Siloxane bridge reaction with

This table shows that a reaction of TCS with siloxane bridges does not occur at reaction temperatures of 423 K. Apparently, this reaction temperature is too low to induce a cleavage of the siloxane bridge. Reaction does occur, however, when the temperature is raised to 623 K. When the Cl content is converted to the amount per nm2 and divided by 3, the amount of TCS that has actually reacted with the siloxane bridges can be calculated (table 9.8, last column). For a silica gel, pretreated at 973 K, approximately 0.3 siloxane groups per nm2 react with trichlorosilane at 623 K. 

The most common method of crosslinking via short bridges is a two-step process involving trimethoxysilane, which is shown in Fig. 18.9. In the first step, we graft siloxane branches onto polyethylene with the aid of peroxy radicals. The second step consists of a condensation reaction, which occurs in the presence of hot water or steam. A siloxane bridge is created and methanol is released. 

It is seen that the trichlorosilane reacts with the silanol groups to form siloxane bridges. Subsequently the residual chlorines are hydrolyzed. Under carefiiUy controlled reaction conditions it is possible to obtain a product in which the hydrocarbonaceous layer at the surface is similar to that in a corresponding monomeric bonded phase. However, the hydrolysis of chlorines that did not react with surface silanbis may result in a silanol concentration at the surface that is higher than that in the silica gel proper used as the starting material for the reaction with alkyltri-chlorosilanes. 

Experiments to determine the mode of bonding of the metal hydrocarbyl with silica dried above 200 °C were not attempted because of Reaction 4. Strained siloxane bridges are not present on silicas dried below 200°C (63). 

In the temperature range above 873 K, the surface is mainly covered with free silanols. Their concentration decreases as a function of increasing temperature, due to a condensation reaction forming siloxane bridges. Some of these condensation reactions occur as interparticle condensations, explaining the collapse of surface area and porosity above 873 K. 

He also suggested a side reaction (N) with so-called strained siloxane bridges. 

The above does not mean that reaction (N) with the siloxane bridges does not occur. 

The reaction of strained siloxane bridges with Lewis bases is not restricted to chlorosilanes. For instance, it will be discussed in part 3, that also ammonia reacts to some extent with strained siloxane bridges. 

A relatively easy way to check the existence of reaction (N) with strained siloxane bridges is to replace and/or to block all surface hydroxyl groups by a reaction with HMDS. This deactivated silica gel is then reacted with trichlorosilane. Kieselgel 60, thermally pretreated at 973 K, was refluxed with HMDS and consequently reacted with TCS at 623 K for 1 h. The Cl-contents on the surface of the silylated samples was determined. The results are presented in table 9.8. 

Comparing the Cl-contents on the silica samples, pretreated at 973 K, with and without a pre-modification with HMDS, the reaction with strained siloxane bridges seems to be quite significant. However, in a real situation, the different reactions are competitive. HMDS modification creates a surface where the reaction with siloxane bridges can occur free of competition. Therefore, these values may overestimate the degree of reaction in a real situation. The scientific relevance of these data is concealed in the unambiguous proof that (1) chlorosilanes do react with strained... 

The authors evidenced, using a methoxylated silica as a substrate, that at these temperatures no reaction occurs of the halogenosilanes with the siloxane bridges of the silica. No pretreatment temperature was mentioned, however. Therefore we cannot exclude a small reaction of the bromo- or iodosilanes with the siloxane bridges of the silica, thermally pretreated at high temperatures. 

In his paper, Haukka did not discuss explicitly the reaction (C) with strained siloxane bridges, although Kunawicz4 stated already in 1970 that these groups have equal, if not greater, nucleophilic reactivity than the hydroxyls . This statement was confirmed in 1983 by Kinney et al.8... 

Most authors agree that also a dissociative reaction with siloxane bridges occurs ... 

The formation of silazane species is probably due to a secondary reaction of the Si-NH2 species with (strained) siloxane bridges, according to reaction (C) 24... 

The anchored Cr(VI) species are not themselves the sites for the propagation reaction in PE formation. In the industrial procedure, the formation of the active centers takes place by direct contacting of the Cr(VI) species with ethene at 373-423 K. The polymerization starts after an induction period, which is attributed to a reduction phase, during which Cr(VI) is reduced to Cr(II), and ethene is oxidized (3,182). Formaldehyde has been found to be the main byproduct, but water and other oxidation products have also been observed in the gas phase (194). These reactive products can themselves react with surface silanols and siloxane bridges, and also with the reduced chromium sites. Consequently, the state of the silica surface and the chromium species after this reduction step is not well known (3). 

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