E. A. Rebrov

Liquid crystalline carbosilane dendrimers: First generation

Abstract An approach to the synthesis of a new class of liquid crystalline (LC) compounds, dendrimers of regular structure with terminal mesogenic groups, was elaborated. LC dendrimers based on the carbosilane dendritic matrix of first generation were synthesized. Cyanobiphenyl, methoxyphenyl benzoate and cholesteryl groups were used as mesogenic fragments. Individuality and structure of all compounds obtained was proved by GPC together with 1H- and 29Si NMR methods. The mesomorphic behaviour and structure of the LC dendrimers synthesized were investigated. It is argued that different mesophases of the smectic type are realized in all cases. It is shown that the mesophase type of these compounds essentially depends on the chemical nature of the mesogenic groups.

Preparation of multi-arm star polymers with polylithiated carbosilane dendrimers†

Polyfunctional dendrimers are employed for the synthesis of multi-arm star-shaped polymers. Terminal allyl groups of a carbosilane dendrimer have been partly extended by hydrosilylation-addition of didecylmethylsilane and partly lithiated by reaction with butyllithium. Due to the peculiar structure of the dendritic polylithium compounds, the carbanionic sites were screened from intermolecular interaction and did not aggregate. Good solubility and accessibility for different monomers allowed the preparation of new star-branched poly(dimethylsiloxane), polystyrene and poly(ethylene oxide) polymers.

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.

Carbosilane liquid crystalline dendrimers with terminal chiral mesogenic groups: structure and properties

This paper presents a systematic study of two series of carbosilane liquid crystalline (LC) dendrimers from first to fifth generations bearing 8, 16, 32, 64 and 128 terminal chiral mesogenic groups, respectively. All the LC dendrimers synthesized are characterized by the same glass transition temperature around -5°C. It has been shown that the LC dendrimers of the lower generations (G-1-G-3) form a ferroelectric SmC* phase over a very broad temperature range up to about 180°C, while the LC dendrimers of the higher generations (G-4 and G-5) display a rectangular columnar mesophase (Colr). Schemes of packing in the SmC* and Colr mesophases formed by the LC dendrimers are suggested and discussed. Electrical measurements on the ferroelectric LC dendrimers have shown that an increase in generation number leads to a decrease in the value of the spontaneous polarization and an increase in switching time.

Polyhydroxycarbosilanes of dendritic structure

Synthesis of a versatile hydroxysilylation agent and its use in the preparation of hydroxy derivatives of a poly(allylcarbosilane) dendrimer and hyperbranched polymer are reported. In a first approximation, the bulk properties of synthesized hydroxy polymers are found to essentially identical. Infrared spectroscopy provides evidence for the build-up of a complicated H-bond network in these polymers. Spectroscopic study of the hydroxyl groups can gain a deeper insight into the properties of branched dendrite-core macromolecules.

CARBOSILANE DENDRIMER OF SECOND GENERATION WITH TERMINAL METHOXYUNDECYLENATE GROUPS

The thermodynamic properties of carbosilane dendrimer of second generation with terminal methoxyundecylene groups were studied between 6 and 340 K by adiabatic vacuum calorimetry: the temperature dependence of the molar heat capacity Cp0 was measured, the physical transformations were established and their thermodynamic characteristics were obtained. The experimental data were used to calculate the thermodynamic functions Cp0 (T), H0(T)-H0(0), S0(T), G0(T)-H0(0) of the compound in the range 0 to 340 K. from the relation Cp0 (T) the fractal dimension of the dendrimer was experimentally determined. The heat capacity of the dendrimer was compared with the corresponding additive values calculated from the properties of its constituents - a dendritic matrix (carbosilane dendrimer of second generation) and the corresponding amount of moles of methyl ester of 11-(tetramethyldisiloxy)undecanoic acid serving as terminal groups.

Synthesis, structure, and phase behavior of carbosilane LC dendrimers with terminal butoxyphenylbenzoate mesogenic groups

Two series of carbosilane LC dendrimers with terminal protonated and deuterated butoxyphenylbenzoate mesogenic groups linked to carbosilane dendritic matrices of the first to fifth generations via an undecylene spacer have been synthesized. The chemical structure of new dendrimers has been studied by 1H NMR spectroscopy and gel-permeation chromatography. The dendrimers of first-fourth generations are characterized by formation of the smectic C mesophase in a wide temperature range, whereas much more complex columnar supramolecular structures are formed in dendrimers of the fifth generation. Structural studied of mesophases by X-ray diffraction and small-angle neutron scattering show that segregation takes place in mixtures of deuterated and protonated LC dendrimers; as a result, huge aggregates composed of hundreds of chemically unbound molecules develop and the sizes of these aggregates reversibly change with temperature.

Synthesis, phase behaviour and structure of liquid crystalline carbosilane dendrimers with methoxyphenyl benzoate terminal mesogenic groups

Abstract For the first time a series of liquid crystalline dendrimers of generations 1–4 containing 8, 16, 32 and 64 terminal methoxyphenyl benzoate mesogenic groups respectively have been synthesised. Mesogenic groups were attached to the carbosilane dendritic matrices with terminal allyl groups through -OOC-(CH2)10-Si(CH3)2OSi(CH3)2- spacer by a hydrosilylation reaction. Structure and purity of all new compounds were confirmed by 1H-NMR spectros-copy and GPC analysis. Phase behaviour of LC dendrimers was investigated by means of polarising optical microscopy, DSC and X-ray methods. All LC dendrimers synthesised display mesophases of smectic types as well as crystalline phases over wide temperature region. Possible structure and arrangement of molecules of LC dendrimers in these mesophases are discussed.

PROBLEMS OF SYNTHESIS OF TETRAHEDRAL ORGANOSILSESQUIOXANES. HYDROLYTIC CONDENSATION OF HEXAFUNCTIONAL BRANCHED ORGANOTETRASILOXANES

The hydrolytic condensation of methyltris(methyldichlorosiloxy)silane (1b), methyl(1a), and vinyltris(methyldiethoxysiloxy)silanes (1c) in dilute homogeneous solutions was studied. It was found by X-ray diffraction and1H NMR methods that, irrespective of reaction conditions, only octaorganooctasilsesquioxanes (T8), namely, octamethyloctasilsesquioxane and 1,4-divinylhexamethyloctasilsesquioxane, were obtained instead of the expected tetraorganotetrasilsesquioxanes (T4). These data suggest that the condensation processes involved in the hydrolysis of1a–c are predominantly intermolecular, and the molecules of the starting oligomers do not undergo rearrangements. Probably, the formation ofT4, whose molecules are built of four strained six-membered organosiloxane cycles, is less favorable thermodynamically than the formation of their homologs (T8) built of eightmembered organosiloxane cycles.

Divergent synthesis of segmented carbosilane dendrimers

A number of carbosilane copolymers of the first through third generations has been synthesized via the divergent approach. Each molecule of these compounds contains two pairs of dendrons—segments that have the same poly(propylenesilyl) carbosilane dendritic architecture but differ in the generation number and the amount and type of terminal groups (n-decyl or allyl). The target dendrimers have been isolated as individual compounds via preparative gel-permeation chromatography, and their structure has been studied by 1H NMR spectroscopy.