Silarylene-siloxane polymers

Siloxane-silarylene polymers Si-O- and Si-(m or p phenylene) CH3CH3 Stiffer backbones useful in some applications... 

Figure 61 The hybrid silarylene-siloxane/carboranylenesiloxane polymers, 99a-e.

Silarylene Polymers. Silarylene polymers can be best described as those polymers which contain silicon atoms and phenyl units in the polymer backbone. Perhaps the best example, and the one which has been given the most scrutiny, is poly(tetramethyl-p-silphenylenesiloxane), Q). The major reason which has been cited for incorporating arylene units into this type of polymer has been to increase the thermal stability of the material while maintaining the low temperature flexibility which is associated with siloxane polymers.3.4,5... 

Silarylene Polymers Containing m-Xvlvlene Units. A majority of the silarylene type siloxane polymers which have been worked on to date contain silicon-phenyl bonds. After an extensive literature search, only one type of polymer was found which contained an arylene group that does not have silphenylene bonds in the polymer backbone. The work, done mainly by Rosenberg and Choe 15-17 of the Air Force Materials Laboratory, is based on polymers which have a general structure as shown below, referred to as poly(m-xylylene-siloxanylenes), (9). 

The first class of polymers which will be discussed is not a true silarylene polymer, but is presented here because its synthesis has led to a number of the silarylenesiloxane polymers discussed previously. These polymers are known as poly(dodecacarborane-siloxanes), the study of which has been reviewed by Peters. The carborane molecule is very interesting and becomes an entire topic in and of itself. For the sake of completeness, the... 

Two remaining polymers have been encountered in the literature which fall under the category of silarylene polymers, but these polymers have no siloxane linkages. The first polymer which will be discussed has been used by Zelei and co-workers 22 as well as Ikeda and co-workers 23 in thermal degradation studies of 1 (poly(tetramethyl-p-silphenylenesilox-ane)). The polymer used was poly(p-dimethylsilphenylene) (101 and has been prepared by a method shown below. The second polymer is poly[l-(dimethylsilyl)-4-(dimethylethyl-ene)silylbenzene]) (H) and was prepared using a well known hydrosilylation mechanism in... 

Finally, silarylene-siloxane-diacetylene polymers were reported by Hom-righausen and KeUer in 2000 [28], as precursors to high temperatiue elastomers. They were obtained as follows ... 

Figure 15.15 Structure of the elastomeric diacetylene-diluted silarylene-siloxane-diacetylene polymer reported by Homrighausen et al.

The third set of silarylene-siloxane-diacetylaie polymers that were reported by Homrighausen et al. also produced elastomeric networks on curing (Figure 15.16) [33]. The precursor polymers were produced by aminosilane-disilanol polycondensation reactions between a series of oligomeric, hydroxy-terminated silarylene-siloxane prepolymers and l,4-bis(dimethylaminodimethylsilyl)butadiyne [(CH3)j N-Si(CH3)3-C -C=C-(CH3)3Si-N(CH3)3]. The oligomeric, hydroxy-terminated... 

Wang, R. Fang, L. Xu, C., Synthesis, Characterization, and Thermal Properties of New Silarylene-Siloxane-Acetylene Polymers. Eur. Polym. J. 2010,46, 465-471. 

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. 

Flexible and oxidatively stable thermosets were prepared by thermally curing linear poly(silarylene-siloxane-acetylene) elastomers at up to 450°C. Thermooxidative weight loss of 3.69% to 7.69% was observed for these crosslinked inorganic-organic hybrid polymers when isothermed at 350°C under air flow. 

In order to improve the solubility of aromatic polymers silicon-containing polymers have also been synthesized, by introduction of siloxane or silarylene linkages into polymer backbones such as polyamides, polybenzimidazoles and polyimides [50]. The resultant polymers show fair solubility in organic solvents but most of them have poorer thermal stability then their analogues without siloxane or silarylene linkages. 

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).

Silarylene-disiloxane polymers, which exhibit a wide range of physical properties depending on their composition and structure, have received considerable academic and industrial interest for nearly 50 years.One of the prominent properties of silarylene-disiloxane polymers is their excellent thermal stability. The typical methods employed for the synthesis of silarylene-siloxane polymers are the selfpolycondensation of arylenedisilanols or the copolycondensation of arylenedisila-nols with other bifunctional compounds. We found that the easily accessible 1,4-bis(dimethylsilyl)benzene (p-BSB) could react with water in the presence of a catalytic amount of transition metal (Pd, Pt, Rh, etc.) compounds at room temperature to afford high molecular weight poly[(oxydimethylsilylene)(l,4-phenylene) (dimethylsilylene)] with the evolution of H2 as the only by-product, as shown in Scheme 4.6. ... 

Recent Advances in High-Temperature Network Polymers of Carboranylenesiloxanes and Silarylene-Siloxanes... 

Silarylene-Siloxane Polymers Containing Thermally CrossUnkable... 

In the case of aU of the developed hybrid silarylene-siloxane and carboranylenesiloxane polymers, an evaluation of the nature of different phases containing aromatic or carborane groups will be illuminating to understand what contribution does each distinct phase has on the elasticity and other material properties of the generated networks. Other in-depth mechanical studies are also in order to better evaluate the high temperature elasticity of the materials. 

The interest in high-temperature elastomeric materials stems from the high demand for such materials for application in advanced technologies, particularly the aerospace, defense, and computer industries. Such materials are expected to have long-term thermal, thermo-oxidative and hydrolytic stability at and above 300 to 350 °C and to also have the ability to maintain pronounced flexibility to well below ambient temperature. The elastomeric crossUnked network polymers of carboran-ylenesiloxanes and silarylene-siloxanes described in this chapter possess similar properties and are therefore ideal candidates for a wide variety of engineering applications under unusual service conditions. In addition, these network polymers also have the utility in other applications that may or may not directly relate to their... 

In summary, recent advances in the area of network polymers of carboranylen-esiloxane and silarylene-siloxanes have produced exceptional high-temperature elastomeric materials as a result of the incorporation of new methodologies in their syntheses. In addition to their high-tanperature properties, the presence of various diverse constituents in the materials has opened avenues for the utilizations of these materials in myriad applications. 

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