Trisiloxanes formation

Fig. 5.5.3. Potential sites of cleavage ( —> ) in the abiotic degradation of the trisiloxane alkylethoxylate surfactant, M2D—C3—O—(EO) —CH3 (1) formation of compounds 2—6.

When a toluene solution of a mixture of cyclotrisilane 34 and cyclohexyl isocyanate (or f-butyl isocyante) was heated at 70 °C, cyclic di- and trisiloxanes 37 and 38, i.e. the cyclic dimer and trimer of the silanone 36, were obtained together with the corresponding isonitrile RN=C. The formation of 37 as well as 38 was completely suppressed in the presence of hexamethylcyclotrisiloxane (19 D3) instead, quantitative conversion of 35 into 39, the formal insertion product of the silanone 36 into the Si—O bond of D3, occurred (Scheme 14). Since neither cyclodisiloxane 37 nor cyclotrisiloxane 38 reacted with D3 under the reaction conditions, the possibility that 37 or 38 is the precursor of 39 was ruled out. Whereas the oxidation of 35 with cyclohexyl and t-butyl isocyanates proceeded with exclusive formation of 37 and 38 (as the silicon-containing compounds) the reaction of 35 with phenyl isocyanate resulted in the formation of 37 in low yield. Furthermore, in this case the presence of D3 did not totally suppress the formation of 37. According to the authors, these results indicate that the oxidation of 35 with cyclohexyl and f-butyl isocyanates appears to use other reaction channels than that with phenyl isocyanate. 

The acyclic compound l-NaphPh(H)SiOCOPh is not significantly converted to benzaldehyde on thermolysis, even at temperatures considerably higher than those (100-160 °C) which are effective with the chelate system. This reaction may proceed via the transient formation of a chelated silanone 82, since the trisiloxane 80 is isolated as the silicon-containing product. Confirmation of this mechanism was obtained in the reaction of 78 with carbon dioxide, from which the silyl ester of formic acid 81 can be isolated and... 

A third way of synthesizing six-membered rings is by intramolecular salt elimination from metallated halogen-substituted trisiloxanols. Lithium 1,1,5,5-tetra-tert-butyl-l-fluoro-5-hydroxy-3,3-dimethyl-trisiloxane is stable in n-hexane. Only by weakening the Li-0 contact by using a donor solvent such as THF, can the elimination of LiF with concomitant formation of a six-membered ring be achieved (Eq. 5). ... 

The Si-H functions of the ferriomethyl-siloxanes 19a-c offer the possibility for further functionalization reactions. For example, photochemical treatment induces elimination of CO and subsequent reaction steps leading to rearrangement of the siloxane backbone. In the case of the ferriomethyl-tri- and -tetrasiloxanes 19a,c, UV irradiation in benzene results in the formation of the six-membered cyclo(ferra)trisiloxanes 20a,b (Scheme 26.1). ... 

Water has a typical surface tension of 72mN/m and, as can be seen fi om the above table, all the surfactants tested reduced the surface tension of the s em, and as a result the aqueous medium wets more efficiently. For the silicone surfactants the best results were achieved with product A, a low molecular weight material. Trisiloxane A gives a very low figure that is only improved upon by the fluorosurfactant. The Critical Micelle Concentration (CMC) is the required level of product to initiate the formation of micelles in the bulk of the liquid. Up to this point the surfactant added to die water migrates to the liquid/air interface to form a film which reduces the surface tension. The low CMC for A shows its high packing efficiency at the intoface, in the much lower level required in comparison to the other products. 

In the case of superwetting trisiloxane surfactants the spreading area is not proportional to surfactant concentration when the experiment is performed in controlled laboratory conditions at 50% relative humidity (Fig. 27A, filled circles) it usually decreases at concentrations above 0.1 wt %. This can be explained by the formation of viscous liquid crystalline... 

The cyclic trisiloxanes are composed exclusively of Q silicons as shown by the MAS and H CPMAS spectra of the 600°C sample in which shifts are observed for the Q peak position, whereas the H CPMAS and hydrated MAS spectra show that the and peak positions are unaffected by the formation or elimination of the D2 species. The H-O distances in the neighboring silanol groups of the precursor (left side Eq. 14) exceeds 5.5 A, causing them to exhibit SiO-H vibrations characteristic of isolated species [164]. 

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