthermore, the similarity of the structures and physical properties of benzene and borazine, suggest that the borazine analogues of many benzene-derived polymers might be possible. In the following sections we outline recent advances in the development of two general classes of borazine-based polymers in which the borazine ring is either a member of the polymer backbone or a pendant on a polymer having an organic or inorganic backbone. Borazine Backbone Polymers In our work two general approaches to forming borazine-backbone polymers are being explored. These involve formation of polyphenylene analogues in which the borazine rings are joined either by direct B-B (2) or B-N (3) linkages or borazine-bridged frameworks with the rings linked via an intervening atom or group, X (4,5). \

/

Β— Β—Ν

/

\ / Ν— Ν—Β

\

/ 2

\

\

B^N

/

\

B^N

Ό - \ 0 / Β—Ν

/

Β

_

Β—Ν

\

/

/

\

3

In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

359

360

I N O R G A N I C A N D O R G A N O M E T A L L I C P O L Y M E R S II

^ B ^ - ^ B ^ ^B^-^B^ Ν Χ Ν

Polyphenylene Analogues. Early work aimed at the development of linked-ring borazine polymers with structures (2-3) has been outlined in a previous review. In particular, it is worth noting that the metathetical reaction of N-lithio pentamethyl borazine with B-chloropentamethyl borazine (eq. 1) led to LiCl metathesis and production of decamethyl-N,B-diborazine. Extensions of this chemistry resulted in formation of a polyborazinyl chains, estimated to contain ten monomer rings, which melted over a broad temperature range. Unfortunately, little follow up on these polymers has been published perhaps because the polymers are not likely to be efficient ceramic precursors due to their expected low char yields and the inclusion of carbon in their ceramic residues. 11


12

Me Me I I Β—Ν

Me Me I I Ν—Β

Me Me I I Β—Ν

M e - ^ f ) B-CI + L i - N ^ f ) B-Me B-N

Ν - β '

Me Me

Me Me

- Me-ISî

Me Me I I Β—Ν

J Β— Νί

)β-Μθ

(1)

W

Me Me

Me Me

The use of other types of elimination reactions for the formation of B-N linked systems has also been used to yield alkylated diborazines. Recently, Niedenzu has reported that the reaction of Et3B3N3H2(SiMe3) with B-haloborazines results in the elimination of trimethylsilyl halide and the formation of several new alkylated borazine dimers and trimers including (6-8). It can be expected that additional studies of this elimination chemistry may produce interesting polymers. 13

14

Ν

l(

R R N W ,N R 53 2

R

2

R

R

)l "

2

ιΓΛΓ

2

R ,>>^fNR NLJI

1

J.

2

S3

R

R

. 1

RB/^\BR l( )l

R B(JI

Β

2

R^X-^NR R

2

R

, 1

B / - S BR R NX-^NR

1

1

_


R

2

2

1

27.


PAINE & SNEDDON

R

R


8

The use of reductive (Wurtz) coupling of borazine rings by combination of B haloborazines with Group 1 metals has also been explored. This approach has recendy been reexamined by Shore who reported that trichloroborazine and Cs metal react explosively at 125°C with formation of a insoluble, perhaps polymeric, solid some of which has a tubular morphology. Most significantly, the tubular structure is retained in the final boron nitride pyrolysis product. Perhaps the earliest examples of backbone polymers derived from the parent, B3N3H6, borazine are those mentioned by Stock and Pohland. They found that pyrolysis of borazine at 500°C produced an undefined insoluble solid with an approximate empirical composition B N H . Somewhat later, Wiberg proposed that the B N H solid consisted of a network structure. Subsequently, Laubengayer specifically proposed that the biphenyl, B g N o H i o , and naphthalene, B5N5H8, analogues formed during the gas phase pyrolysis of borazine. These species were then identified by Margrave in gas phase mass spectrometric analysis. In related studies, Porter ! found that the same species, as well as an insoluble polymeric solid, are formed by photoinduced decomposition of borazine. These early findings clearly suggest that higher linked- or fused-ring polymers based on the parent borazine should be accessable, although no tractable polymeric species were characterized. Processable dehydrocoupled polymers derived from the parent borazine would, of course, be of particular interest as polymeric precursors to boron nitride, owing to their potentially high ceramic yields (95%) and their close structural relationship to h - B N . These considerations stimulated new investigations at Penn of borazine dehydrogenation-polymerization. Initial investigations focused on metal catalyzed synthetic routes; however, during the course of these studies it was discovered that controlled thermolysis of liquid borazine in vacuo at moderate temperatures results in nearly quantitative dehydropolymerizations to form soluble polyborazylenes. In a typical reaction (eq. 2), borazine is heated in vacuo at 70°C with periodic degassing until the liquid becomes sufficiently viscous that stirring cannot continue. The volatile contents of the flask are then vacuum evaporated, leaving a white solid (9) in >90% yield. 15

