Polysilanes
Polymers with silicon-silicon single bonds in the backbone have been known for some time. It was only within the last 10-15 years, however, that high molecular weight materials were developed [165]. Behind the current interest in these materials is a realization that they have various potential applications. These are in ceramic fibers [166], in microlithography [165, 167], in photoconduction [168], and as nonlinear optical materials [169].
The polymers are prepared from disubstitued dichlorosilanes by reacting them with alkali metal dispersions in a reductive coupling process. The polymerizations appear to have the characteristics of chain-growth rather than step-growth reactions [170]:
The above illustrated reaction with sodium dispersions requires greater than 80°C temperatures to proceed satisfactorily. When mixtures of different dialkylsubstituted dichlorosilanes are reacted in this manner, copolymers form [171].
Recently, an ambient temperature sonochemical reductive coupling process was developed [172]. The reaction is carried out in the presence of an ultrasound and results in relatively high (Mn=50,000-100,000) molecular weight materials with narrow molecular weight distributions. In addition, it was reported [172] that polymers can also be formed by anionic ring opening polymerization of cyclotetrasilanes to yield polymers with molecular weights of 10,000-100,000.
