Siloxane Backbone Research Articles

Syntheses of poly(siloxane)‐supported zirconocene catalysts and application to olefin polymerizations

Poly(siloxane)s with bisindenyl, bisfluorenyl, bis(1,2,3,4-tetramethylcyclopentadienyl), bis(2,4,7-trimethylindenyl) and monoindenylmethyl side groups were synthesized by condensation of the corresponding dichlorosilanes and water. For reference, diphenylsilanediol or hydroquinone was also employed in place of water. A series of poly(siloxane)-supported zirconocene catalysts were then prepared from these precursors and applied to ethene and propene polymerizations as well as to the copolymerization of ethene with 1-octene in the presence of methylalumoxane. The polymerization activity of the new supported metallocenes depends drastically upon the substituents in the siloxane backbone. The zirconocene catalysts supported on poly(bisindenylsiloxane) and poly(bisfluorenylsiloxane) give the highest activities for ethene and propene polymerizations, respectively. The weight-average molecular weights of the polymers are also markedly dependent upon the substituents. On the other hand, the molecular mass distributions (MMD) are generally not so sharp, suggesting that the active species formed in these supported catalysts are not uniform. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 421–428, 1998

Syntheses of poly(siloxane)-supported zirconocene catalysts and application to olefin polymerizations

Poly(siloxane)s with bisindenyl, bisfluorenyl, bis(1,2,3,4-tetramethylcyclo-pentadienyl), bis(2,4,7-trimethylindenyl) and monoindenylmethyl side groups were synthesized by condensation of the corresponding dichlorosilanes and water. For reference, diphenylsilanediol or hydroquinone was also employed in place of water. A series of poly(siloxane)-supported zirconocene catalysts were then prepared from these precursors and applied to ethene and propene polymerizations as well as to the copolymerization of ethene with 1-octene in the presence of methylalumoxane. The polymerization activity of the new supported metallocenes depends drastically upon the substituents in the siloxane backbone. The zirconocene catalysts supported on poly bisindenylsiloxane) and poly(bisfluorenylsiloxane) give the highest activities for ethene and propene polymerizations, respectively. The weight-average molecular weights of the polymers are also markedly dependent upon the substituents. On the other hand, the molecular mass distributions ( MMD ) are generally not so sharp, suggesting that the active species formed in these supported catalysts are not uniform.

Synthesis of aldonamide siloxanes by hydrosilylation

The synthesis of hydrophobic/hydrophilic hybrid polymers based on polydimethylsiloxanes (PDMS) and carbohydrate derivatives involving a hydrosilylation step is reported. Starting with poly(dimethyl- co-hydromethyl)siloxanes (PDMS- co-HMS), O-acetylated N-allylaldonamides of various sugars can be attached to the siloxane backbone using platinum catalysts. The characteristics of different transition metal complexes in the polymer-analogous formation of SiC bonds are investigated. A generalized reaction pathway applicable to several reducing carbohydrates is established to synthesize aldonamide siloxanes in a broad variety of molecular weight and composition. The products obtained, “sweet siloxanes”, are characterized by g.p.c., FTi.r. spectrometry and 13C n.m.r. spectroscopy.

Ion scattering and recoiling from liquid surfaces

We have shown that ion beams can be used to probe the surface composition and molecular orientation of liquid surfaces. Time-of-flight analysis of the kinetic energy of scattered inert gas ions and recoil atoms ejected from the surface reveals the identity of atoms in the topmost atomic surface layer of the liquid. In this report we describe the first scattering/recoil experiments on surfaces of a liquid siloxane and glycerol using helium, neon and argon ions in the 2–3 keV energy range. Analysis of peak intensities as a function of experimental parameters can be used to infer average molecular orientations in the surface. Spectra from the liquid siloxane are similar to that reported by Bertrand et al. [J. Phys. Chem. 97, 131 788 (1993)] for the long-chain hydrocarbon hexatriacontane deposited as oriented solid films on Si wafers. Our data indicates that the siloxane backbone is effectively shielded by the attached hydrocarbon groups, and for incoming ions the molecule effectively resembles a hydrocarbon. Recent sum-frequency generation spectroscopy experiments [J. Phys. Chem. B 101, 4607 (1997)] suggest that a glycerol molecule in the liquid is, on average, oriented with the carbon backbone normal to the surface. Our data is consistent with this result, but scattering/recoiling simulations are needed to definitively determine the orientation.

