Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

Hydrosilation reactions represent an important class of chemical transformations and there has been considerable recent interest in expanding the scope of these reactions by developing new catalysts. A major theme to emerge from these investigations is the development of catalysts with electrophilic character that transfer electrophilicity to silicon by Si‐H activation. This type of mechanism has been proposed for catalysts ranging from Group 4 transition metals to Group 15 main group species. Additionally, other electrophilic silicon species, such as silylene complexes and η³‐H₂SiRR′ complexes, have been identified as intermediates in hydrosilation reactions. In this Review, different types of catalysts are compared to highlight the range of hydrosilation mechanisms that feature electrophilic silicon centers. The importance of these catalysts to the development of new hydrosilation reactions is also discussed.     

Evaluation of the dual retention properties of stationary phases based on silica hydride: Perfluorinated bonded material

The synthesis of a new perfluorinated stationary phase based on silica hydride using a hydrosilation reaction was investigated. The material was characterized by elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy and ¹³C cross‐polarization magic‐angle spinning NMR spectroscopy. The retention properties of this new material were tested in the reversed‐phase and normal‐phase mode. Variable buffer strength experiments at two pH conditions for selected polar compounds were used to compare the new phase to hydrophilic interaction liquid chromatography retention. These results and previous data reported in the literature were used to postulate differences in the retention mechanism between hydrophilic interaction liquid chromatography and silica hydride‐based stationary phases.     

On the synthesis of siloxanes. XXIV. Nucleophilic substitution reactions of 2-functional 1,3-dioxa-2-4,7-trisilacylcloheptanes and 1,3-dioxa-2,4,8-trisilacyclooctanes

2-Hydrogen-1,3-dioxa-2,4,7-trisilacycloheptanes and 2-hydrogen-1,3-dioxa-2,4,8-trisiacyclooctanes, each as a mixture of three configurational isomers, were synthesized and halogenated with chlorine and bromine in the presence of pyridine. The stereochemical course of the halogenation reactions was studied by gas chromatography. 2-Chloro-2,4,7-trimethyl-4,7-bis(trimethyl-siloxy)-1,3-dioxa-2,4,7-trisiacycloheptanes and 2-chloro-2,4,7-trimethyl-4,7-diphenyl-1,3-dioxa-2,4,7-trisilacycloheptanes reacted with alcohols in the presence of pyridine, triethylamine, or 2,6-dimethylpyridine. Gas chromatography, and¹H NMR and²⁹Si NMR spectroscopy were used to investigate the stereochemistry of these substitution reactions. It has been found that all reactions proceed with retention of configuration and that the differences of the relative reactivities of the configurational isomers were distinctly smaller than those observed for reactions of the configurations isomers of functional cyclotrisiloxanes.     

Synthesis and properties of vinyl‐terminated and silicon‐containing polysulfones and polyketones

4‐Fluorophenylsulfonylphenyl‐terminated polysulfone and 4‐fluorobenzoylphenyl ketone were prepared with bisphenol A and an excess of bis‐(4‐fluorophenyl)sulfone or 4,4′‐difluorobenzophenone, respectively, at 160 °C using potassium carbonate in N,N‐dimethylacetamide. The resulting polymers were reacted with 4‐hydroxystyrene to synthesize vinyl‐terminated polysulfones and ketones. The silicon‐containing polysulfones and ketones were prepared from the vinyl‐terminated polymer precursor and various H‐functional silanes or siloxanes. The synthesis of silicon‐containing polymers was achieved by hydrosilation with a rhodium catalyst. It was shown that the hydrosilation reaction proceeds with 55:45 chemoselectivity. The resulting polymers were investigated by ¹H NMR spectroscopy, DSC, and thermogravimetric analysis. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2937–2942, 2001     

Extinction coefficients for the β-methallyl radical

The radicals formed in the flash photolysis of 2-methylbut-1-ene and iso-butene and subsequent reactions have been investigated by kinetic spectroscopy and gas liquid chromatography. The extinction coefficients of the β-methallyl radical were measured from the absorption bands. The decay of the radical was second order. The measured rate constant for β-methallyl combination was (2.6±0.3) × 10¹⁰ mol^?1 & s^?1 at 295 ± 2K.     

NMR study on microstructure of polymer produced by hydrosilation of styrene with poly(hydrogenmethylsiloxane)

The microstructure of the polymer formed by the hydrosilation of styrene with poly(hydrogenmethylsiloxane) was studied by ¹H- and ¹³C-NMR spectra. Two modes of addition in hydrosilation were found to occur randomly without the influence of the adjacent monomer unit by statistical analysis of sequence distribution of ²⁹Si-NMR spectrum.     

