Cations, silicon-containing

Crivello, J.V. and Lee, J.L., The synthesis, characterization and photoinitiated cationic polymerization of silicon-containing epoxy resins. J. Polym. Sci. Polym. Chem. Ed., 28, 479-503 (1990). 

Examples of silicon-containing macrocycles are rare and are perhaps incidental to the report of crown-type compounds. Obviously, silicon is more like carbon than a heteroatom in its ability to complex a cation. Nevertheless, there is a recent report of a crown compound which contains silicon as part of the macroring. Liptuga, Irodionova... 

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... 

One section in this review will deal with silyl cations, another with hypervalent silicon compounds. The concept of hypervalent sihcon compounds belongs, strictly speaking, to the class of Lewis base catalysis. However, since a Lewis base forms in situ with a silicon containing reagent or SiCl an intermediate, which functions as a Lewis acid to activate substrates during the reaction, we would also present a few examples in this review. Since silicon is a semimetal we leave it up to the reader to decide whether silicon catalysts should be considered as organocatalysts. 

TABLE 1. Solution 29Si chemical shifts for various compounds with some cationic character containing silicon, together with calculated values and related species for comparison0... 

There have been numerous experimental as well as theoretical studies dealing with the structural and thermochemical properties of cationic silicon hydrides, Si Hm+ , and a detailed discussion of these species would certainly exceed the limited space available. We will therefore confine ourselves to the discussion of only a few exemplary cases. For further information on Si-ion thermochemistry the reader is referred to several reviews on the experimental2-4 as well as computational5 determination of thermodynamic properties of silicon-containing ions. 

There are several reports in recent literature on the application of silicon-containing compounds as the initiators of cationic ring-opening polymerization. This apparently is related to the attempts to prepare block copolymers containing polysiloxane or polysilane segments. (CfL SiCFAgClCL system was used to initiate cationic polymerization of tetrahydrofuran... 

In spite of the fact that in many cases the larger alkaline earth metals can be exchanged for divalent rare earth metals, the latter ones form different varieties of three-dimensional or layered triply bonded disilicides with either the a-XhSi2 or the AIB2 structure. These two structure types are clearly dominated by a trigonal prismatic arrangement of the cations and contain silicon in exclusively trigonal planar coordination (Fig. le and If). 

Photolithography on the basis of the cationic graft-polymerization uses an inert polymer film containing a PAQ coated onto a wafer. Exposing the photoresist to the UV radiation generates acid on its surface, to which a vapor-phase silicon-containing monomer is grafted. RIE in O2 is used as a final step. ° ... 

Jahedshoar el al. (Wella) Silicone surfactant hydrophobic, nonsurfactant silicone basic or cationic N-containing conditioning compound Polyhydric alcohol Optically clear, transparent or translucent hair conditioner leave-in or rinse-off... 

The recent syntheses of zeolitelike crystalline silicas, which contain infinitesimal amounts of aluminum, and the more recent syntheses of zcolitelike crystalline aluminum phosphates, chemical analogues of crystalline silica, have further confused the zeolite versus molecular sieve question. The conservative position contends that these molecular sieve crystals are not zeolites on two counts They are not aluminosilicates, and they do not contain exchangeable cations. Furthermore, some of the relatively high silicon-containing materials have an average of less than one... 

The silaallyl cation structures 229, 230 and 231 were assigned118,119 to the corresponding ions in the mass spectra of trimethylsilylmethanes, and the silicon-containing cations 232 and 233 were suggested119 to be formed via complicated pathways (see Section III.F) from ionized TMS-substituted chlorosilanes. 

The electropositive nature of silicon is one of the reasons why doubly charged cations are very often encountered in the El mass spectra of silicon-containing molecules. As the stability of these dications increases with decreased Coulomb repulsion, i.e. with the increased distance between the formally charged centres, it is understandable that the [M — 2Me]2 + dication is much more pronounced in the El spectrum of the bis-TMS ether 365 in comparison to 366 (reaction 153)76. 

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