Organosiloxane oligomers

Table 2. General structure of (Si—X) terminated organosiloxane oligomers and important functional end groups...

Organosiloxane oligomers, in silicone polymerization, 22 555-556 Organosiloxane polymers, silicone resins as, 22 586-590... 

Starting in the 1950s a process was developed that leads from small-molecule silicon alkoxides such as tetraethoxysilane (tetraethyl orthosilicate), to organosiloxane oligomers and low polymers, and eventually to silica via a low temperature synthesis route.14 24 A simplified outline of the basic chemistry is shown in reactions (l)-(4), where R is an ethyl or higher alkyl unit. Any or all of the Si-OR bonds can be... 

Bead-Shaped Organosiloxane Oligomers and Copolymers with Organocyclotetra(Penta, Hexa)Siloxane Fragments... 

In contrast to the hydrolysis technology, the methanolysis process allows for the one-step synthesis of organosiloxane oligomers and methyl chloride without formation of hydrochloric acid (64,65). The continuous methanolysis can also yield quantitatively linear silanol-stopped oligomers by recycle of the cyclic fraction into the hydrolysis loop. 

Synthesis of Organosiloxane Oligomers with Cyclic Fragments in the Side Chain by the Reaction of Catalytic Dehydrocondensation... 

An important advantage in the preparation of a,eo-functionally terminated siloxane oligomers, over the other telechelic systems, is the flexible polymerization chemistry of cyclic organosiloxane monomers and intermediates. This is mainly due to the partial... 

Siloxane containing block or segmented copolymers can be synthesized either by living anionic polymerization of the cyclic organosiloxane trimers with appropriate vinyl monomer(, 4 ) or by step-growth or condensation copolymerization of preformed a,w-difunctional siloxane oligomers with conventional difunctional... 

As it is well known, the (Si-0) bond in organosiloxanes may be considered to be polar or partially ( 50%) ionic.(12) Therefore, it can be cleaved by the attack of strong acids or bases. This is the main rationale behind the "equilibration" route to the synthesis of a wide variety of functionally terminated siloxane oligomers(12-14) from cyclic siloxanes and a.ordifunctional disiloxanes as shown in Scheme 3. 

Monomer organosilanes and oligomer organosiloxanes belonging to highly flammable liquids. According to the nature of their interaction... 

Hydrosilylation of unsaturated organosilicon compounds has also found several applications in molecular and polymer organosilicon chemistry. In particular, the addition of polyfunctional silicon hydrides to poly(vinyl)organosiloxane, catalyzed exclusively by Pt compounds and providing an activated cure for silicon rubber [10], has been of great practical importance. Hydrosilylation of the vinyl group at silicon seems to be effective synthetic method for preparation of oligomers and polymers with a linear or cyclolinear stmcture (polyhydrosilylation), and can occur either via the addition of dihydro-carbosilanes and -siloxanes to divinyl-silanes and -siloxanes [25, 26] or by intermolecular hydrosilylation [4] (eq. (1)). 

Condensation processes are used to prepare oligomers of organosiloxanes, silazanes, silthianes, borazines, borazanes, metaborates, metaphosphates, phosphazenes, thiazyls, alazanes, metal oxides and so on. 

Different polymers or oligomers can be prepared by the equilibration of oligomers with different side groups. Copolymerization of oligomers is also possible. These techniques are particularly applicable to organosiloxane and organophosphazene systems. 

A well-known example of cyclic oligomer formation by loss of water is the preparation of cyclic organosiloxanes during the hydrolysis of diorganodichlorosilanes ... 

FIGURE 37 Diffractograms of comb-type organosiloxanes with aminohydroxyl groups in the side chain. Where curve 1 corresponds to oligomer XXXI, curve 2-XXX and curve 3 - XXIX. 

A study of the reaction of hydrolytic polycondensation of hexafunctionalized organosiloxanes (2) has shown that this reaction is predominantly intermolecular even in dilute ( 3%) solutions in the presence of catalytic amounts of HCl or AcOH. This fact is favorable to the formation of cubic molecules via interaction of two molecules of the starting oligomers 2. Thus, in the course of hydrolytic polycondensation of vinyl-tris-(diethoxymethylsiloxy)silane (2a) whose molecules bear a vinyl radical at the central silicon atom, a tsirget macromonomer, 1.4-divinylhexamethyloctasilsesquioxane (3), was obtained. 

Alkoxy- or aryloxyphosphazene high polymers (where OR = OCH2CF3 or OC H ) depolymerize to cyclic oligomers at temperatures above 200-250 C. This behavior is reminiscent of the reversion reactions that are characteristic of poly(organosiloxanes). 

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