2-Methylpropyl)hydroxysilyl]-bridged double-decker silsesquioxane was synthesized, and its stereoisomers were obtained by fractional precipitation followed by silica gel column chromatography, and their structures were confirmed by 1 H and 29 Si NMR and single crystal X-RD. The pure cis, trans and mix isomer were polymerized with silane-terminated octamethyltetrasiloxane via cross-dehydrocoupling polycondensation catalyzed by tris(pentafluorophenyl)borane to give completely alternating structure of the double decker silsesquioxane, and linear octamethyltetrasiloxane. All the polymers showed significantly increased glass transition temperature, compared with linear poly(dimethylsiloxane). Excellently transparent organo-soluble polymers gave self-supporting film by simple casting method. IntroductionIt is generally accepted that many of the properties of polysiloxanes are a consequence of the static and dynamic nature of the siloxane backbone. 1 Linear poly(dimethylsiloxane)s have been well-known to show particular characteristics such low glass transition temperature (T g ), thermo-stability, stability against oxidation originating from a strong and flexible main chain, and physiological inertness, with a hydrophobicity based on a mainly methyl side chain. 2,3 However, low T g and easy cleavage of Si-O-Si bonds under acidic or basic condition, 2,3 or by thermal treatment often limits their applications. Introduction of bulky and rigid moieties like phenylene, naphthylene, anthrylene, phenanthrylene or adamantylene moieties in the main chain increases T g and thermal stability of the polysiloxanes. 4-7 These polymers, especially, poly(arylene-dimethylsiloxane)s are a kind of good candidates for high-temperature elastomers because of the property originated from flexible Si-O-Si backbone and increased thermal stability given by the presence of arylene moiety, which can be also the origin of good dielectric and surface properties, and so on. 8-10

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