Carborane-siloxanes

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). 

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 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... 

One of the earliest and most popular systems investigated are the highly heat-resistant poly(w-carborane-siloxane)s. A review of the general literature on the methods used in the synthesis of these materials is reported in the following sections. 

Poly( w-carborane-siloxane) rubbers are most commonly prepared by a ferric chloride-catalyzed bulk condensation copolymerization of dichloro- and dimethoxy-terminated monomers with alkylchloro- or arylchloro-siloxanes.16... 

Scheme 5 Poly( -carborane-siloxane) using ferric chloride.

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

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. 

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 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. 

Figure 2 57Fe Mossbauer spectrum for a poly(w-carborane-siloxane).

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. 

Figure 4 Weight loss vs. temperature for poly( -carborane-siloxane)s in air.

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).

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

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

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. 

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. 

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. 

Temporal trends of PCC in guillemot eggs from the Baltic. Chemosphere 27, 1987-2001. Witte, J., Buthe, A., Ternes, W., 2000. Congener-specific analysis of toxaphene in eggs of seabirds from Germany by HRGC-NCI-MS using a carborane-siloxane copolymer phase (HT-8). Chemosphere 41, 529-539. 

FIG. 14.4 Longitudinal sound speed and absorption for poly(carborane siloxane) at 2 MHz. From Hartman (1996) Courtesy Springer Verlag. 

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