Cyclic Oligosiloxanes Research Articles

Ring-opening polymerization of cyclic oligosiloxanes without producing cyclic oligomers.

A long-standing problem associated with silicone synthesis is contamination of the polymer products with 10 to 15% cyclic oligosiloxanes that results from backbiting reactions at the polymer chain ends. This process, in competition with chain propagation through ring-opening polymerization (ROP) of cyclic monomers, was thought to be unavoidable and routinely leads to a thermodynamically controlled reaction mixture (polymer/cyclic oligosiloxanes = 85/15). Here, we report that simple alcohol coordination to the anionic chain ends prevents the backbiting process and that a well-designed phosphonium cation acts as a self-quenching system in response to loss of coordinating alcohols to stop the reaction before the backbiting process begins. The combination of both effects allows a thermodynamically controlled ROP of the eight-membered siloxane ring D4 without producing undesirable cyclic oligosiloxanes.

Ionic Liquids Containing Silsesquioxane and Cyclic Siloxane Frameworks.

This paper describes the author's recent work on the preparation and properties of thermally stable ionic liquids (ILs) containing siloxane frameworks. Quaternary ammonium and imidazolium salt-type ILs containing random oligosilsesquioxane frameworks were successfully prepared via the hydrolytic condensation of the corresponding organotrialkoxysilanes by using an aqueous superacid bis(trifluoromethanesulfonyl)imide (HNTf2 ) solution as a catalyst and solvent. Imidazolium salt-type ILs containing polyhedral oligomeric silsesquioxane (POSS) frameworks were also prepared through a reaction similar to that described above by using a water/methanol mixed solution of HNTf2 . In addition, amorphous POSSs with two types of ionic groups randomly distributed in the side chain were prepared. These POSSs were ILs exhibiting fluidity at relatively low temperatures. Furthermore, imidazolium and ammonium salt-type ILs containing cyclic oligosiloxane frameworks were prepared through a reaction similar to that of the corresponding organodialkoxysilanes. The thermal decomposition temperatures of the above ILs containing siloxane frameworks were higher than those of general ILs.

Synthesis and Properties of Regioregular Polythiophene Bearing Cyclic Siloxane Moiety at the Side Chain and the Formation of Polysiloxane Gel by Acid Treatment of the Thin Film

Preparation of 2-halo-3-substituted-thiophene bearing a cyclic oligosiloxane moiety in the side chain was carried out and subjected to nickel(II)-catalyzed deprotonative polymerization to give the ...

One‐Pot Sequence‐Controlled Synthesis of Oligosiloxanes

AbstractSilicones (organopolysiloxanes) have found applications in a wide range of research areas, and their unique and valuable properties have rendered these materials virtually irreplaceable. Despite the fact that silicones have been employed industrially for more than 70 years, synthetic routes to generate silicones remain limited, and the sequence‐controlled synthesis of oligo‐ and polysiloxanes still represents a major challenge in silicone chemistry. Described here is a highly selective sequence‐controlled synthesis of linear, branched, and cyclic oligosiloxanes by simple iteration of two reactions, specifically, a B(C6F5)3‐catalyzed dehydrocarbonative cross‐coupling of alkoxysilanes with hydrosilanes and a B(C6F5)3‐catalyzed hydrosilylation of carbonyl compounds, in a single flask. The sequence of the resulting oligosiloxanes can be controlled precisely by the order of addition of the hydrosilane monomers.

One-Pot Sequence-Controlled Synthesis of Oligosiloxanes.

Silicones (organopolysiloxanes) have found applications in a wide range of research areas, and their unique and valuable properties have rendered these materials virtually irreplaceable. Despite the fact that silicones have been employed industrially for more than 70 years, synthetic routes to generate silicones remain limited, and the sequence-controlled synthesis of oligo- and polysiloxanes still represents a major challenge in silicone chemistry. Described here is a highly selective sequence-controlled synthesis of linear, branched, and cyclic oligosiloxanes by simple iteration of two reactions, specifically, a B(C6 F5 )3 -catalyzed dehydrocarbonative cross-coupling of alkoxysilanes with hydrosilanes and a B(C6 F5 )3 -catalyzed hydrosilylation of carbonyl compounds, in a single flask. The sequence of the resulting oligosiloxanes can be controlled precisely by the order of addition of the hydrosilane monomers.