16

17

18

19

20

2

22


361

362

INORGANIC A N D ORGANOMETALLIC POLYMERS Π

Η B

If

X Η

A \ Β

H, Ρ

,R

R=H or borazinyl

70°C

)\

(2)

Η

R

Η


Ν—Β

Ν

Β^

Η

The polymer prepared in this manner is highly soluble in polar solvents and, a typical sample has, according to size exclusion chromatography (SEC)/low angle laser light scattering (LALLS) analysis, Mw 7,600 g/mol and Mn 3,400 g/mol. Polymerizations carried out for shorter times can be used to produce viscous liquid polymers with correspondingly lower molecular weight averages. For example, when the polymerization is carried out for only 24 h, Mw = 2100 and Mn = 980. Elemental analyses of the polymer are consistent with the empirical formula of - B 3 N 3 H 3 , (linear polyborazylene = B 3 N 3 H 4 ) suggesting the formation of a branched-chain or partially crosslinked structure. Evidence of chain branching has also been found in the SEC/LALLS/UV studies. The detailed structure of the polymer has not yet been established; however, since small amounts of both the N:B coupled dimer (1:2'( B 3 N 3 H 5 > 2 ) and borazanaphthalene ( B 5 N 5 H 8 ) were isolated in the volatile materials from the reaction, the polymer may have a complex structure, having linear, branched, and fused chain segments. Because of its composition, high yield synthesis, and excellent solubilities, polyborazylene would appear to be an ideal chemical precursor to boron nitride. Indeed, bulk pyrolyses of die polymer under either argon or ammonia to 1200 °C have been found to result in the formation of white boron nitride powders in excellent purities and ceramic yields 85-93% (theoretical ceramic yield, 95%) (eq. 3). H

Ν—Β

,R-i 1200°C

K

O

BN

(3)

.Ν—B . R H %

9

Thermogravimetric studies of the polyborazylene/boron-nitride ceramic conversion reaction showed that the polymer undergoes an initial (2%) weight loss in a narrow range between 125-300°C, followed by a gradual 4% loss ending by 1100°C. In an effort to determine the steps involved in the ceramic-conversion process, a sample of the polymer heated to 400°C was investigated in more detail. This material proved to be an insoluble white powder having an empirical formula of - ^ Ν β Η ^ suggesting that polymer crosslinking had occurred. This conclusion is supported by diffuse reflectance infrared studies * which show corresponding decreases in the BH and N-H stretching intensities as the polymer is heated to 400°C and then 1200°C. 22

1


27.

PAINE & S N E D D O N


3623)

Recent Developments in Borazine-Based Polymers 363 Poly(borazylene)

40 50 60 2 0 (DEGREES)

70

80

Figure 1. XRD for polyborazylene and pyrolyzates X-ray diffraction studies (Fig. 1) demonstrate that both polyborazylene and the intermediate material obtained by heating to 400°C exhibit patterns similar to those observed for turbostratic boron nitride, indicating that the layer structure of boron nitride is already present in the polymer. These studies suggest the simple process shown in Scheme I for the conversion of the polymer to boron nitride. This involves a dehydrocoupling reaction between the B-H and N-H groups on adjacent polymer strands resulting in a 2-dimensional-crosslinking of the polymer. Because the polymer has a more complex structure than the idealized linear structure depicted in the scheme, the final high-temperature processing step is probably necessary in order to remove residual hydrogen from the B-H and N-H groups that are not in the proper orientation for the initial low-temperature crosslinking reaction.