Supramolecular association of acid terminated polydimethylsiloxanes

Benzoic acid terminated polydimethylsiloxanes (PDMS) have been synthesized by hydrosilylation of allyloxybenzylbenzoate with SiH terminated precursors followed by deprotection. These oligomers have been characterized by 1H, 13C and 29Si NMR, by FTIR and by SEC in tetrahydrofurane. Because of the flexibility and low polarity of the siloxane backbone, they are ideally suited to study the effect of hydrogen bonding on macromolecular properties.

Silicon-Modified Carbohydrate Surfactants V: The Wetting Behaviour of Low-Molecular-Weight Siloxane, Carbosilane, Silane and Polysilane Precursors on Low-Energy Surfaces

The surface tensions, wetting tensions, contact angles and solid/liquid interfacial tensions of defined siloxanes as well as those of analogous carbosilanes, polysilanes and neopentyl substituted silanes were determined. The wetting experiments were carried out on a glass plate coated with perfluoroalkyl methacrylate (FC 722®). The siloxanes possess the lowest surface tensions. Due to the presence of oxygen atoms in the siloxane backbone, a donor–acceptor portion (γ+/−lv) of the surface tension of about 1–2 mN/m was determined. The solid/liquid interfacial tension also contains a donor–acceptor portion (γ+/−sl). Its value is almost identical to that of γ+/−lv. The γ+/−sl differences between individual molecules of the same surface tension are responsible for contact angle differences of up to 4°. © 1997 John Wiley & Sons, Ltd.

Synthesis of Novel Multifunctional Siloxane Oligomers Using Sol−Gel Techniques and Their Photoinitiated Cationic Polymerization

Employing modified sol−gel techniques, trialkoxysilanes bearing either epoxy or 1-propenyl ether functional groups can be condensed under well controlled conditions to yield a series of soluble multifunctional siloxane oligomers. The key to the success of this synthetic approach was the use of weakly acidic ion-exchange resins as catalysts. The novel multifunctional oligomers displayed exceptionally high reactivity in photoinitiated cationic polymerization using diaryliodonium salts as photoinitiators. Polymerization involved not only addition polymerization of the pendant epoxide or 1-propenyl ether groups but also acid-catalyzed hydrolysis and condensation of residual alkoxy groups along the siloxane backbone.

In situ examination of water diffusion to the polypropylene-silane interface using FTIR-ATR

This study investigated the effect of moisture on a model silane coupling agent modified adhesive bond. Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy was used to characterize the transport of moisture to a polypropylene-silane interphase and monitor the resulting chemical changes. The FTIR-ATR method offers the advantage of in-situ examination of the diffusion process, as well as the ability to characterize chemical changes that occur due to the presence of moisture. Experiments were conducted at ambient and elevated temperatures. The results of the real-time measurements demonstrated that moisture will migrate through the polypropylene to the silane interphase. The diffusion behavior was described well by a Fickian model. The apparent diffusion coefficients for water in the polypropylene-silane bilayer were on the order of the diffusion coefficients for water in polypropylene at both test temperatures. Furthermore, changes in the spectra were observed during the diffusion experiments. These changes were indicative of hydrolysis of the siloxane backbone in the silane layer while buried beneath the polypropylene film. This finding is significant as it presents direct evidence of a degradation mechanism in silane-modified adhesive bonds. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1971–1985, 1997

Optimization of pyroelectricity in the smectic-C* phase of a liquid-crystalline terpolymer

The dynamic pyroelectric response in the smectic-C* (SmC*) phase of a novel class of polysiloxane terpolymers is reported. In these terpolymers methyl groups and two distinct types of mesogenic side chains are attached to the siloxane backbone. Our investigations reveal that the magnitude and temperature dependence of the pyroelectric signal, as well as the temperature range of the SmC* phase, can be controlled by varying the relative side-chain composition. This permits optimization of the pyroelectric properties over a temperature range extending to subambient. Specifically, the pyroelectric coefficient p can be made very nearly temperature independent, and the ratio p/ε′ extremely high, making these materials suitable for uncooled infrared detectors.

