Poly-m-carborane-siloxanes

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... 

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. 

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. 

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).

Poly(m-carboranyl-siloxane) elastomers containing a mixture of di-methyl-and methyl(phenyl)-silyl units were synthesised using the Ferric Chloride catalyzed condensation reaction between di-chloro-di-organosilane and 1,7-bis(di-methyl(methoxy)silyl)-w-carborane. Silica Nano tubes synthesised using insitu growth of DL Tartaric acid nanocrystals and concurrent sol-gel hydrolysis and condensation of silane. 

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. 

The two carbons enter the icosahedral cage adjacent to each other (o-carborane). At temperatures 450°C, they migrate away from each other across the surface of the cage to yield m- and p-carboranes. Carborane cages can be linked by siloxane units to give poly(carborane-siloxanes). 

Figure 2.3. Separation of polywax 655 by high temperature gas chromatography on a 6 m x 0.53 mm I. D. open tubular column coated with a 0.1 xm film of a poly(carborane-siloxane) copolymer (equivalent to 5 % phenyl). Initial column temperature -20°C for 1 min, programmed at 10°C/min to 430°C, and final hold 5 min at 430°C. The helium carrier gas flow rate was 20 ml/min. ( )SGE, Inc.)...

Fig. 1. The glass transition for poly(carborane siloxane)— longitudinal sound speed and absorption vs temperature at 1 MHz. Adapted fix>m Ref. 49.

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