NMR of Polysilanes

The Si-NMR spectra of polysilanes show that most of the chemical shifts are in the negative region [4, 72]. Representative examples of polymers and their Si-NMR chemical shifts are given in Table 7.1. 

If the same substituents are present on silicon (in polymers of the type [RzSiJn) a single resonance is seen in the Si-NMR spectrum (for example, see entries 1 and 2, Table 1). On the other hand, if the substituents are different on the silicon, there is a possibility of fully ordered polysilanes. Thus, if all the substituents are on the same side it would be an isotactic polymer (Fig. 7.23). However, such an ordered isotactic polysilane has not yet been prepared. 

Even in atactic polymers where the organization of the side groups is perfectly random, if one considers short segments, say a triad of three successive silicon atoms there are three possibilities of a short-range order that could be present. These are termed isotactic, syndiotactic and heterotactic (Fig. 7.24). It should be emphasized that the terms isotactic, syndiotactic and heterotactic as applied in this context represent the statistical possibil- 

Another way of preparing the ordered copolymer consists of using the masked-disilene method. If the monomer (masked disilene) is designed in such a manner that one of the silicon atoms has two methyl groups and the other silicon has two -hexyl substituents, it would be expected that the polymer obtained from it should have a stereo regular arrangement. This has been accomplished and in the Si NMR of this polymer two distinct 

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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. 

Polysilanes are a unique class of polymers in which the o--electrons are delocalized entirely along the sp -bonded silicon backbone, causing their electronic absorption properties to be strongly dependent on the conformation of silicon backbones [2]. This property has created much interest in the structure of these polymers in the solid state [3]. In spite of the usefulness of solid-state NMR, there are few systematic studies on the Si CP/MAS NMR of polysilanes [4-6]. Most recently, it has been demonstrated that the VT Si CP/MAS NMR experiment is very useful to study the conformational features of polysilanes in the solid state [7j. Measurements of the Si CP/MAS NMR spectra of poly (methylphenylsilane) (PMPS), in the solid state over a wide range of temperatures, are performed and the conformation... 

Fig. 1. Dimethylsilylene region of the C NMR of polysilane prepared a) by anionic polymerization, and b) by Wurtz coupling of of 6 4 mixture of masked disilene.

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. 

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. 

T. Takayama, has reviewed research works on polysilanes and copolymers of silanes and siloxanes. T. Takayama, Solid State NMR of Polymers, 1. Ando and T. Asakura eds., Elsevier Science, Amsterdam, 1998, p. 613. 

Fluorescence spectroscopy in combination with circular dichroism (CD), optical rotatory dispersion. X-ray crystallography, UV and NMR spectroscopy of the main chain is a powerful probe for identifying helical conformation, uniformity, and rigidity in polymers. In recent years, these techniques have been applied extensively to investigate the structures of polysilanes in both the solid state and in solution and it is now clear that after electronic structure main chain helicity is the principal determinant of the properties of polysilanes. In... 

A final comment on the crystallization of SiC from pol)mier derived ACCs is that, more or less, in all papers regarding this problem it is reported that silicon carbide is formed as pSiC, i.e., the polytype 3 C. Our group followed the polytype development of SiC formed by pyrolysis of polysilane in dependence on the pyrolysis temperature [168] and found that, indeed, up to 1800 °C pSiC dominates the XRD, Raman, as well as Si NMR results, but that especially in the case of pyrolysis temperatures in the range 1600-1700 °C non-negligible parts of polytypes of a-SiC also occur which have disappeared again at T = 1800 °C. 

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. 

Proton, 13C and 29Si NMR spectra for polysilanes have been recorded.(32) The proton NMR provide little structural information, but integration of areas under the proton resonances is quite useful for determining the composition of copolymers. 

Polymers with triflate groups react with alcohols to form alkoxysubstituted polysilanes. This reaction occurs readily in the presence of bases. The best results were obtained using triethylamine and hindered pyridine. In Fig. 3c the NMR spectrum of the reaction mixture containing the excess of triethylamine is shown, the methyl groups from the polymer chains absorb in the range typical for alkoxysilanes. Reaction in the presence of unsubstituted pyridine leads to the formation of insoluble polymer probably by attack at the p-C atom in the silylated pyridine. 

During photolysis, the double bond content of the polysilane(P-l)(15mol% in this experiment) decreased to 10mol%, as measured by 1H-NMR spectroscopy. However, the ratio, quantum yield of scission(Q(S))/quantum yield of crosslinking(Q(X)), was not affected by the reaction of the double bond. West and his coworkers have reported that poly((2-(3-cyclohexenyl)-ethyl)methylsilane-co-methylphenylsilane) crosslinked upon irradiation(55). The difference between our results and West s may lie in the amount of the double bond and inhibitation of the radical closslinking by the phenol moiety. Polysilane with a halogen moiety, P-8, photodecomposed rapidly, compared with P-1 or P-3. The introduction of a chloride moiety was effective for the sensitization of the photodegradation. Similar results has already been reported(55). 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

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