Phosphorus-containing polycarbosilazanes: synthesis via dehydrocoupling catalysis and flame-retardant properties

The barium complex [Ba{N(SiMe₃)₂}₂]₂ has been used to catalyse the dehydropolymerisation of the phosphine-functionalised hydrosilane 4-Ph₂P-C₆H₄SiH₃ (A) with the α,ω-diamine 1,4-(CH₂NHMe)₂-C₆H₄ (C), for the production of -[Si(4-C₆H₄PPh₂)H-N(Me)CH₂-C₆H₄-CH₂N(Me)]- polycarbosilazanes that contain dangling phosphino groups along the polymer backbone. The comonomers A and C, specifically prepared for this purpose and comprehensively characterised, lend themselves well to barium-promoted dehydrocoupling catalysis. They allow for the formation of linear, amine-capped polymers with molecular weights in the range 4000-8000 g mol^-1, as estimated by DOSY and ¹H end-group NMR analyses. The terpolymerisation of C with various mixtures of A and PhSiH₂R (R = H or Ph) led to the formation of terpolymers featuring various contents of phosphine-functionalised silazane groups, with an overall composition that reflects well the initial feed ratio of the comonomers. Thermal decomposition and flammability were studied at the microscale on a series of polycarbosilazanes to assess the effect of molecular groups on thermal stability and heat release. The SiCN backbone containing the silazane group significantly contributes to the "charring" of the polymer. By contrast, PPh₂, as other groups bearing a phosphorus atom in a low oxidation state, is not an effective char promoter.