The heterofunctional condensation of 1,3-dichloro-1,3-disila-1,3-diphenyl-2-oxaindane with dihydroxydiphenylsilane at various ratios of initial compounds in the presence of amines was carried out, and alpha,omega-dihydroxy(1,3-disita-1,3-diphenyl-2-oxaindane)-diphenylsilox ane oligomers with various degrees of condensation were obtained. Corresponding block copolymers were obtained by heterofunctional polycondensation of synthesized alpha,omega-dihydroxy(1,3-disila-1,3diphenyl-2-oxaindane)-diphenylsiloxa ne oligomers with alpha,omega-dichlorodimethylsiloxanes in the presence of amines. Thermogravimetry, gel permeation chromatography, differential scanning calorimetry, and wide-angle X-ray analysis were carried out on the synthesized block coplyrners. Differential scanning calorimetry and wide-angle X-ray studies of these copolymers showed that their properties were determined by the ratio of the lengths of the flexible linear poly(dimethylsiloxane) and rigid poly(1,3-disila-1,3-diphenyl-2-oxaindane)-diphenylsiloxane fragments in the main macromolecular chain. Two-phase systems were obtained with specific flexible and rigid fragment length values in synthesized block copolymers. (c) 2006 Wiley Periodicals, Inc.