Trifunctional silanes

By endcapping PTMO with trifunctional silanes, the resulting PTMO molecules will have a functionality, f, of 6. This should reduce the gelation time considerably and potentially enhance the dispersion of the PTMO species. 

Wirth, M.J. and Fatunmbi, H.O., Horizontal polymerization of mixed trifunctional silanes on silica—a potential chromatographic stationary-phase. Anal. Chem., 64, 2783, 1992. 

It is difficult to find crosslinking systems that are ideal in that all functional groups are of equal reactivity and intramolecular cyclization is negligible. The crosslinking of vinyl terminated poly(dimethylsiloxane) polymers with tri- and tetrafunctional silanes appears to be an exception. Thus the calculated and experimental pc values were 0.578 and 0.583, respectively, for the tetrafunctional silane and 0.708 and 0.703, respectively, for the trifunctional silane (with r — 0.999) [Valles and Macosko, 1979]. 

The nomenclature of the RP is not consequent. The RP most often used contains octyl (RP C8) or octadecyl (RP C18) groups. There is no differentiation even when two methyl groups are introduced additionally with the silane (as with monofunctional silanes) or only one (difunctional) or none (trifunctional silane). Some manufacturer use silanes with bulky side groups (e.g., isopropyl groups) to improve the hydrolytic stability of the bonded phases, but here also, only the longest alkyl group is used in nomenclature. RP C8 and RP C18 are the work horses in HPLC. Shorter chains (RP4) are used in protein separations, and special selectivity can be obtained with bonded phenyl, cyano, amino or fluoro groups. 

For synthesis of the commercially available GFF-II version of the Pinkerton s internal surface reversed-phase, monofunctional instead of trifunctional silane has been used in order to prepare more homogenous surfaces (123). Compared with the first version, GFF-II shows significantly improved selectivity, retention capacity, and separation efficiency as demonstrated in the analysis of trimethoprim (124). 

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

From the work of Pratt and co-workers and analysis of the available literature, a reasonable model for hydrolysis mechanisms can be developed. The studies of the hydrolysis of mono-, di-, and trifunctional silane esters in aqueous and aqueous-organic solutions lead to a kinetic expression describing the rate of silane ester disappearance (hydrolysis) ... 

The organosilane chemicals utilized in glass composites are trifunctional silanes—i.e., they contain three hydrolyzable groups per silicon atom. Upon hydrolysis, the silanol group adheres strongly to the glass surface. The mechanism by which this takes place is inherently difficult. The indirect confirmation is much better documented through mechanical property data. 

The most common and convenient methods for the preparation of a variety of Si- and C-substituted silatranes are the reactions of trifunctional silanes with tris(2-hydroxyalkyl)amines (equation 1). Halo, alkoxy, acyloxy and dialkylamino groups can... 

A standard synthesis of 1-organyltribenzosilatranes is the reaction of a trifunctional silane with tris(2-hydroxyphenyl)amine (equation 169)402. 

Silanes react with silanol groups ( SiOH) on silicon surfaces according to the reaction shown in Fig. 10.5 [423] (review Ref. [424]). This reaction, which is usually carried out in organic solvents, is called silanization. Trifunctional silanes bind to the surface silanol groups,... 

Wirth and Fatunmbi8 defined the bonding of trifunctional silanes as self-assembly, when the bonding density of functional groups is made to be close-packed, approximately 8 pmol/m2 (2.2 nm2/chain). Conventional polymeric phases are no more than 5 jtmol/m2. Because of their close packing, these phases are very well suited for chromatographic separations, since all interference of surface hydroxyls is excluded. 

In figure 3.11 a mono-ethoxysilane is used as an example. Alternatively, a trifunctional reagent such as a triethoxy-alkylsilane may be used, to yield what is commonly referred to as a polymeric material. The various possible products from the reaction of trifunctional silane molecules with the silica surface are shown in figure 3.12. 

For steric reasons bifunctional or trifunctional silanes can react with either one only or, at most, two silanol groups on the silica gel surface (second reaction in Fig. 1.8A). Thus, some of the functional (Cl or alkoxy) groups remain unreacted and easily hydrolyse to form new silanol groups. If the reaction mixture contains even traces of water, the hydrolysis occurs during chemical modification of silica and the new silanol groups react with excess molecules of reagents to form a polymerised surface layer (Fig. 1.8B). These bonded phases may be more stable and usually show stronger retention than monomeric phases at low pH. However, the reaction is difficult to reproduce and various batches of the same material may have different properties, so that the reproducibility of separation is poorer than with monomeric phases. Polymeric phases are more resistant to penetration of analytes and may show increased mass-transfer resistance and decreased efficiency (plate number) of separation [- 91. 

There is reason to believe that the molecular structure of crosslinked trifunctional silanes/siloxanes may also influence the durability of the resulting silicone resin network [14, 40 - 42]. Allowance must be made here for the fact that such considerations are models in nature and also that we still do not have adequate knowledge of the molecular principles of fully condensed silicone resin networks [15]. 

The cyano column may be used in the normal- or re-versed-phase mode and can be regarded as the first column of choice for method development when both modes are under consideration. Typically, it is prepared from mono-, di-, or trifunctional silane. A more... 

Monofunctional silane reagents yield efficient stationary phases with flexible furlike or brushlike structure of the chains bonded on the silica surface. When bifunctional or trifunctional silanes are used for modification, Cl or alkoxy groups are introduced into the stationary phase, which are subject to hydrolysis and react with excess molecules of reagents to form a polymerized spongelike bonded phase structure. Stationary phases prepared in that way usually show stronger retention but lower separation efficiency (plate number) than mono-merically bonded stationary phases. 

Depending on the surface modification reversed phase silicas can be grouped into (a) monomeric reversed phase silicas chemically modified with monofunctional silanes and (b) polymeric reversed phase silicas with a polymeric layer made by surface reaction with trifunctional silanes. 

This sorbent is manufactured using trifunctional silane under carefully controlled conditions. The ISOLUTE SPE product has been subjected to the following Q.C. tests. 

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