AbstractPolysilphenylenesiloxanes containing various amounts of vinyl substituents (ranging from partial (25 %) to complete (100 %)) on the silicon atoms were synthesized. 29Si NMR spectroscopy revealed that they had a well defined structure, as designed. Unlike the known crystalline poly(tetramethyl‐p‐silphenylenesiloxane), all polymers containing vinyl side groups were amorphous and showed low Tgs, ranging from −52 to −32 °C (from differential scanning calorimetry (DSC)). Dynamic and isothermal thermogravimetric (TG) analyses indicated that they all possessed a greatly improved thermal stability up to 500 °C, and have ultra‐high residual yields at 1000 °C under both inert and oxidative conditions. Polysilphenylenesiloxanes containing a high percentage of vulcanizable vinyl substituents should be excellent candidates for high‐temperature polymers and have potential applications such as heat‐resistant or flame‐retardant materials. Copyright © 2004 Society of Chemical Industry