Telechelic polydimethylsiloxanes with terminal acetylenic groups prepared by phase-transfer catalysis

Telechelic, hydroxyalkyl terminated polydimethylsiloxanes were obtained under mild conditions via hydrosilylation of allyl alcohol or ethylene glycol monoallyl ether by oligomeric αω-hydride terminated polydimethylsiloxanes, with the use of a platinum catalyst. They were subsequently modified in a phase transfer catalyzed reaction by propargyl bromide, to give telechelic polysiloxanes terminated by acetylenic groups. The synthetic pathway is described and the characterization of the obtained products is presented.     

Platinum-group metal cyclodextrin complexes and their use as command-cure catalysts in silicones

The Command-Cure concept is defined for a curable formulation as one with long work-like at ambient temperature and rapid cure time at elevated temperature. This concept is explored for a curable silicone system, cured via hydrosilylation. CODMCl₂ complexes (COD=1.5-cyclo-octadiene:M=Pt. Pd) are reacted with beta-cyclodextrin (-CD) to make 11 inclusion compounds,M=Pd.2;M=Pt.4. Compounds2 and4 were analyzed by¹H NMR and X-ray powder diffraction. Their catalytic ability was evaluated in a model system as well as a polymeric system that gels upon cure. Surprisingly, the Pd analog2 was a good command-cure catalyst whereas the guest compound CODPdCl₂,1, was not active in the hydrosilylation reaction. The Pt analog,4, was an effective command-cure catalyst while the corresponding guest. CODPtCl₂,3, was too active at low temperature in the hydrosilylation reaction. Additional Pt compounds and one Rh inclusion compound were evaluated as command cure catalysts. These inclusion compounds were: 11 -CD:[CODRhCl]₂,5: 11 -CD:CpPtMe₃,6 (Cp=cyclopentadienyl): 12 -CD:MeCpPtMe₃,7; 12 -CO:CODPtMe₂,8. The effectiveness of4 8 was evaluated in a number of silicone systems.     

Synthesis of cyano-polycarbosilane and investigation of its pyrolysis process

Polycarbonsilane (PCS) is an important precursor of silicon carbide (SiC) fibers and ceramics. The ceramic yield of PCS is relatively low, about 60 %, which may bring some deficiencies in its applications. In this work, a novel precursor cyano-polycarbosilane (PCSCN) is synthesized by hydrosilylation reaction between PCS and acrylonitrile using a rhodium-containing catalyst, although acrylonitrile is generally not easy for hydrosilylation. After introducing tiny amounts of cyano (-C≡N) groups into the PCS molecules, the ceramic yield of PCSCN can increase largely to over 80 %. The ceramization mechanism of PCSCN is investigated by FTIR, TG, XPS, ESR, NMR, Raman and XRD analyses. It is found that some crosslinking structures in PCSCN are formed between SiH bonds and CN groups from about 200 ℃, which can be responsible for the high ceramic yield. The existence of a little more N, O and free C elements in the pyrolysis products may inhibit the growth of crystalline β-SiC. Moreover, the PCSCN precursor can also be melt-spun into continuous fibers by tailoring its molecular weight and softening point. The oxidized PCSCN fiber with relatively low oxygen content can be pyrolyzed without melting, and the final SiC fiber with an oxygen content as low as 8.5 % is obtained.     

Synthesis and properties of polysiloxanes containing mixed functional groups

An effective synthesis of polysiloxanes containing mixed functional groups was carried out using consecutive hydrosilylation of two different olefins. With the functionalization of polysiloxanes, we sought to obtain derivatives containing fluoroalkyl groups (with hydrophobic properties) and reactive groups capable of attaching a polysiloxane to the surface. Glycidyl and trimethoxysilyl groups were chosen for the study. All of the obtained derivatives were isolated and spectroscopically characterized. In the next stage of the study, glass plates were modified with functionalized polysiloxanes, both alone and with added silica sol, and their hydrophobic properties were determined by measuring contact angles. From the results, we determined the effect of functionalized polysiloxane structure on the hydrophobic properties of polysiloxane-modified surfaces.     

Redox-active silica nanoparticles : Part 2. Photochemical hydrosilylation on a hydride modified silica particle surface for the covalent immobilization of ferrocene

The surface of spherical nonporous monodisperse silica particles (diameter: 200 nm) is covalently modified with hydride by means of hydrosilanization. A direct silicon-carbon link between the hydride modified silica surface and 10-undecylenic acid is obtained by photochemical radical initiated hydrosilylation. A ferrocene unit is attached through an amide link to the carboxylic acid modified surface. Solid-state NMR and DRIFT spectroscopy indicate success of the reactions. The electrochemical oxidation and reduction of the ferrocene modified silica particles are observed after their adsorption on a platinum electrode. The cyclic voltammograms indicate, in addition to the charge transfer between ferrocene units on the particle surface, an interparticle charge transfer.     

