Anions and radicals as intermediates in the reductive coupling of disubstituted dichlorosilanes with sodium

Abstract

Polymerization of four monomers with alkenyl groups: allylmethyldichlorosilane, allylphenyldichlorosilane, 5-hexenylphenyldichlorosilane and 5-hexenylmethyldichlorosilane was studied by reductive coupling with sodium in the presence of ultrasound at ambient temperatures. Soluble homopolymers and copolymers with phenylmethyldichlorosilane with molecular weights Mn >10,000 were obtained for the first three monomers. UV and29Si NMR studies indicate a high degree of Si-Si catenation for allyl derivatives. Approximately half of the alkenyl groups remain in the polymer after a few hours. This indicates that after the formation of the polysilane backbone alkenyl groups are predominantly trapped intramolecularly in a process resembling cyclopolymerization of diallylsilanes. The main mechanistic conclusion is that in the reductive coupling process radicals are present as short living intermediates which are rapidly reduced to anions at higher polymerization degrees, but which are efficiently trapped at a monomeric or dimeric level. Thus, electron transfer proceeds in two one-electron transfer steps from a chloroterminated chain to a radical, and then to a macromolecular silyl anion. The latter reacts with a monomer via a nucleophilic substitution. Polymerization is an anionic chain growth process.