Tetrafunctional silane

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 main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

OMe) and hexamethyldisilazane (HMDS). Reactions were performed on hydroxylated-but-anhydrous Ti02 surfaces in the gas phase. IR spectra confirm the presence of a bonded silane layer. Terminal surface OH groups are found to react more readily than bridging OH groups. By-products of the modification adsorb tenaciously to the surface. The various silanes show only small differences in their ability to sequester surface OH groups. Following hydrolysis in moist air, Si-OH groups are only observed for the tetrafunctional silanes. 

Mark and Semiyen, in a series of papers, have studied the mechanism and the effect of trapping cyclics in end-linked elatomeric networks [100-103], Sharp fractions of cyclics of polyfdimethylsiloxane) (PDMS), varying in size from 31 to 517 skeletal atoms, were mixed with linear chains for different periods of time and the linear chains were then end-linked using a tetrafunctional silane. The untrapped cyclics were extracted to determine the amount trapped. It was found that while cyclics with less than 38 skeletal atoms were not at all trapped, for n>38, the percentage of cycUcs trapped increased with size, with 94% trapped in the case of the cychc with 517 skeletal atoms. In effect, the system of trapped cycUcs in the end linked PDMS network is a polymeric catenane. It is thus possible to control the elastomeric properties of the network by incorporating the appropriate sized cyclics. This study has been extended to cyclic PDMS in poly(2,6-dimethyl-l,4-phenylene oxide) [104,105] and cyclic polyesters in PDMS [106]. 

Elastomeric properties are obtained by flghdy cross-flnking the polymer chains. There are two types of rubber material room temperature vulcanised (RTV) and high temperature vulcanised (HTV) polymers. The chemistry used to produce these elastomers is shghtly different. For the RTV the cross-links are created by the reaction of the polymer with a reactive cross-linking agent, usually a hydro-lysable tetrafunctional silane (Figure 7.18). 

The production of these resins starts with a mixture that might include di-, tri- and tetrafunctional silanes, such as monomethyl-, dimethyl-, monophenyl-, diphenyl-, mono-vinyl-, and methylvinylchlorosilane, along with silicon tetrachloride. The tri- and tetrafunctional ingredients are cross-linkers. The resin properties will depend on the chlorosilane mixture, the degree of cure, and processing conditions. 

An interesting branched tetrafunctional epoxy-silicone monomer, VIII, can be readily prepared as shown in the following equation by the platinium catalyzed condensation of the tetrafunctional SI-H compound, tetrakis(dimethylsiloxy)silane, with 3-vinyl-7-bicyclo[4.1.0]heptane. 

In the presence of chloroplatinic acid as the catalyst, silane adds to the double bonds of the tetrafunctional crosshnking agent with the formation of stable Si-C bonds ... 

Monomeric silane derivatives (Wheeler, 2(X)5) used in conservation are trior tetrafunctional derivatives, such as tetraethoxysUane (synonyms ethyl silicate, tetraethyl silicate, TEOS). The polymerization, initiated by traces of water, is by hydrolysis and condensation (Figure 13.2). Not all the silanol Si-OH groups will condense to form cross-links (Amoroso and Fassina, 1983). The polymer formed from a tetrafunctional monomer will be similar to silica gel. Care must be taken to ensure anhydrous storage of silanes to prevent premature polymerization. Trifnnctional monomers such as methyltrimethoxysilane react in a similar fashion bnt retain the Si-C bond. The polymer formed from this monomer is resistant to solvents and photo-oxidation. It is tough but not flexible (Moncrieff, 1976). The silanol groups will react with -OH groups on a surface to create chemical links. 

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