Scheme I


364

I N O R G A N I C A N D O R G A N O M E T A L L I C P O L Y M E R S II

Because of its solubility, low-temperature decomposition, and high ceramic and chemical yields, polyborazylene shows excellent potential for many technological applications requiring a processable BN precursor. Indeed, high quality boron nitride coatings have now been achieved on alumina, silicon nitride, silicon carbide and carbon fibers using polyborazylene. Others have also recently demonstrated that this polymer may be used to generate BN matrices in carbon composites that significantly increase the oxidation stability of the composite. In addition to being of use as a boron nitride precursor, polyborazylene has proven to serve as a useful reagent for the formation of new composite metalnitride/metal-boride materials that have improved properties over the individual pure phase metal boride or nitride. For example, titanium-boride/titanium-nitride composites have been made (eq 4) by dispersing titanium powder into polyborazylene Downloaded by MONASH UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch027

23

(B N H ) + Ti 3

3

4

X

A

»

TiN/TiB + H 2

(4)

2

24

followed by pyrolysis under inert atmosphere. Ceramic bars with excellent shape retention in observed ceramic yields of 91-96% are achieved by heating shaped green bodies to 1450°C. X-ray diffraction and TEM studies of the ceramic conversion reaction show that the material goes through several changes in composition and crystallinity as the processing temperature is increased. The final ceramic contains intimately mixed polycrystals of 500 nm size grains with residual amorphous phase at triple points and along grain boundaries. The dependence of the crystal size and distribution on the reaction/sinter temperature, the evolution of microstructure and the properties of the final composite material, such as conductivity and hardness, are currently under investigation. The reactions of polyborazylene with other metals and soluble metal complexes are also being explored with the goal of developing a range of new metal boride/nitride precursor systems that will allow the formation of both shaped bodies and coatings. Linked-Ring Polymers. Yet another potential route to form borazine backbone polymers involves deamination of B-aminoborazines. Early work on this chemistry by Gerrard and coworkers, and Toeniskoetter and H a l l demonstrated that deamination may be used to form bis-borazinyl amines of structure type (4) with X NR. Extending that work, Taniguchi described the result of heating the monomer, [(H2N)BNPh]3, at 250°C. This gave a meltable polymer that was drawn into fibers. Paciorek also studied the thermally promoted deamination of several organosubstituted B-amino borazines and apparentiy obtained polyborazines. Although these polymers have not been characterized in detail, it appears that they probably contain networks of bridged-ring units such as represented by (4), and that the polymers may be processible and potentially useful in some applications. In seeking to simplify and utilize the deamination-condensation chemistry and obtain polymers with a minimum of organic substituent groups, at New Mexico we reinvestigated the aminolysis of B-tris(dimethylamino)borazine (10). It was initially found that when excess NH3 is added to a cold toluene solution of [(Me2N)BNH]3 an organic solvent insoluble polymer (11) is obtained. It is assumed that this polymer is highly crosslinked as illustrated in (eq. 5). Subsequently, Kimura reported aminolysis of [(Et2N)BNH]3 (12) in liquid NH3 at 25°C (eq. 6)fromwhich was 13b

25

26

27

28

29


27.

