Silicon-Carbon Chains

Germanium-chain polymers (Carbon, silicon)-chain polymers... 

Finally, in 1985, the results of an extensive investigation in which adsorjDtion took place onto an aluminium oxide layer fonned on a film of aluminium deposited in vacuo onto a silicon wafer was published by Allara and Nuzzo 1127, 1281. Various carboxylic acids were dissolved in high-purity hexadecane and allowed to adsorb from this solution onto the prepared aluminium oxide surface. It was found that for chains with more than 12 carbon atoms, chains are nearly in a vertical orientation and are tightly packed. For shorter chains, however, no stable monolayers were found. The kinetic processes involved in layer fonnation can take up to several days. 

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

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. 

Polysiloxanes, or silicones as they are commonly called, are polymers of silicon, not carbon. Their chains are made up of alternating silicon and oxygen bonds and are characteristically very flexible (have very low glass transition temperatures—see Chapter 7). As a result, silicones typically find use as... 

Based on kinetic and stereochemical considerations, Muller revised this mechanistic proposal in favor of a two-step associative mechanism ". The monomer would be added to the a-carbon of the pentacoordinated siliconate chain (49) followed by migration of the sUyl group to the carbonyl of the monomer (equation 48). It is then essential that the exchange of the catalyst between chain-ends is fast compared to chain propagation. 

These rubbers are based on atoms of silicon chains rather than carbon atoms. Their unique structure is responsible for their extreme temperature properties. The most common types of silicone rubbers are specfically polysilaxanes. The Si-O-Si bonds can rotate much more freely than the C-C bond or the C-O bond. So the silicone chain is much more flexible and less affected by temperature. Silicone rubber is vulcanised by the action of peroxides which crosslink the chains by abstracting hydrogen atoms from the methyl side groups, allowing the resulting free radicals to couple into a crosslink. Some varieties of polysiloxanes contain some vinyl methyl siloxane units, which permit sulfur vulcanisation at the double bonds. 

But to form branched rings and chains, an element has to be able to form more than one or two bonds. Hydrogen, fluorine, and chlorine cannot form more than one bond, and so if they bond to another atom they are done. Oxygen and sulfur can form only two bonds they can make chains and rings such as —0—0—0—0— or —S—S—S—S—, but that is all. So, from our original list of a quarter of the elements found in nature, we are left with only four candidates for life nitrogen, carbon, silicon, and phosphorus. 

We are left with only three candidates for the element of life carbon, silicon, and phosphorus. All of our remaining candidates form bonds to themselves, and all prefer to form single bonds so that no bonds are left over for rings and branched chains. 

The other common elements suitable as chain caps are fluorine and chlorine, but they are worse than hydrogen. Carbon, silicon, and phosphorus form extronely strong bonds to fluorine— so strong that silicon-silicon and phosphorus-phosphorus bonds cannot compete. And, except for carbon, bonds to chlorine cannot compete with bonds to oxygai. 

Apart from the carbon chain polymers discussed above, the silicon chain polymers have also been investigated extensively in terms of microstructure. The stereochemistry of polysilanes has been studied using Si-NMR spectroscopy [54,55]. Wolff et al. [56] concluded that for a poly (phenylmethyl silane), the ratio of mm rr mr(rm) to be 3 3 4 and that the spectra of poly(1,2,2-trimethyl -1-phenyldisi-lane) are consistent with approximately equal amounts of head-to-head and head-to-tail sequences and an atactic configuration. 

Over the range 0-200°C the temperature coefficient of viscosity of silicone fluids is only about one tenth of that of mineral oils. In contrast the isoelectronic polymers (Me SiCH ) have normal viscosity characteristics. This means that silicones can be used over very much wider ranges of temperature. Some can still be poured well below — 50°C. Polysiloxane chains are very flexible as noted above for hexamethyldisiloxane, the bond angles in the chains are readily deformed. Moreover there are two mutually perpendicular 2p-3d)n systems which together have approximately cylindrical symmetry about the Si—0 bonds. This means that there is little resistance to torsional motion within the molecule. There is also essentially free rotation of methyl groups about the carbon—silicon bonds. (Barriers to rotation (kjmol" ) About Me—Si, 6.7 Me—C, 15.1 About Si—0, 0.8 C—0.11.3.)... 

A number of systems have been developed for the functionalization of C-H bonds by their conversion into carbon-boron or carbon-silicon bonds. The substrates can be alkenes (Scheme 3.35), arenes (Schemes 3.36-3.38), heteroarenes (Schemes 3.39-3.41)" ° and even alkanes (Scheme 3.42)." With arenes, the electronic properties of the aromatic ring do not always seem to control the regioselectivity, as mixtures of isomers are often formed. Methyl-substituted aromatics may be borylated on the side chain depending on the choice of catalyst." ... 

Nametldn NS, Zav yalov VI, Zelenaya AV et al. (1997) Synthesis of polymers having silicon-carbon main chain and F-atom in pendant groups. Patent SSSR 269485 Bull of inventions 21... 

Desilylbromination. -Silyl ketones are desilylbrominatedto a,/)-unsaturated ketones with CuBr2 in DMF. This occurs spontaneously in cyclic ketones, while with open-chain ketones sodium bicarbonate is required to eliminate HBr from the -bromo ketone thus formed. The carbon-silicon bond in organopentafluorosU-icates prepared from alkenes and alkynes is cleaved with cop-per(II) bromide to give the corresponding alkyl and alkenyl bromides (eq 19). The reaction is stereoselective thus ( )-alkenyl bromides are obtained from ( )-alkenylsilicates. 

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