Silicon trimer

Results of 29Si NMR spectroscopy of the Ti-Beta precursor gels are shown in Figure 1. The spectra exhibit peaks that belong to four major types of silica species, Q°, Q1, Q2, and Q3. Here Q" denotes a silicon environment with n Si-O-Si bonds. By comparison of the measured spectra with the 29Si NMR spectra found in the literature [4, 5] we were able to determine that the peak with the chemical shift of -72.9 ppm corresponds to Si monomer Q°, while the peaks at -80.9 ppm and -81.4 ppm are the peaks of Q1 linear trimer and dimer, respectively. The peaks from -83 ppm to -89.8 ppm were contributed to Q2 silicon oligomers, while the peaks at the chemical shifts from -90.3 ppm to -102 ppm were denoted as the part of the Q3 silica species. 

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Cyclic trimers and tetramers have been reported by Fritz and Raabe128. These were presumed to arise from intermediates formed in the dissociation of tetramethyl silicon by C-H bond rupture. This seems unlikely, but the same precursors could be formed by hydrogen abstraction from the parent compound. The available evidence indicates that the decomposition does not involve long chains, but free radical attack on the alkyl by the reactions... 

The commercial importance of polysiloxanes (silicone polymers) was discussed in Sec. 2-12f. The higher-MW polysiloxanes are synthesized by anionic or cationic polymerizations of cyclic siloxanes [Bostick, 1969 Kendrick et al., 1989 Noll, 1968 Saam, 1989 Wright, 1984]. The most commonly encountered polymerizations are those of the cyclic trimers and tetra-... 

This, however, is an oversimplification, since these monomeric anions are in rapid equilibrium with oligomers (small polymers) such as the dimer (cf. Fig. 7.2b), cyclic trimer (7.2c), and other small silicate oligomers, 22 of which have been identified by silicon-29 magnetic resonance methods and many of which have no counterparts in solid silicate minerals9 ... 

The sharp lines (labeled as NO and N1 in the 0.6 and 95 wt % spectra of Fig. 3 are believed to be the monomer (no additional Si—O bond) and dimer (1 additional Si—O bridging bond). Neat APS by itself shows two silicon environments (not shown here). The broad bands labeled Ol , 02 and 03 are believed to be the Si—O—Si oligomer environments. Ol represents dimer environments (1 additional Si—O bridging bond), 02 is trimers (2 additional Si—O bonds), and 03 is tetramers (3 additional Si—O bonds). Such assignments are consistent with Si-29 studies of silanols/siloxanes which show an approximately 10 ppm decrement or upheld shift (usually to the right as spectra are plotted) for each silanol converted to a siloxane [15]. Recent Si-29 solution work of Besland et al. [ 16] is consistent with such interpretations. 

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. 

Cyclic siloxanes are important precursors in the silicon industry, being formally dimers or trimers of silanone (R2Si=0), a known intermediate. Cyclic siloxanes have been synthesized by four routes, the conventional methods being the condensation of silanediol or the hydrolysis of species such as halosilanes or aminosilanes (Scheme l)16-20. Alternatively, oxidation of disilene by triplet oxygen (equation l)21-27 or oxidation of oxadisiliranes by singlet oxygen (equation 2)28-31 may be utilized. 

Similarly to nonyltrichlorosilane, one can obtain other alkylchlorosi-lanes and other higher radicals at the silicon atom. For example, methyl-nonyldichlorosilane can be obtained from methyldichlorosilane and propylene trimer, hexyltrichlorosilane can be obtained from trichlorosilane and propylene dimer, isobutyltrichlorosilane can be produced from trichlo-... 

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