Tris(pentafluorophenyl)borane‐Catalyzed Reactions of Siloxanes: A Combined Experimental and Computational Study

The reaction of 1,1,3,3‐tetramethyldisiloxane with 1‐octene as a model reaction of silicone curing catalyzed by B(C6F5)3 resulted in the redistribution of the disiloxane into dimethylsilane and cyclic oligosiloxanes, and the subsequent hydrosilylation reaction of dimethylsilane afforded dimethyldioctylsilane. To obtain insights into the reaction mechanism and possibility alter the reaction pathway to favor the hydrosilylation over the redistribution, mechanistic analysis of the reaction between a hydrosiloxane (1,1,3,3‐tetramethyldisiloxane, silox‐H) and a vinylsiloxane (1,1,3,3‐tetramethyl‐1,3‐divinyldisiloxane, silox‐vin) in the presence of B(C6F5)3 was performed through density functional theory calculations. The results of the calculations indicate that the activation of a Si–H bond in silox‐H by B(C6F5)3 initiates the reaction to form the B(C6F5)3–silox‐H complex with a Lewis acidic silicon atom and a hydridic hydrogen atom. The B(C6F5)3–silox‐H complex can undergo two different reaction pathways, that is, trisiloxane formation and the hydrosilylation of silox‐vin by silox‐H. The trisiloxane formation involves trisilyloxonium ions as intermediates and can lead to either the homotrisiloxane of silox‐H or a mixed trisiloxane of silox‐H and silox‐vin. The energetics of the reaction pathways predict the preference of trisiloxane formation over hydrosilylation, and the fine tuning of the steric and electronic natures of the substrates could alter the thermodynamic and kinetic favorability.

Preparation of protic ionic liquids containing cyclic oligosiloxane frameworks

Protic ionic liquids containing cyclic oligosiloxane frameworks, which were prepared by the hydrolytic condensation method, exhibited relatively high thermal stabilities.

Optically isotropic liquid crystal media formulated by doping star-shaped cyclic oligosiloxane liquid crystal surfactants in twin nematic liquid crystals.

The formation of optically isotropic liquid crystal (LC) media has been investigated by doping the star-shaped LC molecular surfactants (SiLC) into the rod-shaped twin LC host molecules (DiLC). The experimental phase diagram was constructed on the basis of differential scanning calorimetry (DSC) and then a theoretical calculation was conducted through a combined Flory-Huggins (FH)/Maier-Saupe-McMillan (MSM)/phase field (PF) model to account for the experimental results. The phase diagram of the SiLC/DiLC mixtures revealed the broad coexistence regions such as smectic A + crystal (SmA1 + Cr2), liquid + crystal (L1 + Cr2), and liquid + nematic (L1 + N2) at the intermediate composition along with the narrow single phase crystal (Cr2), smectic (SmA1), and nematic (N2) regions. The morphologies and structures of these coexistence regions were further confirmed by polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). At the 80/20 SiLC/DiLC composition, the optical anisotropy was induced under an alternating current (AC) electric field above its isotropization temperature. The formation of an optically isotropic LC medium in mixtures of the SiLC molecular surfactants and nematic LC host may allow us to develop new electro-optical devices.

Hierarchical superstructures from a star-shaped molecule consisting of a cyclic oligosiloxane with cyanobiphenyl moieties.

Unconventional star-shaped liquid crystals (abbreviated as SiLCs) were successfully synthesized by chemically connecting four cyanobiphenyl anisotropic mesogens to the periphery of a super-hydrophobic and ultra-flexible cyclic tetramethyltetrasiloxane ring with flexible hexyl chains. Based on the combined experimental techniques of differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM), solid-state carbon-13 ((13)C) nuclear magnetic resonance (NMR) spectroscopy and one-dimensional (1D) wide-angle X-ray diffraction (WAXD), it was found that the SiLC molecule exhibited the monotropic phase transition from a LC phase to a crystalline phase. The crystalline phase was only detected during slow heating processes above its glass transition temperature, while a LC phase was formed both during cooling and during heating processes. The hierarchical superstructures were identified from the structure-sensitive 2D WAXD of the macroscopically oriented SiLC film and confirmed by selected area electron diffraction (SAED) of the SiLC single crystals. The molecular packing symmetry in the monoclinic unit cell was further investigated by computer simulations on the real and reciprocal spaces. Macroscopically oriented SiLC hierarchical superstructures on the different length scales may provide the targeted physical properties, which can allow us to apply SiLC molecules in the fields of electro-optical devices and nonlinear optics.

