Two bis(dimethylamimo)silanes with benzocyclobutene (BCB) groups, bis(dimethylamino)methyl(4'-benzocyclobutenyl)silane (2) and bis(dimethylamino)methyl [2'-(4'-benzocyclobutenyl)vinyl]silane (4), were synthesized from different synthetic routes, which were then employed to prepare two novel silphenylene-siloxane copolymers (SiBu and SiViBu) bearing latent reactive BCB groups by polycondensation procedure with 1,4-bis(hydroxydimethylsilyl)benzene. At elevated temperatures these copolymers were readily converted to highly crosslinked films and molding disks with network structures by polymer chain crosslinking, which followed the first-order kinetic reaction model. The final resins of SiBu and SiViBu demonstrated excellent thermal stability with high glass transition temperatures (218 and 256 degrees C) and high temperatures at 5% weight loss (553 and 526 degrees C in N-2, 530 and 508 degrees C in air). After aging at 300 degrees C in air for 100 h, the cured resins showed weight loss lower than 4%. The films of cured SiBu and SiViBu also exhibited relatively low dielectric constants of 2.66 and 2.64, low dissipation factors of 2.23 and 2.12 x 10(-3), low water absorptions (<= 0.28%), and high transparence in the visible region with cutoff wavelengths of 321 and 314 nrn. Moreover, the aged films exhibited good dielectric properties and low water absorptions. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7868-7881, 2008