Carborane-siloxane polymers

Potential areas of apphcation for carboranes include adhesives, gaskets, and O-rings (27). Research has been reported on improving the oxidation resistance and strength of carbon materials by coating with a carborane siloxane polymer (36). 

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. 

Physical transitions for a scries of linear carborane-siloxane polymers, 1 have been examined and the data appear in... 

Thus a strong technology base now exists for carborane-siloxane polymers which adds measureably to the arsenal of design engineers for advanced systems. The visionary support of the Office of Naval Research has been largely responsible for the important advancements in this area. 

Carborane-siloxane polymers are also prepared from Li2[l,7-C2BioHio] and [ClSi(CH3)20]2Si(CH3)2 in ethyl ether. The thermal stability of a carborane-siloxane polymer containing l,7-C2Bj(,Hio units are improved upon by introducing a ferric oxide stabilizer . ... 

In the US and the Russian munitions field, carborane dicarba-c/oso-dodecaborane(12) [5] has been used to improve the properties of silicones. Some American companies have synthesized carborane-siloxane polymers as high temperature elastomers [6]. Lately, Keller s group has reported a diacetylene-carborane-siloxane system showing one of the highest stabilities in silicon polymers [7]. 

Substantial improvements in the heat-resisting capability of silicone rubbers were achieved with the appearance of the poly(carborane siloxanes). First described in 1966, they were introduced commercially by the Olin Corporation in 1971 as Dexsil. The polymers have the essential structure... 

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... 

After synthesis, the modified carborane-siloxane gums were fabricated into shaped components using standard siloxane vulcanization and fabrication technology. Di-chlorobenzyl peroxide (1% by wt) was used as the cross-linking agent and mixed into the polymer formed in scheme 7. Shaped rubber components were readily prepared by compression molding operations at 70°C. Postcure operations were typically at 120°C for 24 hours. 

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 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. 

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. 

Carborane-siloxanes are a family of polymers which have the linear structure, 1, ... 

The phenyl modified polymers show a significant decrease In weight loss compared to their all methyl analog (6). In a study of the thermal breakdown of phenyl substituted carborane-slloxane polymers, It has been reported that the presence of phenyl groups In carborane-siloxanes leads to cross-linking and less loss of weight ( ) ... 

Properties of carborane-siloxanes can be readily modified through changes in the polymer backbone and by the use of supplementary agents in order to optimize the property-performance profile for different use situations. 

Heteroboranes, compounds where one or more of the cage borons are replaced by a main group element (33), are not themselves commercially available. However, carborane siloxanes containing w-carborane [16986-21-6], C2H12B10, are available under the trade name of Dexsil for the stationary phase in gas—liquid chromotography (qv) (34). The carborane, l,7-dicarba- / o-dodecaborane(10) (35), contributes enhanced chemical and thermal stability to the siloxane polymer. 

The extreme thermal stability of icosahedral carboranes has long attracted the interest of polymer chemists seeking to develop new materials for high-temperature applications.1 Nevertheless, only the elastomeric poly(carborane-siloxane)s, for example 1, have ever been manufactured on an industrial scale.2 This situation reflects not only the very high cost of carborane-based starting materials, but also a failure to develop linkages between carborane cages which can match the stability of the carborane unit itself. 

Incorporation of carborane moieties into silicone chains also leads to improved thermal properties of such semiorganic polymers. The stationary phases based on carborane polymers have been available commercially since the early 1970 s, when the first analytical results were also reported [86] for some biological compounds. Although such phases were believed to be stable up to about 400°C, only moderately successful separations have been reported. While the carborane cages can be differently incorporated into siloxane polymers to influence their consistency, functional groups (phenyl, cyanoethyl, etc.) may also be varied to influence selectivity. 

C2BioHioSi(CH3)20SiR R"0— with molecular weights up to 250,000 . In a related study a high-temperature-resistant polymer is prepared from l,7-C2BioHio-l,7-[Si(CH3)20H]2 and (CH3)2Si[N(CH3)2]2 . A carborane-siloxane block copolymer with 1,7-C2BioHio units is prepared by the reaction of a ureidosilane-terminated polysulfone hard block with a carborane-silanol-terminated carborane-siloxane oligomer . 

Figure 2.2. General structures of poly(siloxane) liquid phases. A, poly(siloxane) polymer B, poly(silarylene-siloxane) copolymer and C, a poly(carborane-siloxane) copolymer ( = carbon and O = BH).

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