Preparation of a liquid boron‐modified polycarbosilane and its ceramic conversion to dense SiC ceramics

A boron‐modified ethynylhydridopolycarbosilane (B‐EHPCS) was successfully prepared via the hydroboration reaction of ethynylhydridopolycarbosilane (EHPCS) with 9‐borabicyclo‐[3.3.1]nonane (9‐BBN). The as‐synthesized B‐EHPCS with a branched structure was characterized by means of gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). The structural evolution of ceramic conversion of B‐EHPCS was investigated by solid‐state NMR. The ¹³C magic angle spinning (MAS) NMR results indicated that the C?C and C?C groups of B‐EHPCS take part in the hydrosilation cross‐linking at a relatively low temperature (170°C). According to the ²⁹Si MAS NMR analysis, the CSiH₃ end groups are most reactive hydride functionality involved in the hydrosilation cross‐linking. With increasing curing temperature, the C₂SiH₂ and CSiH₃ units are completely consumed, while C₃SiH units remain even after curing at 600°C. The TGA results show the 1200°C ceramic yield of B‐EHPCS reaches 86%, which is 10% higher than that of the parent EHPCS (76%). At high temperatures, the introduction of <1 wt% boron significantly inhibits silicon carbide (SiC) crystallization. The 1800°C ceramics derived from B‐EHPCS are found to be significantly denser than that from EHPCS. Copyright © 2010 John Wiley & Sons, Ltd.     

Some new perfluoroethers and copolymers from ‘Dewar’ hexafluorobenzene

The thermal and photolytic reactions of hexafluorobicyclo[2.2.0]hexa-2,5-diene and bis(fluoroxy)difluoromethane yielded four-membered fused-ring-containing perfluoroethers and copolymers. The reaction products were analyzed by gas chromatography, gas chromatography-infrared, gas chromatography-mass spectra, field ionization mass spectra and ¹⁹FNMR. The elucidation of the polymer microstructures was assisted by the ¹⁹FNMR spectroscopy of the related compounds, hexafluorobicyclo[2.2.0]-hexanes and -hexene in the volatile fraction.An increase of the concentration of ‘Dewe’ C₆F₆ in hexafluorobenzene for copolymerization with CF₂(OF)₂ decreased the six-membered ring moieties. At about 90% level of the para-bonded C₆F₆, no six-membered ring-containing perfluoroethers were found in the volatile fraction and only a low concentration of the six-membered ring moieties were in the polymeric fractions. The molecular weights of the copolymers ranged from 1000 to over 10,000. The microstructures of the thermal and photopolymers about 1000 range were suggested.     

Kinetic studies of photopolymerization using real time FT-IR spectroscopy

The real time FT-IR (RT/FT-IR) technique has been recognized as a very vital tool to quantitatively study the curing parameters such as the effects of initiator (or catalyst) type and concentration, accelerator, stabilizer, irradiation wavelength, temperature, and curing environments. Herein, our results in studies of photoinduced polymerizations for adhesive and coating applications are reported. The photoinduced polymerizations studied included anionic and hydrosilation (a polyaddition polymerization) reactions. In photoinduced anionic polymerization our studies for ethyl cyanoacrylate polymerization are described. The effect of the concentration of photoinitiator and inhibitor on the ethyl cyanoacrylate polymerization kinetic rate will be discussed. In photoinduced catalytic hydrosilation reaction studies, the effects of the catalyst concentration and staging irradiation are disclosed. The hydrosilation reaction was monitored using a Si? H silicone hydride stretching band located at 2169 cm^?1. The cyanoacrylate polymerization was monitored using the C?C stretching band occurring at 1617 cm^?1. The hydrosilation conversion was completed with an appropriate formulation. For monofunctional cyanoacrylate monomer, the photoinduced conversion to straight chain polymer was approximately 85% for a 60 s period. The intrinsic rates of the reactions were calculated for kinetic comparisons. For very fast cyanoacrylate polymerization studies, new FT-IR kinetic software was used to collect 204 spectra/min. Some detailed experimental techniques and polymerization reaction mechanisms are also discussed. © 1993 John Wiley & Sons, Inc.     