Relationship between Alkyl Chain Length and Adsorption Energy ofN-Alkyl-N,N-dialkyl-3-(siloxanyl)propylammonium Bromides at the Mercury/Electrolyte Interface

The influence of the variation in alkyl chain length at the nitrogen atom of ammonium group-containing siloxane surfactants on the adsorption energy and lateral interaction energy at the mercury/electrolyte interface has been investigated by means of the concentration dependence of the differential capacity. The results show that for various siloxane backbones, successive addition of methylene groups to the alkyl leads to a corresponding successive increase in the adsorption energy.

Molecular dynamics simulations of siloxane-based side chain liquid crystalline polymers

Abstract Molecular dynamics simulations have been performed to study the intramolecular interactions of biphenyl ([B]) and cholesterol ([C]) substituents on linear oligomeric siloxanes. These interactions are of interest due to the liquid crystalline behaviour of these materials. Molecules with alternating diblock and triblock substituent arrangements were examined, as were unmixed substituents. The orientational behaviour of the mesogens and the stability of [B]-[B], [C]-[C], and [B]-[C] interactions were characterized by an order parameter for the mesogens as a function of simulation time. Results indicate that the [C]-[C] interaction is the strongest, with the [B]-[B] and [B]-[C] interactions being weaker. However, the arrangement of [B] and [C] mesogens in the different molecules strongly influenced the level of mesogen ordering. The flexibilities of the siloxane backbone and the spacer groups used to attach the mesogens give the mesogens a wide range of motion, allowing them to find energetically favourable arrangements and moderate to high order parameters.

Dielectric and electro-optical properties of ferroelectric side-chain liquid-crystalline polysiloxanes containing 4′-(2-chloro-3-methylpentanoyloxy)-4-alkanyloxybiphenyl side groups

Abstract A series of ferroelectric liquid-crystalline polymers (FLCPs) with siloxane backbone were synthesized in this study. The dynamics of these polymers was studied by broad-band dielectric spectroscopy (10 −1 to 10 7 Hz). The collective (Goldstone and soft mode) and molecular (α, β, γ, δ relaxation) relaxation processes were observed. These FLCPs exhibited large values of spontaneous polarization (larger than 190 nC cm −2 ), fast switching time (less than 1 ms) and reversible bistable effects in the chiral smectic C (SmC∗) phase.

29Si MAS NMR investigation of the pyrolysis process of cross-linked polysiloxanes prepared from polymethylhydrosiloxane

Polymethylhydrosiloxanes (PMHSs) crosslinked with hexa-l,5-diene or hexane-l,6-diol have been considered as potential precursors for Si–C–O systems. Their pyrolysis under an argon flow has been investigated mainly by 29Si MAS NMR spectroscopy. The two systems, with similar C/Si but different O/Si ratios, show different pyrolytic behaviour. This is essentially due to different graftings between the siloxane backbone and the alkyl chains, Si–C vs. Si–O–C bonds. In both cases, degradation of the organic chains occurred at 600 °C; but in the case of PMHS cross-linked with hexanediol (DO6), all the C groups are lost, while for PMHS cross-linked with hexadiene (DE6), the C groups bonded to Si remain. As a consequence, the oxycarbide phase obtained at 1000 °C is much richer in C for DE6, compared to DO6, but the amount of free C is also higher. These differences in composition strongly influence the nature of the samples obtained at 1500 °C: crystalline silicon carbide for DE6 and mixture of amorphous silica and silicon carbide for DO6.

