Polymers derived from borazine

Boron nitride can be prepared by many routes such as CVD (NH3 + organoborane), 2 pyrolysis of preceramic polymers derived from borazine, 2 and solid-state metathesis... 

Boron-Containing Polymers Derived from Borazine. 352... 

This chapter will be divided into three parts corresponding to the 3 main class of boron-containing preceramic polymers which are used for BN preparation (1) poly-borazylene and other polymers derived from borazine (2) poly[B-ami-... 

Because of the difficulties encountered for handling borazine due to its high reactivity and volatility at room temperature, very few efforts were devoted to the preparation of borazine-based preceramic polymers. Sneddon et al. have demonstrated that polymers derived from borazine, namely polyborazylenes, could be... 

Alternative synthetic approaches to Si-B-C-N-H polymers starting from functionalized borazine derivatives were published by Srivastava et al. " and Haber-echt et al." Srivastava et al. published a method for synthesizing a soluble borazine-based Si-B-C-N-H polymer by reacting B-chloroborazines with LiSi(Si(CH3)3)3 followed by subsequent polymerization with hexamethyldisilazane. 

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]. 

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 e 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 deriv from these 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 initiate. 

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 HPZ (-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 HPZ, 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 cryst zation in conq)arison to the ceramic derived from the unmodified HPZ. 

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

Polymers included in this category are generally called polyborazylenes. Synthetic route to these polymers have been reviewed [8] thus, we provide an overview of typical synthesis procedures starting from borazine derivatives and leading to BN preceramic polymers. 

It has been confirmed that the presence of bridge-type bonds in such poly (B-alkylaminoborazines) confers flexibility and an inaeased melt-spirmability, thus leading to the conclusions that melt-spinnability inaeases from 1- to 4-daived polymers. In addition, the ceramic yield is reduced with the increased proportion of -NlCHj) in the B-(alkylamino)borazine leading to important shrinkages during the furlha- polymer-to-ceramic conversion of green fibers derived therefrom. Hence, it is relevant to assume... 

---