PAINE & SNEDDON


H -•Ν

NMe

2

H-N/-VN-H l + xsNH, MegN-B . _ B-NMe 'N

U

H-N?

toi. 1. -78°C 2. 25°C -MegNH

2

H

1 1

Ν

Ν*

H

Ι1

II

Η

ΗJ η

11

10 NEt

NH

2


(5)

2

Η-ΝΛΝΝ-Η Et, )N-BX~XB-NI B-NEt Ν

+ xs NH3

-Et NH

HoN-BCs-^ B-NHa Ν

2

2

I

I

Η

Η

12

13

H

(6)

Η

Η

Η

Η

Β

^B

Β

B

HN

ι

NH

NH

2

2

NH

2

14 obtained a crystalline solid, [(H2N)BNH]3 (13). This compound undergoes rapid deamination at 120°C forming a slightly soluble polymer whose structure has been proposed to resemble that illustrated in (14). Further examination of this chemistry has shown that a small fraction (-10%) of the polymer (11) is also soluble in toluene. Further, if the insoluble fraction isredissolvedin liquid NH3 and briefly digested in that solvent at -40°C, the resulting polymeric evaporate is soluble in aromatic hydrocarbons. Full characterization of this soluble polymer continues and practical applications for the solutions have been accomplished. The assembly of borazine rings via bridging groups X as schematically shown by (15-18) offers a particularly flexible approach to formation of both nonfunctionalized and functionalized borazine polymers. The terminology two-point (15,16) and three-point (17,18) is employed to designate the number of bridging groups in the polymer units. Nearly all efforts to date to make representatives of these polymers has occurred with linkages through the boron atom (15, 17), since that substitution chemistry is more facile and precedented in the ΙΐΐβπίΓΐΐΓβ. · 30

31



366

INORGANIC AND ORGANOMETALLIC POLYMERS II

32

Lappert first reported coupling of borazine rings via a bridging group, X = NR, in 1959. That work was followed by numerous related studies in the period 1960 - 70, but few of these polymers were adequately characterized due to their generally low solubility, and it is unclear if practical applications were realized. In 1978, Meller and Fullgrabe briefly reported simple 1:1 reactions between hexamethyldisilazane (HMDS) and 2'alkyl-4,6-dichloro-l,3,5-trimethylborazines (eq. 7). Based upon limited solution NMR data and mass spectra, it was proposed that cyclic two-point oligomers were formed having four andfiveborazine rings in the cyclic structure. 11

33

R Me-N/^v N-Me

'(

)'

+ (MesSifeNH

CI-BVVB-CI

Ν I Me

Μθ-Ν/

benz. Δ -Me^SiCI

N-Me (7)

.

Ν I Η

Β^

>*B>

1 Me

^

1 H J

4,5

That result eventually stimulated further studies of the HMDS/borazine systems by Paciorek and at New Mexico, and the reader is referred a previous review for a detailed summary. In general, it is found that introduction of organic groups on the borazine nitrogen atoms and/or on one boron atom lead in some cases to polymers that have modest to very good organic solubility. Unfortunately most of these polymers have not been extensively characterized since they are generally poor as ceramic 27

34


11

27. PAINE & SNEDDON


precursors. Nonetheless, as interest in organic modified polyborazine systems has broadened, more effort is being given to developing this chemistry. The reaction of HMDS with B-trichloroborazine is quite complex. With excess HMDS, the reaction is reported to give an organic solvent soluble polymer that has been used to produce fibers and coatings. * Reactions performed at New Mexico with 2:1 to 1:1 (HMDS/borazine) mole ratios, (eq. 8) on the other hand, form polymers that are initially insoluble in organic solvents. However, if these polymers are briefly dissolved in liquid NH3 and then dried, they redissolve in organic solvents. It is reasoned that the initial polymer formed is highly cross-linked, but NH3 solvolysis likely breaks down the extended structure leaving more soluble fragments. Indeed it is possible then to process these polymer solutions to form fibers, coatings, dense or porous bodies, xerogels, aerogels, and aerosols all of which are converted to BN in these forms. 11