Some perfluorocarbocyclic ethers and polyethers from hexafluorobenzene

High-resolution nuclear magnetic resonance (NMR) spectroscopy was used to determine the microstructures of some perfluorocarbocyclic ethers and polymers from reactions of bis(fluoroxy)-difluoromethane and hexafluorobenzene. The thermal and photochemical reactions of CF₂(OF)₂ and excess hexafluorobenzene are described. The structures of several new perfluoro-1,3-dioxolanes in which the rings constituted part of a bicyclic system were elucidated by ¹⁹F-NMR, gas chromatograph, gas chromatograph-infrared, and gas chromatograph-mass spectra. The copolymers of molecular weight about 2500 are, with 40% of the difluoromethylenedioxy groups, estimated by ¹⁹F-NMR to be in the polymer chain and the rest as pendant groups to the perfluorocyclohexylene and perfluorocyclohexenylene moieties. The suggested polymer structures are described and the mechanisms discussed.     

Stereochemistry of bromoalkoxylation of 6-phenyl-δ2-dihydropyran and synthesis of 2-alkoxy-6-phenyl-δ3-dihydropyrans

2-Alkoxy-6-phenyl-δ³-dihydropyrans were synthesized by bromoalkoxylation of 6-phenyl-δ [²-dihydropyran] with N-bromosuccinimide in the presence of alcohols and subsequent dehydrobromination of the resulting 3-bromo-2-alkoxy-6-phenyltetrahydropyrans. The structural stereospecificity of the bromoalkoxylation and dehydrobromination reactions was studied by gas-liquid chromatography and PMR spectroscopy, and the configurations and preferred conformations of the compounds were also established.     

Synthesis and Reactivity of Hydroxypoly(ethylene oxide) propyl–b–polydimethylsiloxane–b–propyl hydroxypoly(ethylene oxide)

A series of α, ω–bishydroxyl terminated PDMS, hydroxypoly(ethylene oxide) propyl–b–polydimethylsiloxane–b–propyl hydroxypoly(ethylene oxide) (HPEO–PDMS–HPEO) was prepared by a hydrosilation reaction of monoallyloxy substituted poly(ethylene oxide) with α,ω–bishydrogen terminated PDMS (HPDMS) that obtained via acid–catalyzed ring–opening polymerization of octamethylcyclotetrasiloxane with 1,1,3,3–tetramethyldisiloxane. Chloroplatinic acid was employed as the catalyst of hydrosilation. The molecular weight of HPEO–PDMS–HPEO could be controlled easily by varying the chain length of HPDMS. FTIR and ¹H–NMR spectroscopy were used to identify the structure of HPEO–PDMS–HPEO and HPDMS. The conversion of Si–H bond to Si–C bond was affected by the catalyst amount, reaction time and temperature. It was found that the optimum condition of hydrosilation reaction was the catalyst amount of 22 μg/g and 5 h time at 100°C. Synthesized HPEO–PDMS–HPEO showed good storage stability at ambient temperature. Urethane reaction of OH and NCO group revealed that HPEO–PDMS–HPEO was more reactive toward to diisocyanate than α, ω –bishydroxylbutyl terminated PDMS.     

Product analyses and crosslink mechanism of plasma-polymerized methyl methacrylate

Plasma-polymerized methyl methacrylate (PPMMA) film was produced by a radio-frequency glow discharge of methyl methacrylate (MMA) gas with argon. PPMMA deposited under a plasma of mild energy was separated into a soluble fraction (sol-PPMMA) and an insoluble matrix (insol-PPMMA), which is highly crosslinked, by solvent-extraction and filtration. The chemical structure of the sol-PPMMA was determined by pyrolysis/gas chromatography, IR spectroscopy, ¹H-NMR, ¹³C-NMR, and elemental analysis techniques. On the other hand, the structure of the insol-PPMMA was examined with IR spectroscopy, ¹³C-NMR with CP/MAS method, and elemental analysis. The crosslinking mechanism in the plasma polymerization of MMA is discussed on the basis of product analyses of the sol-PPMMA and the insol-PPMMA.     

Analysis of silica hydride and surface hydrosilation reactions by solid-state NMR in the preparation of<Emphasis Type="Italic"> p</Emphasis>-chlorobenzamide bonded silica phase

²⁹Si and ¹³C CP-MAS NMR spectroscopy was used to follow the conversion of native silica to a p-chlorobenzamide bonded silica material. The benzamide bonded phase was prepared via a hydrosilation reaction of a hydride silica intermediate with p-chloro-N-allylbenzamide. Solid-state NMR was used to show the disappearance of reactive surface hydride species (M_H) and to identify newly formed bonded chemical species on the silica surface. DRIFT spectroscopy, elemental analysis, and specific surface-area determinations (BET) of the prepared phases are also reported.     

