Soluble high-molecular-weight polysilanes

The discovery that soluble high molecular weight polysilanes may be prepared by the reductive coupling of dichlorodialkylsilanes by alkali metals (1,2) has led to considerable work on the properties of this interesting class of polymers (3,4,5). The preparation of the polymers leaves much to be desired as frequently the high polymer is only a minor product. Mechanistic studies of the reaction with a view to improving the relevant yields have been few (6). The major ones by Zeigler (7,8,9) showed that a silylene diradical was not involved in the reaction, and stressed the importance of polymer solvent interactions. 

Soluble polysilane polymers of high molecular weight were first prepared several years ago (1-3). The method commonly used is the reductive coupling of dialkyldichlorosilanes with sodium in refluxing toluene. Relatively few studies of the mechanism of the reaction or discussions of the probable reaction intermediates have been reported. 

Additionally, it is known that cyclic SiaClg can be polymerized in an ROP via radical species to yield an insoluble but moisture sensitive polymer with an estimated degree of polymerization of about 35 and a molecular weight of about 3,500gmor [45]. Due to the insolubility and the high sensitivity to moisture, there have been no further reports on a ring opening type synthesis of perchlorinated polysilanes [42,45]. Nevertheless, a functionalization by reaction with isopropanol has been accomplished which led to an increased solubility and enabled a SEC-analysis [45]. 

The polymerization of these monomers in THF solutions with a catalytic amount of butyllithium gave the corresponding amino-substituted polysilanes 15a-d in high yields. Although polysilanes 15a and 15b were insoluble, 15c was soluble in various organic solvents. Since the solid state Si NMR spectra of 15a and 15b were almost similar to that of 15c, the insoluble polymer 15a and 15b are also pure amino-substituted polysilanes. The molecular weight distribution of 15c was determined by gel permeation chromatography calibrated by polystyrene standards with toluene as eluent Mn = 2.7 x 10", Mw n = 1.7. 

Soluble polysilane high polymers have recently attracted considerable attention because of their unusual electronic properties.(i, 2) Polysilanes absorb strongly in the UV spectral region, typically between 300-410 nm, and are strongly fluorescent. These transitions have been attributed to delocalized Si(o-a ) states. The spectroscopic properties of polysilanes are sensitive to the molecular weight, polymer conformations, and the substituents attached to the silicon backbone.(2-4)... 

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