alt-Copoly[1,9-decaphenylpentasiloxanylene/1,3-bis(ethylene)tetramethyld isiloxanylene], alt-copoly[1,9-decaphenylpentasiloxanylene/1,5-bis(ethylene)hexamethyltr isiloxanylene], alt-copoly[1,9-decaphenylpentasiloxanylene/1,7-bis(ethylene)octamethylte trasiloxanylene], and alt-copoly[1,9-decaphenylpentasiloxanylene/1,9-bis(ethylene)decamethylpe ntasiloxanylene] were synthesized by Pt-catalyzed hydrosilylation reactions of 1,9 divinyldecaphenylpentasiloxanes with a series of oligodimethylsiloxanes. The molecular weights of these copolymers were determined by gel permeation chromatography. Their glass-transition temperatures (T-g's) were obtained by differential scanning calorimetry. The thermal stabilities of the copolymers were measured by thermogravimetric analysis. The structures of the copolymers were verified by H-1, C-13, and Si-29 NMR as well as IR and UV spectroscopy. The copolymers displayed high thermal stabilities and a single T-g, indicating that phase separation between the two short blocks did not occur. (c) 2005 Wiley Periodicals, Inc.