Nickel(0)-Catalyzed Polycondensation of Silafluorene: Control over Molecular Weight and Polymer Growth Mechanism

The catalytic polymerization of silafluorenes with [Ni­(dmpe)₂] (dmpe = 1,2-bis­(dimethylphosphino)­ethane) afforded 2,7-dibutoxy-9H-9-silafluorene polymers with a narrow polydispersity (M_w/M_n < 1.1). In an open vessel, the polymerization ceased shortly after complete consumption of the monomer, but transfer of the reaction mixture into a closed system resumed the polymer growth. Polymerization in a closed system with occasional removal of the evolved H₂ furnished soluble polymers with a maximum molecular weight of M_n = 3860. Treatment of a 2,7-dibutoxy-9H-9-silafluorene tetramer with [Ni­(dmpe)₂] resulted in the initial formation of a mixture of monomers and trimers, which subsequently transformed into oligomers (Si₁–Si₉). This polymerization involves a preferential dehydrogenative condensation between the monomer and an oligomer relative to that between oligomers, while the presence of H₂ regenerates the monomer after its consumption and thus promotes further polymer growth.