M-Carborane

The structure of the disilanol l,7-bis[hydroxy(dimethyl)silylmethyl]-m-carborane has a hydrogen-bonded network reminiscent of the sil-oxanediol type shown in Fig. 10c, in which pairs of molecules are hydrogen-bonded together and then these pairs are further linked to give a double chain (290). 

In Olin s attempts to derivatize dilithiated products of o-carborane with chlorosilanes for further reaction with ammonia, it was observed that cyclic compounds, instead of polymers, were produced by the interaction of the substituents on the adjacent carbon atoms in the o-carborane units.11 However, when a linear dimethoxy intermediate of m-carborane was reacted as an equimolar mixture with dichlorosilane in the presence of the catalyst FeCl3, the quantitative evolution of CH3C1 was observed... 

Figure 56 Two sets of [ ]carboranophanes (92 and 93) derived from the dithiol derivative of m-carborane. (Adapted from ref. 123.)...

A novel cyclooctaphane (94) (Fig. 57) was assembled by the condensation reaction between the C,C -dicopper(I) derivative of m-carborane and l,2-bis(4-iodophenyl)-o-carborane.124 The macrocycle was found to adopt a butterfly (dihedral angle 143°) confirmation with the o-carborane units at the wingtips and the phenylene ring planes roughly perpendicular to the wing planes of the confirmation. 

The synthesis of block polymers of diacetylene-silarylene and diacetylene-carboranylenesiloxane polymers (99a-e) (Fig. 61) by the polycondensation reaction of 1,4-dilithiobutadiyne with l,4-bis(dimethylchlorosilyl) benzene and/or l,7-bis(tetramethylchlorodisiloxane)-m-carborane have been reported by Sundar and Keller.129 These polymers are a hybrid between the carboranylenesiloxane and silarylene-siloxane polymers and have high char yields (up to 85%) at 1000°C in N2 and in air, reflecting the thermal stabilizing effects of the carborane and aromatic units in the polymeric backbone. 

The original FeCl3-catalyzed condensation reaction strategy has been exploited recently by Patel and co-workers for the synthesis of poly(m-carborane-siloxane) rubbers (103) (Fig. 63) in the reactions between dimethoxy-m-carborane terminated monomers and dichlorodimethylsilane.131 They have also synthesized similar polymers... 

Figure 64 Hydrosilation reactions between l,7-bis(vinyltetramethyldisiloxyl)-m-carborane monomer and the polymeric cross-linker, poly(methylhydrosiloxane), producing hard, colorless networked plastics. (Adapted from ref. 133.)...

Ando and co-workers have reported the synthesis of a silyl-carborane hybrid diethynylbenzene-silylene polymer (108) (Fig. 66) possessing high thermal stability.136 The polymer contained Si and —C=C— group in the main chain and m-carborane and vinyl groups in the side chain. The 5% weight-loss temperature of the cured polymer in air was over 1000°C as determined by thermogravimetric analysis. 

During heat treatment between 250 and 500°C, the formation of three-dimensional networks by diene and addition reactions of Ph—C C, C=C, and m-carborane groups in this system has been characterized by nB MQ-MAS NMR and 13C and 29 Si CP-MAS NMR methods. 

A typical polymerization method used at AWE to make poly(m-carborane-siloxane) rubbers is detailed briefly in the following paragraphs. 

Bis-(di-methylmethoxysilyl)-m-carborane was used as the carborane-containing precursor. Briefly, a typical reaction mix included l,7-bis-(di-methylmethoxysilyl)-m-carborane (40 g), di-chloro-di-methylsilane (16 g), and anhydrous ferric chloride (1 mol % of the carborane) as a catalyst. The reactions are shown in scheme 7. The... 

Phenyl and vinyl modified versions of poly(m-carborane-siloxane) were readily prepared using the procedure just described, by introducing the appropriate silane feed into the reaction mix.19 Typically, 1 to 3 mol % di-chloro-methylvinylsilane was added to the di-chlorosilane feed in the syntheses described earlier. The repeat unit of the phenyl modified poly(w-carborane-siloxane) is shown in 5. 

An alternative route to poly(m-carborane-siloxane) rubbers is via the condensation reaction between w-carborane di-hydrocarbyl-disilanol and a bis-ureidosilane.20 This mild reaction allows the incorporation of desired groups into the polymer via both the dihydrocarbyl-disilanol and the bis-ureidosilane (see scheme 8). The first step involves the formation of the carborane silanol from the butyl lithium carborane derivative. The bis-ureidosilane is prepared from the phenyl isocyanate (see step 2), and the final step involves reacting the dihydrocarbyl-disilanol with bis-ureidosilane. 