Accumulation of Silicon in Cacti Native to the United States: Characterization of Silica Bodies and Cyclic Oligosiloxanes in Stenocereus thurberi, Opuntia littoralis, Opuntia ficus-indica, and Opuntia stricta

Four different cactus species growing in the United States, Stenocereus thurberi growing in Organ Pipe Cactus National Monument, Arizona, Opuntia littoralis and Opuntia ficus-indica, growing on Santa Catalina Island, California, and Opuntia stricta, growing in northern Alabama, were examined for the presence of silica bodies (opaline phytoliths). Silica bodies were found in all four of these cactus species, parallelepiped-shaped crystals in S. thurberi, and starburst-shaped crystalline structures in the three Opuntia species. In addition, the essential oils of the four cactus species were obtained by hydrodistillation and analyzed by gas chromatography - mass spectrometry. To our surprise, S. thurberi, O. littoralis, and O. ficus-indica (but not O. stricta) essential oils contained cyclic oligosiloxanes. To our knowledge, cyclic oligosiloxanes have not been previously found as essential oil components.

Optimization of the conditions of acid hydrolysis of tetraethoxysilane and identification of primary products of sol-gel synthesis

The effect of the reactant ratio in the Si(OEt)4-H2O-EtOH-HCl system on the hydrolytic polycondensation of tetraethoxysilane and gelation rate was examined. A mass-spectrometric study showed that the primary products of the sol-gel process are linear and cyclic oligosiloxanes containing hydroxy and ethoxy groups at the silicon atoms.

Silicone as an organosilicon reagent for the palladium-catalyzed cross-coupling reaction

Silicone, poly(diorganosiloxane), serves as an organosilicon reagent for palladium-catalyzed cross-coupling reactions. Treatment of poly[(aryl)methylsiloxane], poly[(alkenyl)methylsiloxane], or cyclic oligosiloxanes with various aryl iodides in the presence of silver(I) oxide (Ag 2O) or tetrabutylammonium fluoride (TBAF) and a catalytic amount of palladium affords the corresponding cross-coupling product in a good to excellent yield. The reaction of silicone with aryl chlorides in the presence of K 2CO 3/H 2O as an activator proceeded to afford biaryl derivatives in moderate to excellent yields. A wide range of aryl chlorides bearing an electron-donating or electron-withdrawing substituent on the aromatic ring are tolerated.

The influence of shape and size of silyl units on the properties of bent-core liquid crystals—from dimers via oligomers and dendrimers to polymers

A series of oligomeric and dendritic liquid crystalline materials combining mesogenic bent-core units and silicon-containing linking units (siloxanes and carbosilanes) was synthesized and investigated. The influence of the size and shape of the silyl units upon the mesophase type and the switching behaviour was investigated. Linear oligosiloxane based dimers and dendritic carbosilanes show ferroelectric and non-classical antiferroelectric switching monolayer smectic phases with dark conglomerate textures coexisting with achiral domains, whereas cyclic oligosiloxane and silsesquioxane derivatives have antiferroelectric switching and birefringent smectic phases with double layer structure. It is concluded that ferroelectricity and layer distortion require a certain size of the silyl groups and a monolayer structure where each layer is decoupled by sublayers formed by the silylated units.

Synthesis and Properties of Liquid-Crystalline Cyclic Siloxanes Containing Azo Dye Groups

The effects of incorporating an azo dye moiety as a nonlinear optically active chromophore with benzoyloxyphenyl-substituted mesogens to a cyclic siloxane backbone are reported. The liquid-crystalline (LC) cyclic oligosiloxanes were characterized by 1H NMR, FT-IR, and UV-Vis spectra. The mesogenic properties of the oligomers were studied by differential thermal analysis, polarizing microscopy, and X-ray diffraction. The results show that all the cyclic oligosiloxanes exhibit the smectic phase except for the cyclic oligosiloxane with azo dye–only substituents, for which the LC state was not evident. The LC cyclic oligosiloxanes, when irradiated at 365 nm in chloroform solution, undergo a trans/cis isomerization of the azo dye moiety.

