Silicones dimeric nature

The kinetic stability of group 14 metalloles towards [4 + 2] dimerization is dependent on the nature of the heteroatom and on the substituents bonded to it and the ring carbons. Like cyclopentadiene, the silole 2 is unstable as a monomer. The silicon methylated derivatives (3 and 4) are also unstable, but two mesityl groups provide stabilization by steric hindrance of the [4 + 2] dimerization transition state15. The same effect has been observed for 1,1-dimesitylgermole24. 

In the absence of anything else to react with, silenes dimerize in either head-to-tail or head-to-head fashion depending on the identity of the substituents at silicon and carbon, and their effect on the natural (<5+Si=C<5 ) polarity of the silenic double bond. The head-to-tail regiochemistry leading to the formation of the corresponding 1,3-disilacyclobutane is most common, while head-to-head dimerization leading to either the corresponding... 

These results show that the Al and Fe present in the cement gel do I not replace Si, except perhaps to a minor extent. This conclusion is reinforced by most of the results of the silicate anion studies described in Section 7.2.6. If there was any significant replacement of silicon at a crystal chemical level, the average anion size and Si04 connectivity would both be. reduced. Further evidence of a similar nature is provided by the observation that the average anion size in C-S-H preparations made in suspension is not significantly affected by the presence of AF (S73). The silicate anion evidence also makes it appear unlikely that there is significant replacement of silicon by sulphur it is, in any case, improbable that such replacement would occur in a dimeric or chain anion, as a Si-O-S link would be readily hydrolysed. 

Recently, the possibility of Si-O-H... phenyl hydrogen-bonds was discovered in arylsilanols [19]. However, the nature and the size of the organic group attached to silicon can limit the development of such networks and lead to an association of a limited number of units forming dimers, trimers,... 

To prove or disprove route (b) we carried out many chemical studies [13] but I will not enter into details and only give an accoimt of the results. Obviously, the dimer of the standard silaethene is formed neither by condensation route (a) alone nor by silaethene dimerization route (b) alone, but half by elimination way (b) than by insertion way (c) and finally half by condensation way (a ) (Scheme 5). In other words, the product in the middle of Scheme 5 is not formed by an SN2-reaction - usual in silicon chemistry - via route (a), but by a 8, 1-reaction - uncommon in silicon chemistry - via routes (b, c). In fact, two reactants compete for the silaethene intermediates so formed the silaethene source, that is an organometal compoimd, and the silaethene itself The latter is defeated. In this connection, it should be remembered that metal organyls are used for the preparation of the sources of standard silaethene. Naturally, these compete, too, for the silaethene and may suppress reaction (c). 

A Py-GC study [7] with pyrolysis done at 500° C showed numerous peaks corresponding to the isoprene dimers, trimers. .. up to hexamers eluting in clusters of peaks. The separation was done on a methyl silicone 5% phenyl silicone type column with FID detection. The results from a Py-GC/MS study [8] where natural rubber was pyrolysed at 580° C in a Curie point Py-GC/MS on-line system showed similar results. The TIC trace of the pyrolysate with the separation done on a 60 m Carbowax column, 0.32 mm i.d., 0.25 pm film thickness, with the temperature gradient of the GC oven between 40°... 

---