The preparation of poly(m-carborane-siloxane) polymers has also been successfully achieved directly from the carborane monomer.22 The reaction used is shown in scheme 9. Here, the direct salt elimination reaction between dilithiocarborane and a dichlorosiloxane (e.g., 1,5-dichlorohexamethyltrisiloxane) results in the formation of linear polymers with a molecular-weight (M ) typically of 6800 dalton. However, the reported literature detailing this approach is very limited indeed, and the reaction has not found significant use. This is most probably because only relatively low molecular-weight polymers can be produced, ultimately restricting the flexibility to produce materials of controlled mechanical properties. 

At AWE, the Lewis acid-catalyzed bulk polymerization route has been the main synthesis route to poly(m-carborane-siloxane) elastomers. Our selection has been based on considerations of safety, availability of key reagents, and ease of scale-up operations. An understanding of the physical and chemical properties of these materials, and how these properties can be modified through the synthesis process, is essential in order to develop materials of controlled characteristics. 

In the following sections, details are provided on a selection of analytical techniques that have been typically used to characterize poly(m-carborane-siloxane) elastomers. 

The presence of four kinds of nuclear magnetic resonance (NMR) observable nuclei ( H, uB, 13C, and 29Si) allows poly(m-carborane-siloxane) to be readily investigated using NMR spectroscopy. In addition, H spin-echo NMR relaxation techniques can provide an insight into polymer segmental chain dynamics and therefore useful information on material viscoelastic characteristics. 

UB and UB H solution-state NMR spectra (see Fig. la) clearly show the different boron environments within the m-carborane unit. The eight peaks in the -10 to 0 ppm range are indicative of the different boron environments in the w-carborane cage, including B—B, B—C, and B—H bonds. The spectrum also contains... 

The 7 g of elastomers synthesized at AWE was generally found to be within the range 30° to 40°C (see Table 2), which is significantly higher than that for standard poly(dimethylsiloxane). The introduction of the bulky m-carborane unit into the siloxane backbone has clearly elevated the Tg. However, although the carborane unit introduces conformational rigidity, the polymer chains retain sufficient flexibility and mobility to have a T% < -30°C. 

On heating in air at 10°C per min, poly(m-carborane-siloxane) shows typically only 4% mass loss at 450°C and 7% mass loss at 600°C (see Fig. 4). In comparison, siloxanes without carborane units, show an approximate 50% mass loss at 450°C. As a consequence of the relatively high boron and carbon content of these materials, pyrolysis is expected to generate ceramic residues of boron carbide/silicon carbide. 

In thick samples, a boron oxide/boron carbide crust has been detected on the surface of the polymer. This inorganic surface layer has a shielding effect on the inner polymer layers, further enhancing the thermal stability of the material. Poly(m-carborane-siloxane)s have therefore been considered as surface coatings for organic materials, providing protection from erosion effects. 

Figure 7 H spin-echo profiles for a poly(m-carborane-siloxane).

CBiiH12 and derivatives Cation M4 (solvate, solvate-carborane C-H X interaction) Fragment M+ in molecule or polymer (M-carborane interactions, solvate, solvate-carborane C-H X interaction)... 

However, the duster compounds differ greatly in their water solubility. Whereas cioso-dodecaborate (as the sodium salt) is readily water soluble, o-carborane and its thermal rearrangement products m-carborane and p-carborane show poor water solubility. The nido-carborane system, because of its negative charge, is water soluble. The azanonaborane duster is neutral, but still shows a certain degree of water solubility. 

Die Synthese von 1-Nitro-m-carboran (Schmp. 199 200°) gelingt durch Oxidation von 1-Amino-m-carboran mit Chrom(VI)-oxid in einem Essigsaure/Schwefelsaure-Gcmisch (Ausbeute 85%)3 ... 

Interaction ofocarborane or m-carborane with tellurium tetrachloride in dichloromethane in the presence of aluminum trichloride formed the 9-carboranyl tellurium trichlorides. These compounds were not isolated but reduced by sodium sulfide in aqueous ethanol to the corresponding dicarboranyl ditellurium derivatives, which in turn were reacted with thionyl chloride in ethanol or bromine in dichloromethane to give 9-carboranyl tellurium trihalidcs2 (see p. 5). 

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