Ceramic precursors borazines

One unique characteristic of the borazine oligomer is that it has a very high mass yield observed upon conversion to BN. Figure 2.11 shows a thermogravimetric analysis (TGA) plot of the borazine oligomer. This data reveals that the mass yield upon conversion to BN is approximately 85% which is very high for a ceramic precursor [27]. Additional examination has revealed evidence for viscoplastic flow in the crosslinked oligomer under the application of pressure. This may increase the effective BN mass yield even further for a bulk structure such that the volume yield is between 60 and 70%. 

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.l2 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. 

Recently we have also extended these low temperature polycondensation synthetic techniques to the preparation of borazine containing polyureas as precursors for BN/G C ceramics.- ... 

The introduction of small amounts of boron into precursors that produce silicon nitride have been known to improve the ceramic yields of silicon nitride and Si—B—C—N ceramics as first reported in 1986.110 Several reports have appeared in the past couple of years alone that utilize borazine precursors such as 2,4-diethylb-orazine and other cyclic boron precursors, such as pinacolborane, 1,3-dimethyl-1, 3-diaza-2-boracyclopentane, for their reactions with silanes, polysilazanes, and polysilylcarbodiimides for the high-yield production of Si—B—N—C ceramics.111... 

There is a great deal of potential interest in borazine as a precursor of boron nitride, since it offers the advantages of being a single source of boron and nitrogen with the correct B/N ratio and a high ceramic yield. In addition, borazine contains the elementary BN building block as its substituted derivatives. This is described later. 

Renewed interest in borazine derivatives has resulted from their possible application as precursors to boron nitride ceramics. For example, the inorganic analogue of styrene, (H->C=CH)B3NjH5i has been polymerized and decomposed to produce BN.72... 

Borazines have the correct B-to-N ratio for the production of this ceramic and it% polymeric precursor may be used to deposit a uniform surface coaling. 

Borazine is isoelectronic and isostructural with benzene. Poly(borazylene), a polymer of borazine and its derivatives, is reported extensively as a precursor of BN-coated ceramic libers.86 Polymeric borazine oxide derivative is claimed to be flame resistant (Figure 9.10).87... 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

This paper reports the synthesis of 5-alkylsilylborazines by rhodium catalyzed hydroboration with A -trialkylborazines. The characterization of these molecules by NMR spectroscopy gives valuable facts about the regioselectivity and completeness of the reactions and should enable a more facile assignment of NMR signals from precursors of Si-B-N-C ceramics based on borazines and vinylsilanes as educts. 

Hydrosilylation of 1 with trichlorosilane has been performed with Pt on charcoal (1% by weight) in quantitative yields (Scheme 1). The B-tris(trichlorosilylvinyl)borazine (2) was obtained with a high regioselectivity of proximately 80% trans hydrosUylation product [4], Pure 2 can be obtained by fiactional crystallization of the synthesized product fiom hexane. For further synthesis, both a- and P-hydrosilylation products can be used. No further hydrosilylation was observed in this case. In order to interconnect the single source precursor molecule 2 to a pre-ceramic polymer, methylamine was added to the solution of 2 in hexane, and a high viscosity, colorless oil was formed. By changing the reaction parameters (excess of CH3NH2, temperature), the viscosity of the polymer can be varied [5]. The obtained polymer (3P) is pure after evaporation of the solvent, which is checked by NMR. Other solvents like thf or toluene are also possible for the reaction, as well as for dissolution of the polymer. Furthermore, ethylamine leads to similar results in the formation of the polymer. 

Scheme 1. Synthesis of silylvinyl-substituted borazines 2 and 4 as molecular precursors for the ceramic materials.

The conversion of the molecular precursors into boron nitride requires a ceramisation under an ammonia flow. Ammonia is used as a reductive atmosphere, and its ability to replace halo or amino groups on the borazine framework is well known [12,13]. Ammonia is also a curing reagent in the borazine polymerisation [25]. So, for the preparation of BN from molecular precursors, one usually needs to perform a low temperature ceramisation, up to 600°C at least, under an ammonia flow. The conversion of each precursor into BN has been studied using TGA up to 1000°C in order to optimise the ceramisation conditions and the properties of the obtained ceramic. The measurements were realised in a pure ammonia flow up to 650°C and then under nitrogen. The three tested compounds presented original behaviours related to their formulation and their reactivity towards ammonia. 

The behaviour of the precursors P II and P III was tested under ammonia under the same experimental conditions used for precursor P I. No significant reaction took place at room temperature, and Fig. 3 shows the results of a ceramisation. For both precursors, the reaction was very slow up to 100°C. For the precursor P II, the reaction with ammonia became very important in the range of temperature from 200 to 300 C. The total weight loss for precursor P II was more important than expected (the boron content of the polymer was consistent with a ceramic yield of 52 %, while the weight loss reached 55 %). This could be explained by a stripping off of volatile borazine from the precursor [25]. 

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