Borazine structures, formation

Generally, during hydroboration polymerization of dicyano compounds, the formation of the borazine structures that have a six-membered boron-nitrogen ring (scheme 19a) and dihydroborated end groups (scheme 19b) as a structural defect is unavoidable. The borazine cross-linked structures often cause the gelation, and dehydroboration causes a decrease in molecular weight. 

Initially, BN was synthesized by the pyrolysis of boric acid and urea in the presence of ammonia [27]. In this reaction, boric acid presumably reacts with urea to form a urea-boric acid complex gel, which on pyrolysis in an ammonia atmosphere results in the formation of BN [28]. Precursors for BN recently received the greatest attention, with emphasis on borazine-derived polymers. It has been recognized that borazine (1) can form polymeric chains of B and N, which can form BN ceramics on thermal pyrolysis [29]. The polymerization gives a graphitelike hexagonal layered structure (2) ... 

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. 

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 recently been reexamined by Shore who reported tiiat 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... 

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