Hyperbranched Polycarbosilanes via Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involving organomagnesium (Grignard) [1] and organolithium reagents have been used extensively for many years to form Si—C bonds (see Reaction Scheme 12.1). However, their use for the construction of hyperbranched polymers whose backbone contains, as a major structural component, silicon—carbon bonds, i.e., polycarbosilanes [2] is relatively more recent.

$$\begin{array}{l} {\rm{R}}_3 {\rm{SiX + MR'}} \to {\rm{R}}_3 {\rm{SiR' + MX}} \\ \left({{\rm{R,R' = alkyl}}\,{\rm{or aryl;}}\,{\rm{M = Mg(X),}}\,{\rm{Li,}}\,{\rm{Na}};{\rm{X = halogen, OR''}}} \right) \\ \end{array}$$

((12.1))

This chapter focuses on the application of such nucleophilic substitution reactions toward the synthesis of hyperbranched polycarbosilanes, with particular emphasis on those preparations that have resulted in relatively well characterized products. These syntheses are organized by the type of AB_n monomer unit used (see Section 1.2), where A and B refer to the (C)X and (Si)X_n, respectively, functional ends of the monomer unit and where the nature of the coupling reaction leads to entirely or primarily Si—C bond formation. In most cases, these are “one-pot” reactions that employ monomers that bear halogen or alkoxy groups on the C and Si ends of the unit. Indeed, hyperbranched polycarbosilanes have been described, in general, as “obtained in one synthetic step via a random, one-pot polymerization of multifunctional monomers of AB _n type” [2]. Treatment of the AB_n monomer with either elemental Mg or an organolithium reagent, ideally (but not always) forms a complexed carbanion (the nucleophile) by reaction with the C–X end of the monomer unit, resulting in an intermediate of the type, (X_xM)CSiX_n, where M = Mg or Li, X = halogen or alkoxy, and x = 1 (Mg) or 0 (Li). Self-coupling of this reagent via reactions of the type shown in Reaction Scheme 12.1 leads to oligomeric and polymeric products that are connected primarily through Si—C bonds and yield an inorganic MX_x by-product.