Silanization kinetics

From an initial understanding of the silane kinetics, very little decomposition of the silane was expected at this (relatively) low surface temperature. Calculate the silane number-density field, assuming the nominal silane partial pressure at the inlet and for a temperature of 550°C. The measured number density just above the surface was 6 x 1015 molecules/cm3. What is the percent difference between the measured and ideal-gas result ... 

The results of x-ray structure analysis and neutron diffraction, as well as spectroscopic experiments (J(HSi) = 70.8 Hz for 30), can be interpreted in the sense mentioned above. The observed reactivity of 30 is also consistent with this view, the coordinated silanes can be displaced smoothly by phosphines, according to first-order reaction kinetics. 

Recently, this work has been extended and further developed by Brown-Wensley into a preparative method for the synthesis of disilanes. The results of competitive reactions with several silanes allow insight into the reaction kinetics, in particular the relative rates of disilane formation versus hydrosilation (Table 5a, b) [61]. 

Most of the kinetic studies have been carried out by Eaborn and by Benkeser and their coworkers, and the first study was that of Eaborn656, who measured, spectrophotometrically, the rates of cleavage of 4-methoxyphenyltrimethylsilane by hydrochloric and perchloric acids in methanol, and by hydrochloric acid in aqueous dioxan, both at 47.9 °C (Table 226). The reaction was first-order in silane... 

Evidence for a cyclic transition state in iododesilylation of phenyltrimethyl-silane by iodine monochloride (reaction (311), X2 = IC1 giving Phi and SiMe3Cl) has been obtained by comparing rates of chlorodesilylation and iododesilylation in acetic acid at 25 °C743. Good second-order kinetics for the latter reaction were obtained by application of equation (312)... 

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

The hydrogen abstraction from the Si-H moiety of silanes is fundamentally important for these reactions. Kinetic studies have been performed with many types of silicon hydrides and with a large variety of radicals and been reviewed periodically. The data can be interpreted in terms of the electronic properties of the silanes imparted by substituents for each attacking radical. In brevity, we compared in Figure 1 the rate constants of hydrogen abstraction from a variety of reducing systems by primary alkyl radicals at ca. 80°C. ... 

The fact that the rate of silanization is influenced by the moisture content of the sUica supports the mechanism wherein a hydrolysis step is involved. The reaction follows pseudo first-order kinetics. Figure 29.1 shows the mechanism of the primary reaction. 

ZnO influences the characteristics of a compound twofold Primarily, ZnO influences the kinetics of the silica-silane reaction. Secondly, ZnO enhances the scorching behavior of a sulfur-containing coupling agent. This effect is clearly visible when looking at the G at 100% strain and the 300% modulus. 

To summarize, the kinetics of the silanization reaction are strongly influenced by the efficiency of the devolatilization process. The degree of devolatilization mainly depends on processing conditions (e.g., rotor speed and fill factor), mixer design (e.g., number of rotor flights, size of the mixer), and material characteristics. The diffusion coefficient of the volatile component in the polymeric matrix is of minor influence. 

Kinetic hindrance of the silanization reaction due to high concentration of the reaction product, ethanol, in the compound... 

Nienhuis et al. [189, 191] have developed a self-consistent fluid model that describes the electron kinetics, the silane-hydrogen chemistry, and the deposition... 

As a first attempt to modify the code to be able to run simulations on SiH4-H2 discharges, a hybrid PlC/MC-fluid code was developed [264, 265]. It turned out in the simulations of the silane-hydrogen discharge that the PIC/MC method is computationally too expensive to allow for extensive parameter scans. The hybrid code combines the PIC/MC method and the fluid method. The electrons in the discharge were handled by the fluid method, and the ions by the PIC/MC method. In this way a large gain in computational effort is achieved, whereas kinetic information of the ions is still obtained. 

The analysis of the neutral gas composition in a discharge yields useful information on the mechanisms and kinetics of silane dissociation. However, it should be borne in mind that with mass-spectrometric analysis one only detects the final products of a possibly long chain of reactions. 

Mass-spectrometric research on silane decomposition kinetics has been performed for flowing [298, 302-306] and static discharges [197, 307]. In a dc discharge of silane it is found that the reaction rate for the depletion of silane is a linear function of the dc current in the discharge, which allows one to determine a first-order reaction mechanism in electron density and temperature [302, 304]. For an RF discharge, similar results are found [303, 305]. Also, the depletion and production rates were found to be temperature-dependent [306]. Further, the depletion of silane and the production of disilane and trisilane are found to depend on the dwell time in the reactor [298]. The increase of di- and trisilane concentration at short dwell times (<0.5 s) corresponds to the decrease of silane concentration. At long dwell times, the decomposition of di- and trisilane produces... 

The increase in the deposition rate rj (Fig. 63d) corresponds to the increase in the ion flux (Fig. 63c) the fraction of arriving ions per deposited atom, / ,, is constant at about 0.25. Such observations have also been reported by Heintze and Zedlitz [236], who furthermore suggested that the deposition rate may well be controlled by tbe ion flux. The kinetic ion energy per deposited atom, max, is also constant and amounts to about 5 eV. As was shown in Section 1.6.2.3, the material quality as reflected in the refractive index 2 eV (Fig. 63e) and the microstructure parameter R (Fig. 63f) is good 2 cv is around 4.25, and R is low (<0.1). The depletion of the silane stays constant at a value of 4.0 0.4 seem in this frequency range. The partial pressures of silane, hydrogen, disilane (1.3 x 10 - mbar), and trisilane (2 x 10 mbar) in the plasma are also independent of frequency. Similar... 

Reduction of ketones to triphenylsilyl ethers is effected by the unique Lewis acid perfluorotriphenylborane. Mechanistic and kinetic studies have provided considerable insight into the mechanism of this reaction.186 The salient conclusion is that the hydride is delivered from a borohydride ion, not directly from the silane. Although the borane forms a Lewis acid-base complex with the ketone, its key function is in delivery of the hydride. 

Using the titanocene-catalyzed co-hydrogenation of cyclohexene, we have studied the kinetics of the polymerization of a number of primary silanes ( 20 ). The rate law was found to be ... 

Like many homogeneously catalyzed reactions, the overall cycle (or cycles) in these polymerization reactions probably contains too many steps to be easily analyzed by any single approach. Both kinetics and model compound studies have thrown light on some of the steps. However, as indicated above, many of the model compounds isolated from the reactions of primary silanes with metallocene alkyls and hydrides are too unreactive to explain the polymerization results. 

SiH4 is heavier than CH4 however, both molecules have the same average kinetic energy. This is due to the fact that methane molecules have an average speed which is 1.4 times faster than that of silane molecules. 

To obtain reproducible kinetic data, Reiksfel d had to add styrene to a solution of [PhCH=CH2PtCl2]2 before MeCl2SiH was added. When the silane was added first and styrene was withheld for about 10 minutes, the rate of hydrosilation was noticeably retarded. 

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