Systematic parallel investigation of RAFT polymerizations for eight different (meth)acrylates: A basis for the designed synthesis of block and random copolymers

Poly(acrylate)s as well as poly(methacrylate)s were successfully synthesized via reversible addition‐fragmentation chain‐transfer (RAFT) polymerizations using 2‐cyano‐2‐butyl dithiobenzoate (CBDB) as RAFT‐agent. Four different ratios of RAFT to initiator were screened for four acrylates and four methacrylates using automated parallel synthesizer robots. The reactions were monitored by gel permeation chromatography (GPC) and gas chromatography (GC). The knowledge obtained during this screening was used for the designed synthesis of block and random copolymers containing a water‐ and a non water‐soluble monomer. The results obtained from GPC analysis together with ¹H NMR spectroscopy demonstrate the possibility to design and prepare well‐defined block and random copolymers. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3831–3839, 2005     

Photolytic and radiolytic polymerization of N-ethylmaleimide in cyclohexene

Irradiation of N-ethylmaleimide (NEMI) in cyclohexene (CH) with ultraviolet light or γ rays gives rise to polymer formation. Separation of the components by liquid chromatography and determination of their structure of pyrolysis—gas chromatography (GC)–mass spectrometry (MS) coupling, infrared spectroscopy, and ¹³C- and ¹H-resonance spectroscopy show that the polymer produced in photolysis as well as in radiolysis consists of an almost alternating copolymer of NEMI and CH and small amounts of a NEMI homopolymer. Based on the experimental results, we propose a radical mechanism for copolymerization and an anionic mechanism for homopolymerization.     

The iterative synthesis of discrete dimethylsiloxane oligomers: A practical guide

Discrete dimethylsiloxane oligomers are interesting building blocks for the synthesis of high χ–low N block co-oligomers (BCOs) forming highly organized nanostructures. Here, a practical guide to the synthesis of molecularly defined oligodimethylsiloxanes (oDMS) from 7-mer to 40-mer via a linear growth strategy is described. The iteration of a hydroxylation reaction and the condensation of mono- or bifunctional hydroxysiloxanes with chloro-octamethyltetrasiloxane results in asymmetric and symmetric siloxanes, respectively. The synthesis contains critical washing and purification steps to remove minor amounts of low and high-molecular weight byproducts, which are detected using Fourier transform infrared spectrometry, gas chromatography–mass spectrometry, and size-exclusion chromatography. The oligomers are obtained on a multigram scale in yields of 94–50% and in high purity with only one molar mass detected. The formation of the chloride, hydroxide or hydride functional groups is adequately analyzed using ²⁹Si NMR spectroscopy. The hydride terminated siloxane oligomers are used in Karstedt catalyzed hydrosilylation reactions with alkene-functional substrates to obtain oDMS-based oligomers and BCOs. Byproduct formation as a result of isomerization and reduction are followed by ¹H NMR spectroscopy and minimized using dry conditions and low-catalyst loadings.     

A New Nanocomposite Based on Pt-rGO Embedded Polymelamine Formaldehyde Nanocomposite for Reduction of Carbon Dioxide

Here, polymelamine formaldehyde was decorated on the surface of reduced graphene oxide whose surface was then electrodeposited with a sub-monolayer of platinum nanoparticles. The nanocomposite thus prepared was characterized using several spectroscopic methods. Using the nanocomposite as a potential electrocatalyst for carbon dioxide reduction, the products were detected by Raman spectroscopy, gas chromatography, ¹³C-NMR spectroscopy, and gas chromatography-mass spectrometry. The analytical results identified methanol as the main product of CO₂ reduction. Moreover, analysis of the liquid products confirmed methanol as the predominant product with a current density of 0.4 mA/cm and a Faradaic efficiency of 93 %.     

Reaction rate studies of atomic germanium (3P0,1) and silicon (3PJ) with various oxidizers

The gas phase reactions of Ge(³P_0,1) and Si(³P_J) with O₂, NO and N₂O have been studied in a flow tube system at 350 K. Atomic Ge and Si were produced by flowing GeH₄ and SiH₄ through a hollow cathode discharge. The subsequent disappearance of the Ge and Si atoms was followed with atomic absorption spectroscopy. Rate constants were determined for the reactions at 4 and 5 torr pressures.