Metal-free polymerization of phenylsilane: Tris(pentafluorophenyl)borane- catalyzed synthesis of branched polysilanes at elevated temperatures

The strong organoborane Lewis acid B(C₆F₅) ₃ catalyzes the polymerization of phenylsilane at elevated temperatures forming benzene and SiH₄ as side-products. The resulting polymer is a branched polysilane with an irregular substitution pattern, as revealed by 2D NMR spectroscopy. Having explored the mechanism of this novel metal-free polymerization by computational chemistry methods at the DFT level, we have suggested that unusual cationic active species, namely monomer-stabilized silyl cations, propagate the polymerization. Hydride abstraction of SiH₃ moiety by the catalyst in the initiation step was found to be kinetically preferred by around 9 kcal mol^-1 over activation by coordination of the monomer at the aromatic ring. The formation of linear Si-Si bonds during propagation was calculated to be less favorable than branching and ligand scrambling, which accounts for the branched and highly substituted form of the polymer that was obtained. This novel type of polymerization bears the potential for further optimization with respect to degree of polymerization and structure control for both primary as well as secondary silanes, which can be polymerized by sterically less hindered boranes. Polysilanes without metals! The strong organoborane Lewis acid B(C ₆F₅)₃ catalyzes the polymerization of phenylsilane at elevated temperatures (see scheme). 2D NMR spectroscopy revealed the resulting polymer to have a branched and highly substituted structure. DFT calculations confirmed the experimental results of this metal-free synthesis with substitution and branching being kinetically more favorable processes than Si-Si bond formation for linear growth.