Polysilane polymers configuration

Less is known about the configurations of polysilane polymers. Dialkylpolysilanes such as (MeSi-n-Bu) made by sodium condensation appear to be atactic, from Si NMR studies, but arylalkylpolysilanes made by the same procedure appear to be partially tactic. Anionic ringopening polymerization of (PhSiMe)4 (Section 7.3) leads to a (PhSiMe) polymer with greater, but still incomplete, stereoregularity. 

The results for arylsilanes are not fully understood, but the spectra may reflect partial tacticity in these polymers. Further work is needed studies of model compounds with known relative configuration would be particularly helpful. Silicon-29 NMR of polysilane copolymers also shows great promise, especially for distinguishing block-like from fully random copolymers. 

Although main-chain chirality refers to both polymers with stereogenic centers in the main chain (configurational main-chain chirality) and polymers with main chains consisting of helical stereogenic bonds induced by chiral side groups or end termini (conformational main-chain chirality), in this chapter we mainly focus on polysilanes exhibiting the latter type of chirality. 

Various methods may be used to examine configurations of polysilanes, but 29Si NMR spectroscopy has been the most useful. Silicon-29, like carbon-13, has spin 1/2 and a relatively low abundance, 4.7%. Nuclear magnetic resonance (NMR) spectroscopy using 29Si has been important for the characterization of siloxane polymers, and is proving to be equally useful for polysilanes. 

Apart from the carbon chain polymers discussed above, the silicon chain polymers have also been investigated extensively in terms of microstructure. The stereochemistry of polysilanes has been studied using Si-NMR spectroscopy [54,55]. Wolff et al. [56] concluded that for a poly (phenylmethyl silane), the ratio of mm rr mr(rm) to be 3 3 4 and that the spectra of poly(1,2,2-trimethyl -1-phenyldisi-lane) are consistent with approximately equal amounts of head-to-head and head-to-tail sequences and an atactic configuration. 

Because the electronic properties of polysilanes are dependent on the conformation of the backbone,which may depend on the configuration of the substituents, it is necessary to prepare polymers with well-defined microstructures. In ring opening polymerization, the resulting polymer microstructure depends on two factors. The fust one is the configuration of the substituents in the cycfic monomers. The second factor is the mechanism of die ring opening polymerization. This paper is mainly focused on the discussion of stereochemistry in the cyclotetrasilanes. 

---