Hydrolyzation polymerization, silicon alkoxides

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... 

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...

Research groups have expressed a new interest in the preparation of amorphous oxides by a process known as sol-gel (1-4). The advantages arc compositional homogeneity and the low processing temperature. The process consists of the hydrolysis of a silicon alkoxide, which under appropriate conditions, is polymerized. Normally the alkoxide is dissolved in alcohol and hydrolyzed by the addition of water under acidic, neutral or basic conditions. The final result is SiOj ... 

Sol-gel chemistry is frequently employed in designing random mesoporous structures of silicates (Brinker and Scherer 1990 Corma 1997). Liquid silicon alkoxide precursors (Si(OR) ) are hydrolyzed and condensed to form siloxane bridges, a process that is often described as inorganic polymerization and is represented below ... 

Silica sol is prepared by hydrolysis of a metal alkoxide and subsequent or simultaneous polycondensation (polymerization), which can be either acid or base catalyzed. The acid catalyzed hydrolysis proceeds through an electrophilic attack of the H+ ions. This means that the reactivity decreases as the number of OR groups decreases with the progression of hydrolysis [40-42]. The probability of formation of fully hydrolyzed silicon, Si(OH)4, is thus very small. Since the condensation reaction starts before the silicon alkoxide is completely hydrolyzed, silicon alkoxide molecules will polymerize with non-hydrolyzed alkoxyl groups, in which the degree of crosslinking is low. The gyration radius of these small molecules is typically in the order of 1.5 - 1.7 nm [43]. 

The cosolvent should be compatible with the precursors as well as with the hydrolyzed intermediates and oligomers that are formed during polymerization, otherwise phase separation will occur. This is not always simple because most sol-gel precursors are hydrophilic or become hydrophilic after the initial hydrolysis step (which replaces the hydrophobic silicon alkoxide by a more hydrophilic silanol), whereas many of the desirable polymer dopants are hydrophobic in nature. Hence, successful cosolvation for the first stage of polymerization does not necessarily guarantee that the inorganic phase will not precipitate at an advanced stage of the sol-gel process. 

Currently, microelectronics relies on bulk sihca as an important dielectric material that is often used as an insulating template for further reactivity. On the nanoscale, sUica can be synthesized by polymerizing silicic acid in an aqueous system, or through hydrolysis and condensation of silicon alkoxides in the Stober synthesis [51]. The mechanism of these two methods is unique. The first method is dominated by monomers and tetra-functionalized species, such that the resultant sihcate sols are uniform, which means that they are fully hydrolyzed and grow by monomer addition. In contrast, for the second method, di- and tri-functionalized species are dominant for alkoxides. Regardless of the synthesis used, these particles induce a fractal interior with minimal morphological control due to their common template, ammonium hydroxide [51]. 

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. 

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. 

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