Chemically bonded phenylsilicone stationary phases for the liquid chromatographic separation of polycyclic aromatic hydrocarbons and cyclosiloxanes

Chemically bonded phenylsilicone stationary phases, prepared from polymethylphenylsiloxane (PMPS) bonded silica gel, have been developed for use in high-performance liquid chromatography (HPLC) to separate compounds containing aryl groups through π-π interaction. Silanol-terminated PMPS was synthesized through living anionic polymerization of methylphenylcyclotrisiloxane. The PMPS bonded phases, with different degrees of polymerization (DP), were prepared by bonding silanol-terminated PMPS directly onto silica gel. The stationary phases thus prepared were characterized by elemental analysis, gel permeation chromatography, 1R, 1H NMR and 29Si NMR. Evaluation of their chromatographic behavior in relation to small PAHs (polycyclic aromatic hydrocarbons) and various cyclosiloxanes showed them to have reasonable separation ability in both normal-phase and reversed-phase. Copolymerization reactions of trifluoropropylmethylcyclotrisilioxane (F 3) and hexaphenylcyclotrisiloxane (P 3) were followed on these stationary phases, and their performance in the retention of phenyl-containing siloxane copolymers with different compositions was also evaluated. Comparison of these PMPS phases with a commercial ODS (C 18) phase in analyses of PAHs and cyclosiloxanes indicates clearly the significant contribution made by the phenyl moiety on the siloxane backbone to the π-π interaction in the HPLC separation process.

Dynamic Mechanical Relaxation Behavior of Low Molecular Weight Side-Chain Cyclic Liquid Crystalline Compounds near the Glass Transition Temperature

In this paper, we report the mechanical relaxation behavior of low molecular weight glass-forming side-chain liquid crystalline compounds with cyclic cores and their linear polymeric analogues near the glass transition temperature. We examined two systems : one based on cyclic and linear siloxane backbones and one based on cyclic (cyclohexane) and linear aliphatic backbones. Dynamic mechanical spectroscopy is used to measure the dynamic shear moduli and the complex viscosity near but above T g . The temperature dependence of the zero-shear viscosity of the cyclic compounds is well described by the Vogel-Tammann-Fulcher (VTF) equation. The strong temperature dependence of the viscosity along with the values of the fitted parameters of the VTF equation shows that the cyclic LC compounds are fragile liquids. All cyclic LC compounds, regardless of chemical structure, show identical relaxation behavior when viscosity is plotted versus normalized temperature (T g /T), where T g is the temperature obtained calorimetrically at a heating rate of 10 K/min. All cyclic compounds show lower viscosity than their linear analogues when plotted versus T g /T. The difference in viscosity between the cyclic and linear siloxane compounds is much more pronounced than the difference observed in the aliphatic compounds. For the cyclic compounds, master curves of G' and G can be described by a single Maxwell mode. The linear compounds exhibit much broader mechanical spectra, suggesting a more complex relaxation phenomenon is taking place. Our results show that, while there is little difference in relaxation behavior among low molecular weight cyclic liquid crystalline compounds, the behavior of the linear polymeric systems is quite different.

Comparative investigations of linear and cyclic siloxanes with a swallow-tailed liquid crystal as side chain

Abstract Molecular ‘zip-fasteners’ consisting of short linear and cyclic siloxane backbones and the same swallow-tailed liquid crystal unit as side group were synthesized. The closing and opening mechanism was studied by dielectric measurements. Thereby an abrupt decrease of the static dielectric constant e0∥ was observed indicating the antiparallel order of the side groups in the ‘closed’ state. The dielectric relaxation frequencies of the δ- and α-relaxation for the respective oligomers do not depend on the degree of polymerization which could be the result of the high flexibility of the main chain and the relatively low mobility of the liquid crystalline unit.

