Hydrolysis and condensation of alkyl

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

Tospearl is a spherical silicone resin particle made by the controlled hydrolysis and condensation of alkyl trialkoxysilanes (equation 1) and produced by Toshiba Silicones, a joint venture of GE Silicones. The first report of these materials appeared in 1985 when the aqueous amine catalyzed condensation of methyl trimethoxysilane was described (1). Since the initial disclosure, a number of other patents have issued in this area (2-8). The product is a tluee-dimensional network that is intermediate between inorganic and organic particles. This paper describes the physical and chemical properties of this network as well as its utility in a number of applications. 

This is a very diverse group of polymeric materials. Particles have mostly spherical form. Spherical silicone resin (commercial name - Tospearl) is produced in several grades, most of which are spherical (some are elongated and irregular). Controlled hydrolysis and condensation of alkyl trialkoxysilane permits tight control of properties." Particles of some grades are monodisperse, containing only a small admixture of particles of other sizes than attempted in s mthesis. 

The sol-gel process is based on the hydrolysis and condensation of molecular precursors [11]. The most versatile precursors are the metal organic compounds, metal alkoxides, M(OR) . The transition metal is represented by M, n is the valence of the metal, and R is an alkyl group such as methyl, ethyl, propyl, etc. 

As in the case of inorganic silica systems [Eqs. (8)-(ll)], both hydrolysis and condensation of alkoxides may be acid- or base-catalyzed [9,24,37]. The relative contributions of the four reactions—hydrolysis, olation, oxolation, and alcoxolation—determine the physical and chemical characteristics of the resulting oxide material. These contributions are in turn determined by the nature of the metal alkoxide (i.e., nature of the metal atom, nature of the alkyl groups) and the characteristics of the chosen experimental conditions (i.e., the water/alkoxide molar ratio, the nature and concentration of the catalyst, the nature of the solvent, and temperature). A major factor is the hydrolysis ratio, h, defined as the water/alkoxide molar ratio ... 

Methylsuccinic acid has been prepared by the pyrolysis of tartaric acid from 1,2-dibromopropane or allyl halides by the action of potassium cyanide followed by hydrolysis by reduction of itaconic, citraconic, and mesaconic acids by hydrolysis of ketovalerolactonecarboxylic acid by decarboxylation of 1,1,2-propane tricarboxylic acid by oxidation of /3-methylcyclo-hexanone by fusion of gamboge with alkali by hydrog. nation and condensation of sodium lactate over nickel oxide from acetoacetic ester by successive alkylation with a methyl halide and a monohaloacetic ester by hydrolysis of oi-methyl-o -oxalosuccinic ester or a-methyl-a -acetosuccinic ester by action of hot, concentrated potassium hydroxide upon methyl-succinaldehyde dioxime from the ammonium salt of a-methyl-butyric acid by oxidation with. hydrogen peroxide from /9-methyllevulinic acid by oxidation with dilute nitric acid or hypobromite from /J-methyladipic acid and from the decomposition products of glyceric acid and pyruvic acid. The method described above is a modification of that of Higginbotham and Lapworth. ... 

Much interesting work has been done in the last ten years on the bridging of pyrrole and piperidine rings. Early in their work on this subject Clemo and Metcalfe (1937) prepared quinuclidine (V) by the reduction of 3-ketoquinuclidine (IV), the latter resulting from the hydrolysis and decarboxylation of the product (III) of a Dieckmann internal alkylation, applied to ethyl piperidine-l-acetate-4-carboxylate (II), itself made by condensing ethyl piperidine-4-carboxylate (I) with ethyl chloroacetate. 

In a tetralkoxysilane/alkyltrialkoxysilane system, the hydrolysis and condensation reactions proceed not only between similar precursor molecules but also between the two different precursor species. Figure 8.3 illustrates a simplified scheme of hydrolysis and condensafion reactions in a hybrid system containing TMOS and an alkyltrimethoxysilane. Notice the difference from the scheme in Figure 8.1 the alkyl group, represented by R, does not participate in the hydrolysis and condensation reactions that lead to the silica monolith. The sol-gel process kinetics in a hybrid sol-gel... 

FIGURE 8.3 Simplified hydrolysis and condensation reactions of TMOS with alkyl-triaUcoxysilane during sol-gel processing. 

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

Hydrolysis and Condensation. The rate of hydrolysis of the tetraalkyl titanates is governed by the nature of the alkoxy groups. The lower titanium alkoxides, with the exception of tetramethyl titanate [992-92-7], are rapidly hydrolyzed by moist air or water, giving a series of condensed titanoxanes, (Ti— O—Ti— O—) (17). As the chain length of the alkyl group increases, the rate of hydrolysis decreases. Titanium methoxides, aryloxides, and C-10 and higher alkyl titanates are hydrolyzed much more slowly. 

The relative rates of reaction for the hydrolysis and condensation dictate the structure and properties of an alkoxide gel. These reaction rates are schematically described in Figure 8.18 [43] for the example of a silicon ethoxide. In acidic solutions, hydrolysis is achieved by a bimolecular displacement mechanism that substitutes a hydronium ion (H" ) for an alkyl [44]. Under these conditions the hydroljreis is rapid compared to the condensation of the hydrolyzed monomers and promotes the development of larger and more linear molecules, as is described in Figure 8.19. Under basic conditions, hydrolysis occurs by nucleophilic substitution of hydrojgrl ions (OH ) for alkyl groups [45]. Here the condensation is rapid relative to hydrolysis, promoting the precipitation of three-dimensional colloidal particles as shown in Figure 8.17(b) and 8.19. 

Relatively few studies on the synthesis of mesoporous alumina have been reported to date [8]. One of the limitations of the reported synthetic strategies is that the rate of hydrolysis (and condensation) reaction of aluminum alkoxide are much faster than that of silicon alkoxide. In this study, we proposed a novel method to prepare bimodal porous aluminas with meso- and macropores with narrow pore size distribution and well-defined pore channels. The fiamewoik of the porous alumina is prepared via a chemical templating method using alkyl caiboxylates. Here, self-assemblied micelles of carboxylic acid were used as a chemical template. Mesoporous aluminas were prepared through carefiil control of the reactants pH, while the procedures are reported elsewhere [9]. 

A detailed account of the hydrolysis and condensation reactions of alkyl orthosilicates (554) and alkoxides of other metals (555) was presented by Brinker and Scherer (546). 

Hydrolysis of alkyl silicates produces silicic acids with varying degrees of condensation and ultimately silicon dioxide. The reaction time increases with increasing size of the alkyl group. Elevated temperature and high moisture content accelerate hydrolysis and condensation. 

On application, the paint layer hardens within a short time due to evaporation of the solvent, progressive hydrolysis of ester groups after uptake of atmospheric moisture, reaction of silanol groups with the zinc and substrate, and condensation of silanol groups to form sparingly soluble, cross-linked silica gel [2.186]. Conditions are generally chosen such that the film is hard enough to allow transportation after 4 h and a topcoat after 24 h. Alkyl silicate zinc dust paints are particularly suitable... 

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