Silicon chains

The alternative open-chain structure for the product is suggested by the marked unsaturation of one modification of the tetrasilane. Both types are attacked and split by aqueous alkalies. 

It follows that, even though suitable procedures were to be found for the alkylation of the longer silicon chains, the products would be subject to oxidation, to thermal dissociation, and to hydrolysis in the presence of alkalies. It seems unlikely that any combination of substituents could stabilize these chains sufficiently to allow their practical application as polymeric materials. 

---

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom. The two groups attached to the silicon chain... 

C06-0129. Use average bond energies (see Table 6-2) to estimate the net energy change per mole of silicon for the conversion of a silicon chain into an Si—O—Si chain. Repeat this calculation to estimate the net energy change per mole of carbon for the conversion of a carbon chain into a C—O—C chain. 

For short-chain oligomers the absorption maxima also depend on molecular weight, reaching a limiting value in silicon chains about 40-50 atoms in length. See ref. 17. 

The other mechanism, if in fact there is one, is perhaps via a radical anion as per the SET mechanism suggested by Heitz (14,15) for other polymerizations. The ability of the long silicon chain to delocalize electrons would assist in stabilizing either radical anions or straight anionic species. A simple radical is preferred by Zeigler (9) for the formation of the highest MW fraction in the polymerization of PMDS, a product for whose route no evidence has accrued In this work. 

Figure 10.67 indicates the probable distribution of a silicone containing the optimum content of aminoethyliminopropyl groups when applied to a polyester fibre surface. In this case the attachment is through hydrophobic polymer-fibre interaction and the mobility of the silicone chain segments is increased by electrostatic repulsion between neighbouring cationic groups. Dependence of softness of the treated polyester fabric on the proportion of... 

The reaction product, with a dozen or more diol groups per silicone chain, is a viscous fluid exhibiting remarkable properties in surfactant chemistry. 

The dynamics of polysilane dendrimer excited states were also studied by Watanabe as mentioned above in a comparison with polysilynes,360,364 and suggested that a configuration coordinate model is applicable to the photophysics of branched silicon chains. Calculations showed a distorted geometry of the excited state localized at a branching point. 

Ladder polysilanes constitute a special case of fused polycyclic silicon macromolecules, in which cyclotetrasilane rings systematically catenate to form a silicon double helix, comprising two multi-linked silicon chains. Work in this area was initiated in this area by Matsumoto in 1987, and now comprises an integral part of the literature on higher-dimensionality polysilanes. 

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom (Structure 1). The groups R and R attached to the silicon chain can be of a large variety. Polysilanes with alkyl and/or aryl substituents have been the most thoroughly investigated [1-3], whereas polysilanes having at least a heteroatom substitution such as H, Cl, OR, NR2 have received much less attention [4]. The number of silicon atoms is usually from several hundreds to several thousands. 

Tachikawa (1999) also analyzed mobilities of carriers along the silicon chain, and his results should be mentioned separately. As it turned out, the mobility obtained for a positive charge (hole) was several times larger than that for an excess electron. This result suggests that the localization mechanism of a hole and that of an electron are different from each other. Probably, an excess electron is trapped in the defect of the main chain, whereas a hole is not trapped. The defects are mainly structural ones, such as branching points and oxidized sites (Seki et al. 1999). This can lead to a different electron conductivity. Continuation of the polysilane ion radical studies will hopefully result in some important technical applications. 

In recent years polysilane dendrimers have been synthesized in several laboratories. Two examples are shown in schemes 5.7128 and 5.8.129 The dendrimers have electronic absorption spectra similar to those of the longest silicon chains present, but with intensity about ten times as great. Like the network polysilanes, these dendrimers display weak visible emission spectra, perhaps resulting from localized excitations at the branching points. The field of polysilane dendrimers in the subject of a recent authoritative review.130... 

Wurtz coupling of dibutylaminotrimethyl-l-2-dichlorodisilane forms a partially networked polymer.79 This networked polymer shows an absorption maximum at 360 nm, that is 30 nm red shifted relative to the absorption of poly (dialkylsilanes). The shift is due to the nonbonding electron pair of the amino substituents extending the a-conjugation of the silicon backbone. Two broad emission bands at 440 nm and 400 nm are observed and assigned to the network silicon units and the linear silicon chains, respectively. The unusual photophysical properties arise from both the amino side groups and the networked structure. 

Additional CaSi reactions were carried out with halogens, PC15, and other halogenating agents. In all cases a decomposition of the silicon chains to monomeric silane derivatives occurred before reaction of the calcium was complete, so no definable products could be obtained. 

Figure 8.4 Typical variation of the standard NMR parameter, %c, as a function of the variable of percolation e the different states of gelation were obtained by quenching the polymer in ice, at different times during the kinetics of synthesis of the network which occurs at 150 °C. The polymer is a slightly modified silicone chain which bears randomly distributed vinyl groups as comonomeric units (the mean number of vinyl groups along one chain is Cvi = 2 x 10"3). Links are created between vinyl and methyl groups the three symbols correspond to different catalyst concentrations (redrawn from [18])...

In summary, although there are some indications that the stereochemistry of silane dehydrocoupling may not be completely random, the effect is not dramatic and can be rationalized by any of the mechanisms discussed previously. In the case of radical coupling, steric interaction between the substituents on the end silicons would tend to favor syndiotactic enchainment. Coupling in the coordination sphere of the catalyst would involve interactions between catalyst substituents and substituents at the end of the silicon chain. 

---