E. V. Matukhina

Phenylsilanetriol — synthesis, stability, and reactivity

Abstract Crystalline phenylsilanetriol, PhSi(OH)3 (1) was obtained by smooth hydrolysis of phenyltrimethoxysilane and characterised by different analytical methods (wide angle X-ray powder diffraction, NMR, IR, thermogravimetric analysis). The title-compound is surprisingly stable in the solid state but slowly condenses in acetone solution. Reaction of 1 with Me3SiCl and Me2(CH2Cl)SiCl gave the corresponding tris-triorganylsiloxy substituted derivatives in high yields.

New mesomorphic organocyclosiloxanes II. Thermal behaviour and mesophase structure of organocyclohexasiloxanes

Investigations of thermotropic phase transitions performed on organocyclosiloxanes [PhSi(O)OSiR]6, where R is Me3, Me2(CH2Cl) or Me2(CH≃CH2), have revealed that all these hexamers are mesomorphic compounds. The hexamers exhibit uncommon polymesomorphic behaviour forming two quite different mesomorphic structures. The molecular arrangement in the low temperature (LT) modification is characterized by two-dimensional (2D) long-range order with hexagonal packing. The X-ray diffraction pattern and peculiarities of molecular packing in the crystal lead us to suggest that the LT-mesophase is columnar, presumably of the Colhd type. The LT-mesophase is formed by dimeric moieties, which associate with each other in column-like substructures, the ring planes not orthogonal to the stack axis. The high temperature (HT) mesophase is a plastic crystal (3D-order), where molecules take up positions in a face-centred cubic lattice. This is a very uncommon example of thermal behaviour for plastic crystals that provides a unique opportunity to bridge the gap between plastic crystalline and liquid crystalline mesomorphic behaviour. The thermal and structural properties of the mesophases depend upon the type of side groups of the hexamers. The size of the ring also affects the phase behaviour and the mesomorphic structure. This conclusion is consistent with data obtained by us earlier for cyclotetrasiloxanes.

Synthesis and mesomorphic properties of cis-penta[(phenyl)(trimethylsiloxy)]cyclopentasiloxane

New stereoregular cis-penta[(phenyl)(trimethylsiloxy)]cyclopentasiloxane cis-[PhSi(O)(OSiMe3)]5 was synthesized. According to the data from DSC, X-ray diffraction, and polarization microscopy, the noncrystallizable cyclopentasiloxane exists in the mesomorphic state throughout the temperature range below the temperature of destruction and is transformed into mesomorphic glass below the glass transition temperature. This compound possesses the polymesomorphic properties and forms two mesomorphic modifications. The type of mesomorphic ordering for these modifications was determined.

New stereoregular liquid-crystalline phenylcyclosiloxanes

Representatives of stereoregular phenylcyclotetra(hexa)siloxanes with mesogenic cyanobiphenylyl groups cis-oriented relatively to the siloxane ring have been synthesized for the first time. The ability of the compounds to transit to the liquid-crystalline state was confirmed by the thermooptical, X-ray diffraction, and calorimetric methods. The temperatures and enthalpies of the phase transitions were determined, and their reversibility was shown. The bilayered molecular packing with antiparallel arrangement of molecules is the most probable for the smectic A phase of the cyclosiloxanes studied. Taking into account differences in the optical textures and interlayer distances, we cannot exclude completely the possibility of formation of liquid-crystalline phases of other types.

Dimesogenic liquid crystalline organosiloxanes

Liquid crystalline (LC) organosiloxanes with two terminal cyanobiphenylyl groups attached to a linear or cyclic siloxane center through an aliphatic spacer (CH2)n with i = 10 were synthesized. The ability of compounds to pass into the LC state was confirmed by thermooptical, X-ray diffraction, and calorimetric measurements. The temperatures and the enthalpies of phase transitions were determined. The types of LC structures and the capability of one compound for polymesomorphism to form the chiral SmC* phase without a chiral center in the mesogenic group were established. The temperatures and the enthalpies of the reversible phase transitions, crystal ⇌ SmC ⇌ SmA ⇌ melt and crystal ⇌ SmA ⇌ melt, for linear and cyclic LC organosiloxanes, respectively, were determined. Models of molecular packing in the SmA and SmC* phases were proposed based on X-ray diffraction data. A specific feature of the SmA phases of new LC organosiloxanes is a negative gradient of the temperature dependence of the interlayer spacing.

STRUCTURAL CHARACTERIZATION OF THERMOTROPIC LIQUID CRYSTALLINE DIHYDROXYDICARBOSILANE MESOPHASE

Molecular mechanics (MM) calculations of the structure formed by bis(hydroxydimethylsilyl)methane [(CH3)2(HO)Si]2CH2(MDCS) molecules have been performed. The calculations lead us to conclude that MDCS can form a columnar liquid crystalline phase which consists of supramolecular H-bonded associates similar to those found earlier for siloxane compounds of the same type. Synthesis of MDCS was performed. IR, NMR, DSC and XRD (powder and single crystal) investigations have shown the existence of a LC phase in a narrow temperature range. The computational and experimental results suggest the existence of similar columnar LC phases in a wider temperature range for Et-, Pr-, and Bu-substituted bis(hydroxydialkylsilyl)methane.

Effect of the method used for crystal structure formation on the rheological behavior of organocyclotetrasiloxane in the plastically crystalline state

The rheological properties of cis-cyclotetrasiloxane, cis-[PhSi(O)(OSiMe3)]4, in the plastically crystalline state were investigated. The yield stress and non-Newtonian character of the flow indicates that cis-[PhSi(O)(OSiMe3)]4 is a viscoplastic material with respect to its rheological behavior. The conditions of crystal structure formation determine the rheological properties of organocyclotetrasiloxane in the mesophase. The temperature and stress during capillary flow were shown to affect the size and orientation of crystallites formed upon cooling of extrudates.

Modeling of structures of poly(oxyhexaorganocyclotetrasiloxy-2,6-diyl) mesophases

The idealized liquid-crystal structures of two cyclolinear organosilicon polymers, viz., poly(oxyhexaorganocyclotetrasiloxy-2,6-diyls), where the alkyl substituent is Et (1b) or Pr (1c), were modeled. The columns of monomer units in polymers have different conformations, viz., a distorted long boat in 1b and a distorted long chair in 1c. Both polymers have columnar structures with the most probable antiparallel arrangement of the columns.