The cross-linked polysiloxanes were directly synthesized by anionic ring-opening copolymerization of Ph-12-POSS as multifunctional monomer with D-4 and Ph8D4 under KOH or Me4NOH siloxanolate. And the influences of the polar additive DMAc on gelation time were investigated. The results of gel content and swelling ratio, GPC, solid-state Si-29 and C-13 NMR, FT-IR, WAXD showed that Ph-12-POSS was reacted and most of the product was cross-linked. The DSC and TG results indicated that the cross-linked polysiloxanes exhibited distinct glass transition temperatures (T-g) and excellent thermal stability. Compared to that with KOH siloxanolate, the cross-linked polysiloxane synthesized with Me4NOH siloxanolate has more preferable thermal stability. (c) 2005 Elsevier Ltd. All rights reserved.