N. V. Demchenko

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

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 .

Inorganic core/organic shell hybrid nanoparticles: Synthesis and characterization

Core/shell molecular particles, promising components of polymer nanocomposites, were synthesized by condensation of tetraethoxysilane (TEOS) in acetic acid at elevated temperature with subsequent blocking of reactive SiOH groups by trimethylsiloxy groups. Particle diameters ranged from 1 to 10 nm depending on the condensation time until the blocking agent was added. The solubilities of the nanocomposites in THF, toluene, hexane, and other organic solvents made it possible to characterize them by the following physicochemical methods: gel permeation chromatography (GPC), viscometry, dynamic light scattering (DLS), and monolayer compression at the water-air interface. As a result, the particles were found to have compact spherical shapes. The physical state of the particles varied from viscous flow to crystallike depending on the sizes and hardness of their silica cores.

Synthesis and Characterization of Hybrid Core–Shell Systems Based on Molecular Silicasols

New hybrid “rigid inorganic core–soft organic shell” systems based on molecular silicasols were synthesized by applying different synthetic schemes. Inorganic core was composed of molecular silicasols, which were synthesized from hyperbranched polyethoxysiloxane and tetraethoxysilane by polymer chemistry methods. Different organic modifiers were used to form soft shell of the hybrid particles. Obtained compounds were characterized by elemental analysis, GPC, IR and NMR spectroscopy. These systems will be designated for use as model objects for investigation of nanoparticles–polymer matrix interactions in polymer nanocomposites.

Synthesis of fluorine-containing organosilicon copolymers and their use for the preparation of stable hydrophobic coatings based on the epoxy binder

A series of novel fluorine-containing organosilicon copolymers was synthesized in an excess of acetic acid. The efficiency of their use as hydrophobic agents for epoxy film-forming materials was shown.

Synthesis of diethoxy(phenyl)silane and its polycondensation in acetic acid

Low-temperature reaction between phenylmagnesium chloride and triethoxysilane at lowered affords diethoxy(phenyl)silane, whose polycondensation in acetic acid gives oligomeric (phenyl)hydrosiloxanes.

Synthesis and Characterization of Organo-Inorganic Nanoobjects Based on Hyperbranched Polyethoxysiloxanes

The synthetic scheme for obtaining modified molecular silicasols based on hyperbranched polyethoxysiloxane (HPEOS) is presented in this work. The synthetic scheme includes several stages; the first and most important one represents polycyclization and simultaneous modification of HPEOS with either organosilanols or disiloxanes. The following stages are polymer-analogous reactions affecting only the external layer. The obtained nanoparticles are nanogels with specific morphology of inorganic silica core soluble in anhydrous organic solvents. All the hybrid systems were characterized by means of elemental analysis, GPC, IR and NMR spectroscopy. Some physical and chemical parameters were determined for all samples after fractionation. Synthesized molecular silicasols are ready-made universal models for investigations of interactions between macromolecules of polymer matrix and nanoparticles, and evaluation of various factors, such as the chemical nature of the external layer or particle core size, which have a great influence upon properties of nanocomposites.

Polycondensation of Diethoxydimethylsilane in Active Medium

Polycondensation of diethoxydimethylsilane (DEDMS) in an active medium containing an excess of acetic acid was studied. It has been shown that the process selectivity could be well managed only if water was generated in the reaction mixture. We have found that both linear oligomers and cyclosiloxanes could be obtained with high selectivity and 80 % yield at least under conditions of the active medium. Further condensation of the linear oligomers led to the formation of α,ω-dihydroxypolydimethylsiloxanes with the molecular weight ranging from 3500 to 70000 Da. The obtained polydimethylsiloxane samples having hydroxyl end groups correspond to the industrial samples of liquid siloxane rubbers in terms of molecular weight parameters and virtually do not contain any low molecular cyclosiloxane impurities.

Synthesis and thermal and rheological properties of carboxyl-containing polydimethylsiloxanes

A number of polydimethylsiloxanes modified with fragments of undecenoic acid and its esters were synthesized. The structures of polymers were confirmed by 1H and 29Si NMR spectroscopy. The rheological properties of obtained polymers were studied, as well as their thermal properties were studied by DSC.

Synthesis of dimethylcyclosiloxanes in the active medium

The process of condensation of dimethyldiethoxysilane in the active medium in a presence of acetyl chloride, trifluoroacetic acid, and sulfocationites has been investigated. Their impact on the rate and selectivity of the process has been estimated. The prospects of the application of sulfocationites for the polycondensation of dimethyldiethoxysilane in anhydrous acetic acid with an 99% yield have been demonstrated.

Synthesis and thermal properties of polydimethylsiloxanes modified by decyl and methylundecylenate substituents

A series of polydimethylsiloxanes modified by long-chain hydrocarbon substituents, including those containing terminal ester groups, was synthesized. The structures of the copolymers were confirmed by the data of 1H and 29Si NMR spectroscopy. The thermal properties of the copolymers were studied by differential scanning calorimetry. The change in the behavior of the copolymers is observed at the content of modifying units is >5 mol.% and the fluidity of the polymers is retained at the content of these units <15 mol.%.