Liquid crystalline cyclic oligosiloxanes with polar end groups

The series of cyclic oligosiloxanes having different attached mesogenic rigid cores and the polar end groups have been reported. The novel oligomers exhibit smectic A and E phases. The effects of the various central linking units and end terminal groups on mesomorphic properties are discussed. Mesophases were characterised by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction.

Synthesis and characterization of new ferroelectric liquid‐crystalline cyclic oligosiloxanes containing methyleneoxymethylene spacers

AbstractThe synthesis of side‐chain liquid‐crystalline oligocyclosiloxanes containing methyleneoxymethylene spacers, two different mesogenic cores (including biphenyl phenyl carboxylate or biphenyl fluorophenyl carboxylate groups) and chiral tails is presented. Differential scanning calorimetry, optical polarizing microscopy and X‐ray diffraction measurements reveal liquid‐crystalline properties for all of the synthesized monomers and oligomers. All cyclic oligosiloxanes exhibit enantiotropic smectic A and chiral smectic C phases. The mesogenic properties of the cyclic oligomers and side‐chain liquid‐crystalline precursor are discussed. Copyright © 2005 Society of Chemical Industry

Thermal Behaviour and Mesophase Structure of Cyclic Oligosiloxanes With Chiral Mesogenic Groups

Chiral, smectogenic pendant groups have been covalently attached to cyclic oligosiloxanes by hydrosilylation. Differential scanning calorimetry, optical polarising microscopy, and X-ray diffraction measurements reveal liquid crystalline properties for all synthesized monomers and oligomers. The monomers, which differ by the nature of the rigid mesogenic core or end groups, showed enantiotropic smectic A phases. Almost all of cyclic oligosiloxanes present enantiotropic chiral smectic C phases. The mesomorphic range of these oligomers is greatly enhanced relative to the type of monomers and the ring size as well.

Hydrolysis of Chloro(silyl)olefins in Neutral and Acidic Media

Action of water on chloro[chloro(alkoxy)silyl]olefins with various number of chlorines in the unsaturated moiety was studied in acidic and neutral media. Linear and cyclic oligosiloxanes with unsaturated chlorine-containing groups were obtained. Correlation between the hydrolytic stability of the Si-Csp2 bond and the structure of the starting chlorosilylolefin was explored.

Syntheses and properties of sila-functional oligosiloxanes: Synthesis, structure, and spectra of linear and cyclic isocyanato(methyl)oligosiloxanes

Synthesis, structure, and spectra of novel linear and cyclic isocyanato(methyl)oligosiloxanes with the silicon number up to eight were investigated. The linear and cyclic oligosiloxanes were synthesized selectively in good or appreciable yields by vapor phase and liquid phase hydrolysis, respectively. They were isolated as liquids by distillation under reduced pressure except for cyclic tetrasiloxane and pentasiloxane which were provided as solids. The cyclic trisiloxane was obtained as amixture of isomers as confirmed by nuclear magnetic resonance (NMR) spectra. The X-ray analysis of cyclic tetra- and pentasiloxane obtainedrevealed that they have cis-trans-cis andpropeller-like structure, respectively. The infrared (IR) spectra of cyclic siloxanes showed a sharp peak due to νSi−O−Si and a remarkable shift fromthe tri- to the tetrasiloxane while peak broadening and almost no shift were observed for the higher homologues. On the other hand, the sharp peak for linear disiloxanes became broad and split into two peaks for the higher homologues, while the peak at low wavenumber shifted even lower as the siloxane linkages increased, yet no corresponding shift is observed for thepeak at higher wavenumber. In the 29Si NMR spectra of linearoligosiloxanes, the signals appeared separately at low and high fields in narrow regions, which are ascribed to the terminal silicon atom and the silicon associatedwith the silylmethyl groups, respectively. These regions hardly changed, regardless of the length of siloxane linkage. Similar trends were observed in the 1H and 13C NMR spectra.

Silicone as a New Class of Organosilicon Reagent for the Palladium-Catalyzed Cross-Coupling Reaction

Silicone, polyorganosiloxane, serves as an organosilicon reagent for palladium-catalyzed cross-coupling reactions. Treatment of polymethylphenylsiloxane, poly(alkenyl)methylsiloxane, or cyclic oligosiloxanes with various aryl iodides in the presence of silver(I) oxide or tetrabutylammonium fluoride and a catalytic amount of Pd(0) affords the corresponding coupling product in a good to excellent yield.