Doklady Chemistry 

About: Doklady Chemistry is an academic journal published by MAIK Nauka/Interperiodica. The journal publishes majorly in the area(s): Catalysis & Crystal structure. It has an ISSN identifier of 0012-5008. Over the lifetime, 1633 publications have been published receiving 7001 citations. The journal is also known as: Chemistry.

Polycondensation of alkoxysilanes in an active medium as a versatile method for the preparation of polyorganosiloxanes

Abstract: Hydrolytic polycondensation of functional derivatives of silicon is an important method for preparation of polysiloxanes of different structure ranging from organocyclosiloxanes to high-molecular-weight linear, cyclolinear, or branched polymers. The most widely used version of hydrolytic polycondensation is based on the use of organochlorosilanes. More than a five decade history of the industrial use of this process resulted in a broad range of organosilicon compounds without which modern technology cannot even be imagined [1]. However, until now, it was impossible to solve two fundamental problems, namely, eliminate the use of chlorosilanes in polysiloxane preparation processes and carry out hydrolytic polycondensation under homogeneous conditions. The significance of the first issue is obvious in view of the need to decrease the environmental pressure. Currently, the use of alkoxy derivatives instead of organochlorosilanes is held up not only by the lack of industrial direct synthesis of organoalkoxysilanes but also by the difficulty to control the polymer production processes from these raw materials. The solution of the latter problem would markedly increase the process controllability, most of all, through control of the product structures. A study of the reaction of organoalkoxysilanes with an excess of anhydrous acetic acid has shown that the process can be accompanied by complete conversion of alkoxysilyl groups within polyfunctional oligomers or their mixtures with alkoxysilanes. Acetic acid was used as the active reaction medium. A fundamental difference between the active medium and common organic solvents is that the former does not merely dissolve the reactants and products but is also a coreactant. Analysis of the literature concerning this pair of reactants has shown that acetic acid either functioned as an active solvent [2‐4] (in this case, water was added to the reaction mixture for hydrolysis) or as a reactant [5] (in this case, complete conversion of alkoxysilyl groups could not be attained). The authors were the first to demonstrate that an excess of anhydrous acetic acid induces the process to follow the hydrolytic polycondensation mechanism, the required water being generated in the reaction system in amounts needed for complete conversion of the alkoxysilyl groups. The key studies were performed for the reaction of acetic acid with dimethyldimethoxysilane. For the convenience of monitoring the reaction, it was carried out in deuterated acetic acid ( CD 3 COOD ). In this case, the variation of the reactant concentrations could be monitored by recording the 1 H NMR spectra of samples taken directly from the reaction mixture without any additional treatment. The signals for different methoxygroup protons (Fig. 1) were assigned on the basis of preliminary experiments including measuring the individual spectra of the major components of the reaction mixture. The absence of acetic acid protons in the spectrum made it possible to calculate the relative concentrations of functional groups in the reactants and products from the intensities I of the proton signals of these groups in the methoxy region (3.3‐3.7 ppm). In view of the fact that the sum of integral intensities in this region ( Σ I ) remains constant during the reaction, the relative concentrations of functional groups ( c rel ) in this spectral region were calculated using the formula c rel = I / Σ I .