Poly carborane siloxanes

The m-dilithium compound, LiCBjoHioCLi, is obtained by reaction of m-carborane with butyl lithium. After a subsequent polycondensation with water, the acid-catalyzed reaction of this compound with dichloro-disilozane gives polymers with a molecular weight of 15,000-30,000  

The polycondensation only occurs in the presence of siloxane groups, that is, for example, it does not occur with dichlorosilane. In such a case, the dichloro compounds are first converted to the respective dimethoxy derivatives (from the reaction of the dichloro compounds with methanol)  

The polymer produced has a melting point of 464 C and a glass-transition temperature of 77°C. If, on the other hand, the dimethoxy compound is condensed with dichlorodimethyl silane under the same conditions, then elastomers with melting points of 15 PC and glass-transition temperatures of — 22°C are produced. 

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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... 

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

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

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

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. 

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). 

Polycarbosllanes, Polysilarylenes, Polycarbosiloxanes and Poly(carborane-siloxanes). 

The strong interest in poly(carborane-siloxanes) is primarily due to their thermal stabilityThe synthesis of elastomers with a 50°C gain in the use temperature over corresponding polysiloxanes has been a technical success, yet, owing to their high cost, industrial applicability is doubtful. They are synthesized by condensation of the carborane-siloxane halide with a diether ... 

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).

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.)...

Roller and Gillham reported on the thermomechanical behavior 1n nitrogen (19) and in air (20) of a systematic series of linear poly(carborane-siloxane) s containing icosahedral (-CB,qH,(,C-) cages in place of some of the chain oxygen atoms ... 

Poly(carborane-siloxanes) have a linear structure in which R and R" can be alkyl, fluoroalkyl or aryl groups. The main chain contains the carborane polyhedra. Most commonly C2B10H10 is the carborane used. 

ATOMIC OXYGEN RESISTANT POLY(CARBORANE SILOXANE) COATINGS... 

The coatings provided excellent protection to various substrates. P2 protected silver mirror was estimated to lose only 2.5 % of its initial solar reflectance when extrapolated to 1000 hours ashing time. For the poly(carborane siloxane) P2 coating, the mass loss rate of Kapton was reduced to half that of the best commercial coating. 

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