Structural isomers of thermo-oxidatively stable poly(carborane-siloxane-arylacetylene) (PCSAA), namely, m-PCSAA and p-PCSAA, were synthesized by the reaction of the dimagnesium salts of m-diethynylbenzene or p-diethynylbenzene with 1,7-bis(chlorotetramethyldisiloxyl)-m-carborane. The developed polymers have exceptional thermo-oxidative properties similar to their diacetylene counterpart poly(carborane-siloxane-acetylene), PCSA. Thermal treatment of either of the PCSAAs results in a fully crosslinked thermoset by 500 degrees C resulting from the cycloaddition reactions involving the acetylene and aryl functionalities and subsequent formation of bridging disilylmethylene entities as discerned from Fourier transform infrared, 13C and 29Si solid-state NMR, and XPS studies. X-ray diffraction analysis revealed that the thermosets obtained from p-PCSAA possess enhanced crystallinity when compared to that obtained from m-PCSAA possibly due to more efficient packing interactions of the p-diethynylbenzene groups during thermoset formation. The presence of the aryl groups in the backbone of the PCSAAs' chains appeared to have enhanced the storage and bulk moduli of their thermosets when compared to the thermoset of PCSA. Dielectric studies of m-PCSAA and p-PCSAA revealed segmental relaxation peaks, , above their glass transition temperatures with p-PCSAA exhibiting a broader peak with a slower relaxation rate than m-PCSAA. (c) 2013 Wiley Periodicals, Inc.dagger J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 26382650