Enantioselective Lewis‐Base‐Catalyzed Asymmetric Hydrosilylation of Substituted Benzophenone N‐Aryl Imines: Efficient Synthesis of Chiral (Diarylmethyl)amines

Lewis‐base‐catalyzed asymmetric hydrosilylation of substituted benzophenone N‐aryl imines was investigated. Among various chiral Lewis‐base catalysts, a catalyst derived from L ‐Serine was found to be the most favorable one which promote the reaction to afford a series of (diarylmethyl)amines with high yields (up to 97 %) in moderate to good enantioselectivities (up to 97 % ee). The absolute configuration of the product was determined by the X‐ray crystallographic analysis.

     

Facile synthesis of a carbon-rich SiAlCN precursor and investigation of its structural evolution during the polymer-ceramic conversion process

A liquid carbon-rich SiAlCN precursor is facilely synthetized by hydrosilylation between liquid polyaluminocarbosilane (LPACS) and 1,3,5,7-tetravinyl- 1,3,5,7-tetramethylcyclotetrasilazane {[CH₃(CH₂CH₂)SiNH]₄} (TeVSZ). The structural evolution during the polymer-ceramic conversion process is investigated by various methods. The results show that the main cured mechanism is β-addition on hydrosilylation, although α-addition on hydrosilylation, polymerization of vinyl groups and dehydrocoupling reaction between N–H bonds also occur during the cured process. During the pyrolysis process, dehydrogenation and dehydrocarbonation condensation reactions, transamination reactions occur, leading to formation of a three-dimensional network inorganic structure at 400–800 °C, where part of Al–O bonds convert to Al–N bonds. Then the network inorganic structure undergoes demixing and separation into amorphous SiAlCN(O) phase, where the amorphous turbostratic free carbon phase also form at 800–1200 °C. With demixing and decomposition of the amorphous carbon-rich SiAlCN(O) phase, the crystalline β-SiC and graphitic carbon start to form at about 1400 °C, the crystalline sizes of them both enlarge with increasing temperature. However, the crystal growth of β-SiC is distinctly inhibited due to existence of the rich carbon phase, tiny amounts of Al₂O₃ and AlN. In addition, a small amount of AlN can promote the formation of α-SiC at 1800 °C.     

Pd-catalyzed hydrosilylation polymerization of a dihydrosilane with diyne/triyne mixed systems affording crosslinked silylene-divinylene polymers and their properties

Treatment of a dihydrosilane (methylphenylsilane, 1) with mixtures of a diyne (p- or m-diethynylbenzene, 2a or 2b) and a triyne (1,3,5-triethynylbenzene, 3a or B,B′,B″-triethynyl-N,N′,N″-trimethylborazine, 3b; 1:2:3=100:95:5, 100:90:10, 100:80:20) in the presence of Pd-PCy₃ (Cy=cyclohexyl) catalyst gave new crosslinked silylenedivinylene polycarbosilanes. In TGA the resulting crosslinked polymers tended to show higher Td₅ values and higher char yields than the corresponding linear polymers. On the other hand, UV/vis absorption spectra of the crosslinked polymers obtained in the reactions of 2a or 2b with 3a exhibited increased broad peaks around 390 nm for 2a or 360 nm for 2b. Coincidently, their fluorescence spectra showed significant increase of the emission peaks in 400-550 nm. The crosslinked polymer derived from 2a and 3b, however, showed decrease of the absorption peak around 390 nm and profound depression of fluorescence peaks in 400-550 nm.     

Silica-supported Karstedt-type catalyst for hydrosilylation reactions

The preparation, properties, catalytic kinetics and use as a hydrosilylation catalyst of a silica-supported Karstedt (Pt)-type catalyst are reported. This type of catalyst has high catalytic activity at room temperature under an atmospheric pressure, and the catalyst can be reused five times without any appreciable loss in the catalytic activity.     

Synthesis of refractive index tunable silazane networks for transparent glass fiber reinforced composite

Organic-inorganic hybrid polymers have recently attracted attention as matrix materials for transparent glass fiber-reinforced composites because of their optical and thermal properties. In this study, a transparent glass fiber-reinforced composite composed of organic-inorganic structures was prepared by hydrosilylation of polysilazanes. To obtain the transparent composite, the refractive index of the silazane matrix was controlled by phenyl groups. Fourier transform infrared spectroscopy was used to identify the chemical structure of the cross-linked polysilazanes. The transmittance of the composite was 89.36%, close to that of the silazane matrix. The 10% weight loss temperature of the matrix resin was determined as 531.9 °C. To observe the mechanical properties of the composite, a three-point bending test was performed. The flexural strength and the flexural modulus of the composite increased by 65% and 52% compared to those of the silazane matrix, respectively. These results suggest that polysilazanes can be utilized for transparent glass fiber-reinforced composites as alternatives to organic polymers.