27

35

36


38

39

N-H

CI 1

H-N^~N>H

Ό

I

Η

H-N

I

+ xs(Me Si) NH 3

CI-BN-/B-CI Ν

37

40

2

'N I Η

I

Η

Ν I Η

The pyrolytic decomposition of the three point poly(borazmylarnine) has been followed by TGA, thermal desorption mass spectrometry and isotope distribution analysis. Although the full details of the solid state decomposition and lamellar structure assembly are not yet revealed, it can be concluded that borazine ring opening is a crucial reaction at low temperatures and some degree of solid state lamellar order is found for samples heated between 400° C and 500°C. In fact, these samples show xray patterns similar to those shown in Fig. 1. High temperature pyrolysis is required to remove the last hydrogen atoms in this structure as well. The early stage of the decomposition is illustrated schematically in Scheme Π. Some additional attempts have recently been made to prepare hybrid and copolymer materials containing boron and nitrogen by utilizing inherent reactivity of borazine monomers or chemistry that likely transits through borazine ring formation. That work includes blending of vinylic polysilane (VPS) with polyborazmylamine and reaction of (EtNH)3B with Et2AlNH2 and (Me2N)4Ti to give viscous polymers. At New Mexico, we have also found that the structure and reactivity of two-point polyborazinylamines (19) can be modified by varying the nature of the substituent group, X , e.g. X = H, Me2N, NH2, N3. Further, the reactive sites are ideal for introducing chemical constituents for the formation of new copolymers and eventually composites. As an example, it was found that reaction of (19) (X = NH2) with metal amides, e.g., (Me2N)4Ti, followed by treatment with NH3 produced a hybrid titanyl borazinylarnine polymer. This polymer can be pyrolyzed at modest temperatures and 41

42


368

INORGANIC AND ORGANOMETALLIC POLYMERS II I ΗΝ I

I NH I

I

ΗΝ

-**

ΗΝ H* ft ΗΝ Β ' ' HN^ ^NH HN'^NH HN^^NH

i

Î B^

"N H

JB^ ÊL

H

H

HN

H

t

U M

^^

HN

W

HN' H N -NH

NH

3

i

1

ί H ï Ν N I ΗΝ" ^B>» NH >" ^ ^ ^ °>s. ^°x, ^B>. Ν Ν Ν Ν Ν Ν NH B\ B HN' ^NH HN*^ NH

L H

"NH N

UK

I

H

H

κ ι

H

H

H

X

H


H

H

ι

J HN

H

I N. I

ι

H

H

I N^

I Β" I

1

I

/

.H \

ι Scheme Π Χ I

H-N/-VN-H

U

.Β.

19 nano-composites of TiN/BN are obtained. Similar metal nitride dispersions in BN have been achieved with V, Nb, Ta Zr and Al. It is also possible to add non-metals to this polymer, e.g., S1CI4, that lead to growth of a polysilaborazinylamine that converts to S13N4/BN composites. Obviously this is also a rich area for continued development, and we are currently engaged in the extensions of the work. Borazine Pendant Group Polymers As the field of organic polymer chemistry matured, many investigators turned their attention to construction of more complex organic hydrid and copolymer systems. This has proven to be a very fruitful venture as some of the organic materials have proven to have unusual and enhanced properties such as mechanical strength, shock resistance, chemical resistance and unique opto-electric behavior. With this background, it is logical to expect that hybrid inorganic/organic and inorganic/inorganic polymers may be prepared, and these materials should display


27.

PAINE & SNEDDON


special properties with unlimited applications. Indeed, in the last few years there have been a number of reports of the synthesis of silicate/organic copolymer systems (ormsils, ceramers, etc), ' as well as a host of systems involving polycarbosilanes/organic polymers, polysiloxane/organic polymers and polycarbosilane/polysilazane copolymers. The limited results available clearly indicate that this field deserves serious attention. Two general approaches can be envisioned to attain a polymer containing a borazine pendant group: (1) polymerization or copolymerization of a suitable borazine monomer or (2) controlled functionalization by a borazine reagent of a preformed organic or inorganic polymer. Both of these approaches are currently being explored as illustrated by the examples discussed below.


