Polycarbosilazane and Related Dendrimers and Hyperbranched Polymers

Linear polysilazanes and polycarbosilazanes are well-known members of the organosilicon polymer family and can be prepared by a variety of methods [1, 2]. These polymers are characterized by having either a –Si–N– backbone (polysilazanes) or a –R–Si–N– backbone (polycarbosilazanes). On the other hand, dendritic analogs are relatively rare. Polysilazane dendrimers are essentially nonexistent, and the syntheses of polycarbosilazane dendrimers have been reported by only two groups. A primary reason for the paucity of examples is the relative reactivity of the Si–N bond. Since many reagents involved in conventional organosilicon dendrimer synthesis would react with Si–N bonds, syntheses of these systems are difficult to design. This problem has been partially circumvented in polycarbosilazane dendrimers in which the nitrogen atoms are bonded to three silicon atoms, a bonding situation that is considerably less reactive than nitrogen bonded to only one or two silicon atoms

Despite the synthetic difficulties, organosilicon dendrimers with nitrogen in the structure are of interest from a fundamental perspective. First, the nitrogen atoms represent potential binding sites, as in the poly(amidoamine), PAMAM, poly(amidoamine-organosilicon), PAMAMOS (see Chapter 11), or poly(ethylene imine), PEI, dendrimer systems. Secondly, the lability of Si–N bonds raises the possibility of controlled degradation of the dendrimers. Furthermore, the presence of planar trisilyl-substituted amine groups [3–5] throughout the structure would impose some rigidity and interesting configurational constraints on the dendrimer. As a consequence, when combining these factors with the underlying synthetic challenges involved, one can see that this field should hold much interest for the synthetic dendrimer chemist