Hydrolyzation polymerization, silicon

Silcones are important products of silicon. They may be prepared by hydrolyzing a silicon organic chloride, such as dimethyl silicon chloride. Hydrolysis and condensation of various substituted chlorosilanes can be used to produce a very great number of polymeric products, or silicones, ranging from liquids to hard, glasslike solids with many useful properties. 

A similar interpretation may be based upon the results of Schwarz and coworkers580 581>. These authors hydrolyzed higher silicon chlorides and found solid white polymeric compounds in which the original Si-Si framework is retained. An irregular network containing Si-O-Si bond systems is also formed, so that the earlier postulated structural formulas are certainly incorrect. 

Boron differs from aluminum in showing almost no metallic properties and its resemblance to silicon is greater. Both boron and silicon form volatile, very reactive hydrides the hydride of aluminum is a polymeric solid. The halides (except BF3) hydrolyze to form boric acid and silicic acid. The oxygen chemistry of the borutes and silicates also has certain resemblances. 

It follows that in this method of cocondensation one may employ even an unsubstituted silicon tetrahalide or orthoester in the mixture as a source of tetrafunctional groups. The unsubstituted SiX4 (where X is a halogen or ester group) hydrolyzes to the hypothetical ortho-silicic acid, Si(OH)4, and this cocondenses with the equally hypothetical organosilanediol to incorporate the silica unit within the polymeric structure ... 

Silicon alkoxides are rapidly hydrolyzed by water, eventually to hydrous silica, but polymeric hydroxo alkoxo intermediates occur. Organo alkoxides such as the silyl ester (MeO)3SiMe and its derivatives are widely known.72... 

The siloxides of aluminum can be prepared in the same maimer as the alkoxides and exhibit similar stractural features. Interest in these compounds arose from a desire for sol-gel or polymeric precursors for aluminosilicates. Unfortunately, these compounds extensively hydrolyze, losing most of the silicon as the silanol. However, the investigation of the hydrolysis of (Et3SiO)3Al has generated considerable insight into the stracture and composition of aUcoxy- and siloxy-substitutedalumoxanepolymers, (ROAIO) These macromolecules are now believed to have a structure related to that of boehmite. 

Figure 4 Polymer-silica hybrid materials hydrogen bonding between silanol groups of hydrolyzed silicon alkoxides and carboxylate groups of preformed poly(A, A-dimethylacrylamide) (a) or in situ polymerized A, A-dimethylacrylamide/methylene bisacrylamide precursors (b) favors the formation of homogeneous organic-inorganic networks...

These observations are easily explained by another simple reaction mechanism, nucleophilic substitution of an alkoxide on silicon (12). In this case, the basic alkoxide oxygens tend to repel the nucleophile, OH, and the bulkier alkyl groups tend to crowd it. Therefore, more highly hydrolyzed silicons are more prone to attack. Because this mechanism would have a pentacoordinated silicon atom in the activated complex, hydrolysis of a polymer would be more sterically hindered than hydrolysis of a monomer. Reesterification would be much more difficult in alkaline solution than in acidic solution, because silanols are more acidic than the hydroxyl protons of alcohols and would be deprotonated and negatively charged at a pH lower than the point at which the nucleophile concentration becomes significant (ii). Thus, although hydrolysis in alkaline solution is slow, it still tends to be complete and irreversible, if extensive polymerization does not occur first. 

This indirect measurement of the d ree of hydrolysis of the silicon species during the emulsion polymerization is in close agreement with the conclusions from the Si NMR spectra. Both indicate that there is a significant fiaction of alkoxy groups that are not hydrolyzed, and both support the conclusion that the vinyl silane hydrolyzed to a much greater extent during the course of the reaction. 

Summary Several linear and cyclic silanes (four-, five- and six-membered rings) with silicon-halogen or silicon-triflate functions were prepared and hydrolyzed to polymeric structures similar to Wohler siloxene and Kautsky siloxene. Optical investigations on the fluorescence of these polymers were carried out. The color and the fluorescence of the polymers are influenced by the ring size and the kind of substituents. Depending on the starting material the fluorescence maxima range from 400 to 550 nm. 

---