Passive damping has been defined as a key element in vibration control. It is believed that the approach to passive damping could be addressed through the use of carefully designed viscoelastic polymeric materials. This dissertation describes the synthesis and characterization of multiphase, transparent block copolymers that are potential candidates for passive damping applications in large space structures. Relatively high molecular weight polysiloxane-polyarylester block copolymers were prepared by two different synthetic routes. A solution technique was used to synthesize well-defined, perfectly alternating block copolymers by reacting a difunctional silylamine—terminated siloxane oligomer with a difunctional hydroxyl-terminated polyarylester oligomer. A second approach involved the preparation of a segmented (or random) block copolymer by an interfacial, phase—transfer technique in which various polyarylester block lengths are formed <u>during</u> the copolymerization by reacting bisphenol-A, terephthaloyl chloride, and isophthaloyl chloride with a difunctional aminopropyl-terminated siloxane oligomer. To vary the miscibility of the siloxane and ester phases, and in turn the physical properties of the block copolymers, the block molecular weights and the siloxane block compositions (dimethyl, dimethyl-diphenyl, or dimethyl-trifluoropropylmethyl) were controlled. Structure analysis by NMR (proton and silicon) and FTIR verified that the desired starting oligomers and block copolymers were successfully prepared. Intrinsic viscosity measurements, size exclusion chromatography, and the fact that tough transparent films could be solution cast and compression molded indicated that relatively high molecular weight materials were prepared. Due to the high degree of incompatibility of the "soft" siloxane segments and the "hard" ester segments in the block polymers, a two-phase microstructure developed at relatively low block molecular weights. In addition to microphase separation, partial phase mixing was apparent from thermal, mechanical, and microscopic characterization techniques. Compared to a polyarylester homopolymer, the siloxane modified polyarylester block polymers displayed improved resistance to atomic oxygen degradation as seen from x-ray photoelectron spectroscopy and scanning electron microscopy. All physical properties were found to be dependent upon siloxane block composition and copolymer block molecular weights. In conclusion, new siloxane-ester block copolymers were prepared and characterized. They are believed to be potentially useful materials for passive damping applications in the space environment. / Ph. D. / incomplete_metadata