43

44

Poly(vinylborazine). Initial efforts at Penn to generate borazine-substituted polymers focused on the construction of polystyrene analogues in which the borazine would be bound as a pendant group on a conventional polyethylene polymer backbone. Because of their similarity to conventional organic vinyl polymers, these types of borazine polymers might be both soluble and processable. Vinylborazine, CH2=CH-B3N3H5, which could be considered the inorganic analogue of styrene, could serve as the repeating unit in the construction of these new polystyrene-type polymers. Vinyl derivatives of alkylated borazines have been prepared; however, the presence of the ring substituents was found to inhibit polymerization. Furthermore, even if polymers based on these compounds could be made, their high carbon contents would make them less desirable as boron nitride precursors. The parent B-trivinylborazine has also been synthesized. Although the polymerization reactions of this compound have not been reported, it would be expected, due to the three vinyl substituents, to form only insoluble crosslinked polymers and have little preceramic utility. Therefore, at the outset of this work there were no general synthetic routes for the formation of suitable B-alkenylborazine monomers and our initial goal was to develop an efficient synthesis of the parent B-vinylborazine. Since our previous work has demonstrated that transition metal reagents, similar to those widely employed in organic and organometallic chemistry, may be used to catalyze a variety of transformations involving B-H activation reactions, we investigated the use transition metals to catalyze the alkyne-addition and olefin-substitution reactions of borazine. We found (eq. 9) that B-vinylborazine may be readily obtained via the RhH(CO)(PPh3)3 catalyzed reaction of borazine with acetylene and this reaction has allowed the large scale production of this monomer. 45

46

47

48

H Insoluble polymeric materials result from thermal polymerizations of neat liquid B-vinylborazine, but soluble poly(B-vinylborazine) homopolymers are readily obtained by solution polymerization with the free radical initiator A1BN (eq. 10). Thus, when the polymerization of B-vinylborazine is carried out in benzene solution 49


369

370


H H H

—c-c-

y -\ c

.ISL

H

H. (10)

AIBN

70°C H'

V

B

.B

"

H


H

NI H

H

J X

for 20 h in the presence of 1.6 mol% AIBN, a white polymeric solid is obtained that is completely soluble in solvents such as benzene or ethers. Molecular weight studies using SEC/LALLS show Mw = 18,000 and Mn = 10,700 for the polymer prepared in this manner. The molecular weight distribution of these polymers is generally skewed in the high molecular weight end, suggesting that some crosslinking or branching may occur, perhaps by the formation of B-N linkages such as found in the poly(borazylene) polymer. The molecular weight of the polymer may also be controlled by varying the AIBN concentrations. These new polyvinylborazine polymers have now been used to produce, depending upon pyrolysis conditions, a variety of ceramic materials, ranging from black, high carbon content materials to white h-BN. In each case, however, the polymer/ceramic conversions are found to take place with both high ceramic and chemical yields and give materials with B/N ratios of 1.0. Best results for the production of pure h-BN are obtained from pyrolyses under an ammonia atmosphere, which produce white, crystalline h-BN with a composition of Bi.ooNi.oiCo.o06Ho.04Poly(styrene-co-B-vinylborazine) Copolymers. The discovery that soluble poly(B-vinylborazine) homopolymers could be readily formed in solution at 80 °C with the aid of AIBN initiation has enabled the systematic production of new soluble organic/inorganic hybrid polymers. In initial studies at Penn, we demonstrated the production of a new family of poly(styrene-co-B-vinylborazine) copolymers (eq.ll). 49

H

ι

η

Styrene AIBN

, N \ _ / Ν H

'Η Η

CHCHg-f—

CHCH, •

H

80°C

Η

"

^ B ^ ,H

'(

>

Η

*• (11)

)' Ν Η

Η

Jχ L

Elemental analyses and the spectroscopic data are consistent with the styreneco-B-vinylborazine formulations. By adjusting the monomer feed ratios and the initiator concentrations, copolymers ranging in composition from 10 to 90% styrene and molecular weights from ~1,000 to >1,000,000 can be producted. The use of these AIBN initiated olefin copolymerization reactions are now being explored as routes to a wide variety of borazme-containing hybrid organic polymers.


