Polysiloxanes containing thermally curable benzoxazine units in the main chain have been synthesized. For this purpose, first the diallyl functional benzoxazine monomer is synthesized through the Mannich and respective ring closing reactions of 4,4'-isopropylidenediphenol (bisphenol A), formaldehyde, and allyamine. Subsequent hydrosilylation reaction of the resulting allylic monomer (B-ala) with 1,1,3,3-tetramethyldi-siloxane (TMDS) in the presence of Pt catalyst yields the corresponding oligo(B-ala-tetramethyldisiloxane)s (OBTMDS). Using the anionic polymerization route, OBTMDS was then converted to poly(bisbenzoxazinedimethylsiloxane)s (PBDMSs) by reacting with readily available cyclic oligomer octamethylcyclotetrasiloxane (D4) or decamethylcyclopentasiloxane (D5) in the presence of tetrabutylammonium hydroxide as catalyst. The structures of the precursor diallyl monomer, the intermediate oligomer, and the resulting polymers are confirmed by Fourier transform infrared and H-1 NMR analysis. Curing behavior of the products at various stages has also been studied by differential scanning calorimetry. Flexible transparent films of the PBDMSs are obtained by solvent casting. Thermal properties of the cured polymers are also investigated by thermogravimetric analysis. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5156-5162, 2010