Novel poly(tetramethyl-1,4-silphenylenesiloxane) derivatives having 2-methoxy-ethoxy or 2-(2-methoxyethoxy)ethoxy substituents at both 2-and 5-positions on phenylene moieties were synthesized and characterized by differential scanning calorimetry and thermogravimetry analyses. Poly(tetramethyl-1,4-silphenylene-siloxane) derivatives were obtained by condensation polymerization of the corresponding disilanol derivatives, i.e., 1,4-bis(dimethylhydroxysilyl)-2,5-bis(2-methoxyethoxy)benzene and 1,4-bis(dimethylhydroxysilyl)-2,5-bis[2-(2-methoxy-ethoxy)ethoxy]benzene , which were prepared by the Grignard reaction using chlorodimethylsilane and the corresponding dibromobenzene derivatives followed by the hydrolyses, catalyzed by palladium on charcoal. The introduction of 2-methoxyethoxy groups on the phenylene moiety made the melting point high, compared with poly(tetramethyl-1,4-silphenylenesiloxane); however, that of 2-(2-methoxyethoxy)ethoxy groups made it low, indicating the longer oxyethylene moiety induced the lowering of the melting point. There were no significant differences in the thermostabilities of both present polymers, suggesting the length of oxyethylene moiety would not affect the thermostability, though the introduction of polar oxyethylene group onto the phenylene moiety induced a decline of thermostability.