Hydrolysis-polycondensation reactions

It was assumed that linear polymeric particles are formed in the low water content solutions which show spinnability on the way of progressing hydrolysis-polycondensation reaction. In order to confirm this, the molecular weights and intrinsic viscosities of the solutions listed in Table 1 have been measured (2.). Figure 2 shows the log Mjj versus log[ri] plots. The slope of the plot a is larger than 0.5, that is, 0.75 and 0.64 respectively for solutions 1 and 2 of... 

Among the few examples of hybrid materials using the metal-oxygen core for their construction is the composite obtained from BuSn(OPri)J. The hydrolysis-polycondensation reactions are quite fast for the derivatives of tin (as the increase in the coordination number takes place), but the Sn-C bond is quite stable against the attacks of nucleophilic reagents like water. It makes possible the addition of the organic molecules to the oxopolymers — the tin-... 

The formation of a sol-gel porous material is through a hydrolysis-polycondensation reaction. An example is given in equation 1 with the methoxide of silicon (tetramethyl-orthosilicate, TMOS), but many other alkoxides, aryl oxides and acyl oxides can be used, as well as Si—N and Si—Cl compounds. 

Hydrolysis reactions of alkoxides proceed by nucleophilic attack of the metal [Eqs. (7)], and polymerization occurs via the reactive hydroxoalkoxides [Eqs. (7b) and (7c)]. Hydrolysis-polycondensation reactions are governed by numerous factors (Table 3). The metals involved in the composition of electrooptical ceramics are electropositive and oxophilic, and thus the hydrolysis of their ho-... 

Metal salts, such as nitrates or acetates, can also be added to the solution, increasing the flexibility of the process. The kinetics of the hydrolysis/ polycondensation reactions and the nature of the polymer can be tailored by using catalysts and/or by changing the functionality of the alkoxide species and the H20/M(OR)n ratio [6]. Acid catalysts, e.g., mineral acids, afford chain polymers with little branching. In contrast, basic catalysts, e.g., ammonia, yield highly branched polymer networks and subsequently dense colloidal particles. Further, when working with a solution of several alkoxides, inhomogeneous products can result from different hydrolysis or condensation reaction rates. 

Colloidal sols can be prepared from aluminum chloride (or nitrate) by dissolving AlCb (or AI(N03)3-9H20) and aluminum pellets in water under reflux, followed by filtration [27]. The hydrolysis/polycondensation reaction is continued until the viscosity of the precursor is suitable for dry spinning. The green fibers are dried, prefired to 800°C and sintered at 1300°C in air. Crystallization of the gel occurs above 700°C. First transition aluminas are formed and then converted to a-alumina [27],... 

In Class-II materials components are chemically linked by strong covalent or iono-covalent bonds. The molecules used as starting building blocks possess two distinct functionalities alkoxy groups (R-O-M bonds) and metal-carbon (M-C) links. The alkoxy groups can be formed into an oxo-polymer network by hydrolysis-polycondensation reactions in a sol-gel. Hybrids can be obtained from organically modified silicon alkoxides such as polyfunctional or polymer functionalised alkoxysilanes. The network-forming functionalities can be covalently connected in a sol-gel in several ways ... 

CTA" -sepiolite and the rapid hydrolysis-polycondensation reaction gives rise to a silica network in which sepiolite fibers become entrapped [74],... 

The use of ionic precursors in template-directed hydrolysis-polycondensation reactions clearly indicated a strong contribution of the precursor toward the formation of structured mesophases, based on ionic precursor-surfactant interactions. The work of Che and coworkers beautifully illustrates that templating methods involving surfactant-precursor ion pairs allow accessing highly structured siuface-functionalized silica materials. However, this work exclusively focused on the use of monosilylated costructure-directing agents and the formation of siuface-fimctionalized mesoporous silica phases. 

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