Polymers boron-nitrogen

Several examples of well-characterised boron-nitrogen polymers are known. Polyborazylene 9.16 (with molecular weights up to ca 7600 and up to ca 

3400) is a cyclolinear polymer based on six-membered borazine rings, which has been prepared via the thermally induced dehydropolymerisation of borazine [eqn (9.8)].52 

Processing the polymer 9.16 followed by pyrolysis provides an excellent method for preparing this valuable ceramic in shaped forms such as fibres and monoliths. Derivatisation of 9.16 by a thermal dehydrocoupling reaction with amines NHR2 to replace -H substituents on boron by -NR2 groups can be achieved.Examples of void-free BN fibres prepared from polyaminoborazylene precursors formed by 

Reproduced with permission from T. Wideman et al., Chem. Mater., 1998, 10, 412.  

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There is great interest in developing molecular precursors for boron-nitrogen polymers and boron nitride solid state materials, and one general procedure is described in this report. Combinations of B-trichloroborazene and hexamethyldisilazane lead to formation of a gel which, upon thermolysis, gives hexagonal boron nitride. The BN has been characterized by infrared spectroscopy, x-ray powder diffraction and transmission electron microscopy. 

Detailed investigations are being conducted in the field of boron-nitrogen polymers with N— B coordinative bonds due to their hydrolytic stability (superior to that of the polymers containing tricoordinate boron). Such plastics and elastomers are likely to become candidates for special applications because of the high cross-section of 10B for the (n, a) reaction.171... 

Myers et al. reported that partially dehydrated APB is an effective intumescent flame retardant in thermoplastic polyurethane.77 APB at 5-10 phr loading in TPU can provide 7- to 10-fold improvement in burn-through test. It is believed that in the temperature range of 230°C-450°C, the dehydrated APB and its released boric oxide/boric acid may react with the diol and/or isocyanate, the decomposed fragments from TPU, to produce a highly cross-linked borate ester and possibly boron-nitrogen polymer that can reduce the rate of formation of flammable volatiles and result in intumescent char. 

I. B. Atkinson and B. R. Currell, Boron-nitrogen polymers, Inorg. Macromol. Rev. 1, 203... 

The same phenomenon is observed in boron-nitrogen polymers. Borazines with a - BR—NR -)- chain are heat stable up to 400-500 C but are readily hydrolyzed. The parquet polymer, boronitride (BN) , on the other hand, is very resistant to hydrolysis and oxidation and can be used up to temperatures of 2000°C. Boron-oxygen compounds are heat stable but are readily hydrolyzed. Boron-carbon compounds are similarly heat stable but are very prone to oxidation. 

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. 

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