27. PAINE & SNEDDON


371

Borazine-Modified Hydridopolysilazanes. Polysilazanes have been shown to be excellent polymeric precursors to silicon nitride or SiNC ceramic materials. Chemical modification of polysilazanes has also been proposed as a means of modifying and enhancing the properties of the polymer and/or the final ceramic materials. For example, the incorporation of boron in the polysilazane has been claimed to decrease the crystallinity of the silicon nitride derivedfromthese polymers and thereby extend the effective use-temperature of these ceramics. Given the potential importance of SiNCB ceramics, investigations of the generation of new classes of hybrid polymers that might serve as processable precursors to such composites were initiated. It was found that the high yield synthesis of hybrid borazine-substituted hydridopolysilazanes can be achieved by the reaction of the preformed silicon-based preceramic polymer, hydridopolysilazane ( H P Z ) , with liquid borazine at moderate temperatures (eq.12). The amounts of borazine incorporated into the hydridopolysilazane can be readily controlled. For example, polymers of compositions (B3N3H5)o.02(HSi)o.30(Me3Si)0.19(NH)o.26No.23 and (B3N3H5)0.07(HSi)034(Me3Si)0.18(NH)0.13N0.28 were prepared by heating H P Z in excess borazine at 73°C for 2.2 h and 7.0 h, respectively. The spectroscopic data for the new polymers indicate that they retain their hydridopolysilazane backbones, but are substituted with pendant borazines by means of a borazine-boron to polymernitrogen linkage. Such linkages may form by either hydrogen or trimethylsilane elimination reactions.


50

H HB^sBH

S

Μ

κ,

Η SiMe H 3

u π HN^NH Λ H B

>ff H

B H

H N U N H Β •

" I

H N A N H •ί J l HBV^BH Ν Η

(12)

Studies of the thermolytic reactions of these polymers demonstrate that they are converted in high ceramic and chemical yields to composite SiNCB ceramic materials. The ceramic yields (-70%) to 1400°C are increased over that observed for the unmodified H P Z (-57%) due to both the retention of boron and nitrogen in the final ceramic, and a borazine-crosslinking reaction which retards the loss of polymer backbone components. It is also very significant that, in contrast to the ceramic materials obtained from unmodified H P Z , the borazine-modified polymers yield ceramics that are amorphous to 1400°C. Pyrolyses carried out to higher temperatures also demonstrate that the boron-modified ceramics are more stable with respect to nitrogen loss and exhibit significant differences in the onset, phases and extent of crystallization in comparison to the ceramic derived from the unmodified H P Z . Because of their ease of synthesis and processability, as well as the increased thermal stabilities and reduced crystallinity of their derived ceramics, these new hybrid borazine-modified hydridopolysilazanes polymers may prove to be reasonable


372


precursors for enhanced SiNCB fibers, matrices for ceramic-ceramic or carbon-carbon composites and/or binders for nitrogen based ceramic powders. These applications are presently under investigation. Conclusions


The diverse collection of boron-nitrogen polymer chemistry summarized here points out that efforts in this area are fruitful and worthy of continuing attention. Furthermore, the considerable progress made in processing some of the polymers into useful ceramic compositions and forms demonstrates that fundamental inorganic polymer chemistry has practical and, perhaps in the near future, commercial utility. Acknowledgements. At the University of New Mexico we thank the NSF Center for Microengineered Ceramics for their support. At the University of Pennsylvania we thank the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, and the National Science Foundation Materials Research Laboratory at the University of Pennsylvania for their support of this research. We also thank the many coworkers and collaborators that are cited in the references for their contributions to the work described in this chapter. References 1.

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R E C E I V E D April 8, 1994


Recent Developments in Borazine-Based Polymers - American

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