DEXSIL polymer

Figure 8.16. Basic structural unit of Dexsil polymers.

PI, the acetoxy end capped Dexsil polymer, reacts on exposure to moisture to form acetic acid and silanol end groups which self condense. The coatings were not transparent because some crystallization occurred before cure was completed. To correct for this, a highly functionalized coating was synthesized by reaction of methyltriacetoxy silane and 1,7 bis(hydroxydimethylsilyl)—m earborane which produced a prepolymer with one acetoxy crosslinking site per repeat (Scheme 1). 

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... 

Figure 2 Schematic representations of the Olin polymers 2 (with —P—O—P— linkages), 3 and 4 (DEXSIL carborane polymers), and 5 (carborane polymers with tin bridges). (Adapted from ref. 7.)...

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. 

Another relatively nonpolar liquid, but one with a very high temperature limit, is a carborane silicone polymer called Dexsil, first synthesized by Olin Chemical in 1964 and now manufactured by Analabs and by a company bearing the Dexsil name. Its structure is shown in Figure 8.16. Three polymers are available that differ in the substitution on the methyl silicone backbone in a way similar to the silicones discussed above Dexsil 300 is dimethyl silicone, as shown in the figure Dexsil 400 is the methyl-phenyl analog and Dexsil 410 is the methyl-cyanopropyl analog. The temperature range over which they can be used is approximately 50 to 400°C. 

Jl hermally stable elastomers and polymers include fluorocarbon elastomers, polyorganophosphazenes, silicone elastomers, siloxane block copolymers (including the dodecacarborane block copolymers [Dexsil elasto-... 

Procedures for Oxidative Pyrolysis. All oxidative pyrolysis experiments were performed with the IPGCS system. About 5 mg of polymer were pyrolyzed in an air atmosphere at 240°, 264°, and 289°C. The same Chromosorb 102 column was used for the separation of low molecular weight products, but a Carbowax 20M on Chromosorb P column (12 ft X % in. OD) replaces the Dexsil column for the separation of high molecular weight products. The products were characterized primarily by the Norcon 201 spectrophotometer and mass spectrometry. 

Recent studies concern the utilization of a polycarboranesiloxane for a thermally stable liquid phase in packed gas chromatographic columns identification of some polycarboranesiloxanes using pyrolysis gas chromatography , Dexsil 202 polymer pyrolysis and the structure and oxidative thermal stabilities of carborane-containing siloxanes . 

Figure 7. Chromatograph of CHCl3 extraction on a 55-day in vitro LVAD bladder. Column conditions 1% Dexsil 300 100°-350°C at 8°C/min and carrier-30 mL/min helium. Key , polymer derived.

The linear polymers were readily formed by both methods. A salient feature in both of the syntheses was the in situ production of a desired carboranylenesiloxane oligomer by the reaction of a lithiated carborane and a chlorinated siloxane species rather than the use of the commercial Dexsil monomer as in the case of Henderson et al. [25] and Houser et al. [29], This has afforded an enormous degree of variation in the compositions of diacetylene-containing carboranylenesiloxane polymers that is not available by the conventional route of Henderson et al. [25]. The effect of the reduction in the diacetylene concentration was apparent in the positions and the intensities of the DSC peak maxima of the crossUnking endotherms of 5a-c. Thus, it was observed that as the concentration of the diacetylene units in the polymer... 

A series of carborane silicone polymers (see Fig. 4.8) has been designed especially for high temperature work. They were first synthesized in 1964 by Olin Chemical and are sold under the trade name Dexsil. Dexsil 300 is the least polar, having all methyl groups in the chain. It can be used up to 400°C. 

Substantial improvements in heat stability were achieved in the 1960s by the incorporation of the m-carborane moiety (Fig. 15.3) into the siloxane backbone and commercial polymers (e.g. Dexsil 200) were introduced. Because of poor linearity early polymers were somewhat difficult to process and cure and this has led to more novel methods of producing more linear materials (Peters et ai, 1975). 

The extreme thermal stability of icosahedral carboranes attracted the interest of polymer chemists immediately after these compounds were discovered in the early 1960s. In the mid 1960s several groups of polymer chemists studied the effect of incorporation of carborane cages into chais and networks of almost any known type of polymers [412]. The best results were achieved with the family of poly(ffj-carborane-siloxane)s [413,414] that have even been manufaetured on an industrial scale under the trade name DEXSIL (Figure 31). Some of these... 

---