E. A. Tatarinova

Macromolecular nano-objects as a promising direction of polymer chemistry

The main features of the manifestation of polymer characteristics of macromolecular nanoobjects are summarized and analyzed in comparison with classical linear systems. This study is primarily focused on dendrimers that exhibit qualitative changes in their characteristics after passing from lower to higher generations within the same homologous series. The above changes are shown to be typical for other representatives of a similar class of polymers.

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 .

Carbosilane dendrimers with diundecylsilyl, diundecylsiloxane, and tetrasiloxane terminal groups: Synthesis and properties

Three derivatives of poly(allylcarbosilane) dendrimers of the fifth generation with different terminal groups are synthesized. The influence of terminal groups on the properties of the dendrimers in bulk and solution is investigated by viscometry, precision adiabatic vacuum and differential scanning calorimetry, dynamic light scattering, and atomic force microscopy. It is shown that the surface layers of the dendrimers substantially affect their properties and behavior. The existence of the second relaxation transition and its dependence on the nature and structure of terminal groups are established. The experimental data indirectly confirm the assumed formation of intermolecular entanglement networks for higher generation dendrimers.

Complementarity of small-angle neutron and X-ray scattering methods for the quantitative structural and dynamical specification of dendritic macromolecules

The structural characteristics of polycarbosilane dendrimers with different molecular architecture were determined in solutions by small angle neutron and X-ray scattering. The same linear dimensions were sized up for the dendrimers both in benzene and chloroform. A solvent molecules penetration inside dendrimer structure in amount up to 30 vol.-% was found from the comparison of the partial and effective scattering volume for the dendrimers in solution.

Thermodynamic properties of the first to fifth generations of carbosilane dendrimers with allyl terminal groups

Temperature dependences of the specific heats, characteristic temperatures, and enthalpies of physical transformations of the first to fifth generations of carbosilane dendrimers with allyl terminal groups were studied using an adiabatic vacuum calorimeter in the temperature range 6—340 K. The error of measurements was, as a rule, about 0.2%. Thermodynamic characteristics of physical transformations of the dendrimers were determined and their thermodynamic functions Cp°(T), H°(T)—H°(0), S°(T)—S°(0), and G°(T)—H°(0) were calculated for the temperature range 0—340 K. The thermodynamic functions of the dendrimers are linearly related to their molecular weights, the number of allyl groups on their outer spheres, and the number of moles of diallylmethylsilane per mole of the dendrimers formed. Additive dependence of the properties of the dendrimers on their chemical composition and structure indicates that the energy of interaction between structural fragments of the dendrimers is independent of the dendrimer generation number. The fractal dimensions, D, of all dendrimers studied in this work are 1.2—1.3 in the temperature range 30—50 K, thus indicating a chain-layered structure of the dendrimer glasses.

Synthesis and properties of fluorinated derivatives of carbosilane dendrimers of high generations

Fluorinated derivatives containing different amounts of fluorocarbon groups in the surface layer of the molecular structure have been synthesized by the chemical modification of a polyallylcarbosilane dendrimer of the sixth generation via hydrosilylation and heterofunctional condensation procedures. The fluorocontaining dendrimers are well soluble in organic solvents and supercritical CO2. Their properties have been studied by DSC, viscometry, dynamic light scattering, and atomic force microscopy.

Rheology of carbosilane dendrimers with various types of end groups

The rheological properties of high-generation carbosilane dendrimers carrying different kinds of terminal groups are studied. It is shown that the nonlinear viscoelastic behavior of dendrimers and the high-temperature relaxation transition in dendrimers are interrelated and result from the reversible breakdown of the supramolecular structure formed by the system of contacts of exterior shells of dendrimers. The strength of the supramolecular structure is dependent on the specific interaction of terminal groups of dendrimers and their mobility. The dendrimers under study demonstrate the dualism of macromolecule-particle properties: They behave as both polymer melts and colloidal systems.

Thermodynamic Properties of Carbosilane Dendrimers of the Sixth Generation with Ethylene Oxide Terminal Groups.

The temperature dependences of heat capacities of carbosilane dendrimers of the sixth generation with ethyleneoxide terminal groups, denoted as G6[(OCH2CH2)1OCH3]256 and G6[(OCH2CH2)3OCH3]256, were measured in the temperature range from T = (6 to 520) K by precision adiabatic calorimetry and differential scanning calorimetry (DSC). In the above temperature range the physical transformations, such as glass transition and high-temperature relaxation transition, were detected. The standard thermodynamic characteristics of the revealed transformations were determined and analyzed. The standard thermodynamic functions, namely, heat capacity Cp°(T), enthalpy H°(T) - H°(0), entropy S°(T) - S°(0), and Gibbs energy G°(T) - H°(0) for the range from T → 0 to 520 K, and the standard entropies of formation ΔfS° of the investigated dendrimers in the devitrified state at T = 298.15 K, were calculated per corresponding moles of the notional structural units. The standard thermodynamic properties of dendrimers under study were discussed and compared with literature data for carbosilane dendrimers with different functional terminal groups.

Self-diffusion and nuclear magnetic relaxation of dendritic macromolecules in solutions

The self-diffusion and nuclear magnetic relaxation of poly(butylcarbosilane) and poly(allylcarbosilane) dendrimers dissolved in deuterated chloroform and poly(amidoamine) dendrimers with hydroxyl surface groups in solutions with methanol have been studied. The diffusion rates(D) have been measured by the pulsed-field-gradient nuclear magnetic resonance. It is shown that experimental concentration dependencesD(ϕ) obtained for macromolecules in the dendrimer-solvent systems studied can be reduced to a unified view, and thus, the generalized concentration dependence of the normalized diffusion rates of dendrimers can be obtained. In the macromolecular volume concentration range from 0.01 up to 0.55, the generalized dependence of the normalized diffusion rates for dendrimers coincides with the analogous dependence for globular proteins in aqueous solutions; the last result suggests that self-diffusion features of dendrimers and globular proteins are in general similar. It is also shown that the experimental data obtained permit one to characterize the changes of the own monomer density of dendrimers depending on their molecular weight and, as a consequence, to make a conclusion about the swelling of dendritic macromolecules in the solutions studied.

Synthesis and properties of carbosilane dendrimers of the third and sixth generations with the ethylene oxide surface layer in bulk and in monolayers at the air-water interface

A number of carbosilane dendrimers with the ethylene oxide surface layer was synthesized. The density of the surface layer determines their capability to form a physical network due to intermolecular entanglements. The specific interactions of the ethylene oxide fragments exert a minor effect on the thermal behavior of dendritic macromolecules. The compression-expansion isotherms of Langmuir films together with Brewster angle microscopy data show that an increase in the core rigidity with increasing the generation number favors the formation of ordered molecular multilayers. The appearance of a pronounced hysteresis in the compression-expansion cycles is a common phenomenon for amphiphilic dendrimers of high generations.