Variation of selectivity among the poly(siloxane) stationary phases for gas chromatography

AbstractThe solvation parameter model is used to characterize the solvent properties of 12 poly(siloxane) stationary phases of varied structure. Selectivity differences are explained in terms of differences in the cohesive energy of the solvents and their capacity for dispersion, dipole‐type, hydrogen bonding, and electron pair complexation interactions. None of the poly(siloxane) stationary phases exhibited significant hydrogen‐bond acid character. Principal component analysis indicates that replacing a methyl group by a phenyl group in the poly(siloxane) backbone predominantly produces incremental increases in the capacity of the stationary phase for dipole‐type interactions (up to about 50 mol% phenyl groups with only small changes at higher phenyl composition). The poly(methylphenylsiloxanes), however, only span a small sector of the selectivity space and provide limited opportunities for selectivity optimization. OV—275 and QF‐1 are identified as important phases with unique characteristics—OV‐275 because of its high cohesive energy and great capacity for dipole‐type and solvent hydrogen‐bond base interactions and QF‐1 because of its unusually small hydrogen‐bond basicity for a phase with an intermediate capacity for dipole‐type interactions as well as a low capacity for n‐ and π‐electron complexation interactions. From these studies it is possible to speculate on the needs for new phases to explore the full selectivity potential of gas chromatography and to address the issue of temperature and its relationship to selectivity. © 1995 John Wiley & Sons, Inc.

Siloxane Coupling Agents

New siloxane coupling agents have been prepared by the platinum-catalyzed addition of CH 2 =CHSi(OR) 3 silanes (R=CH 3 or CH 2 CH 2 OCH 3 ) to four different siloxanes, poly[(methylhydrogen)siloxane-co-dimethylsiloxane], poly[(methylhydrogen)siloxane] (dp= 33), tetramethyldisiloxane, and tetramethylcyclotetrasiloxane. These new hydrophobic materials offer a distinct alternative to conventional silane coupling agents, prevalent in many industrial applications. The new preparations allow for control of molecular weight, the extent of coupling functionality, and the distribution of coupling groups on the siloxane backbone. The use of 2D NMR experiments, COSY, and HETCOR indicated two modes of addition to the Si-H groups had occurred, Markovnikov and anti-rnarkovnikov. GPC, FTIR, and 1 H and 13 C NMR were used to thoroughly characterize all the reaction products

Structural characterization of the smectic polymorphism of chiral liquid crystalline polysiloxanes containing biphenyl mesogens

Abstract A structural characterization of chiral side chain siloxanes with different average degrees of polymerization, DP n, was performed by X-ray diffraction experiments on powder and oriented fibre specimens. Polymers (DP n = 35) and oligomers (DP n = 4) contained the 4,4′-biphenylene unit with either an (S)-2-methylbutoxy (An, Bn) or an (S)-2-chloro-3-methylbutanoyloxy substituent (Cll). The spacer segment connected to the siloxane backbone had a variable number, n, of methylene groups (n = 5, 8, or 11). Independent of the spacer length and the chiral tail nature, the polysiloxanes underwent the same sequence of phases: C-SF1 (or SI1)-SC1-SA1-I, whereas in the oligosiloxanes the sequence C-SB1-SA1-I (B11) or C-SF1-SC1-I (B5) occurred. The influence of the structure of the polysiloxanes on the formation of the smectic (tilted or orthogonal) mesophases was elucidated. The rather large number of reflections (three or four) detected in the X-ray patterns at low angles, allowed a drawing of the projection...

Photochromic liquid crystalline cyclic siloxanes containing spiropyran groups

The effects of covalently incorporating a T-shaped photochromic spiropyran molecule with cholesterol- and biphenyl-substituted mesogens to a cyclic siloxane backbone on the liquid crystalline behavior of the resulting macromolecular siloxanes are reported. The siloxanes containing up to SOfa spiropyran are liquid crystalline, whereas at higher compositions the products are amorphous. With increasing spiropyran content, the selective relection band characteristic of the cholesteric mesophase shifted to lower wavelengths. The photochromic reaction leading to the blue merocyanine formation resulted in a narrowing of the reflection bandwidth