Equilibrations cyclic siloxanes

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

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. 

The reaction scheme for the preparation of aminopropyl-terminated di-fimctionalized oligomers is illustrated in Scheme II. The reaction proceeds by the anionic equilibration of the cyclic siloxane tetramer, D4, in the presence of l,3-bis(3-aminopropyl)tetramethyldisiloxane. The equilibration process begins immediately upon addition of the siloxanolate catalyst, and samples were removed as a function of time for the purpose of the kinetic study. 

Eq. (6) with oji = 2x. A rotational isomeric state model for polyphosphate chains (2 76,187) was used to compute values of < rj >o required by Eq. (6). Comparison of the experimental values with the theoretical values, as well as with the values for the corresponding cyclic siloxanes (R(CH3)SiO)x [see Section III.(A)5] stronj y supports the contention that polyphos diate chains in sodium phosphate melts adopt random-coil conformations. Further evidence for the structure of sodium phosphate melts would come from precise data for macrocyclic concentrations in quenched sodium phosphate ring-chain equilibrates, if these became available. 

The manufacture of pure octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), which arc either marketed as such or are used as raw materials in the production of polydimethylsiloxanes by the polymerization process, is carried out by the so-called cyclization process. The hydrolysis or methanolysis product is heated in a suspension of potassium hydroxide and an inert liquid (e.g. mineral oil). This method is chosen to hinder polymerization of the siloxanes to highly viscous liquids. The potassium hydroxide catalyzes an equilibrium reaction in which the Si-O-Si bonds are cleaved and newly made (equilibration). Since in this process the, by comparison with the linear siloxanes, more volatile octamethylcyclotetrasiloxane and decamethylcyclopenta-siloxane are continuously distilled off from the siloxane mixture, the equilibrium is shifted in a direction favoring the desired cyclic siloxane thereby enabling all of the siloxane to be so converted. 

The discussion following will deal mainly with the important equilibrations and polymerizations of cyclic siloxanes. For further information concerning these methods and for excellent discussions of other types of siloxane polymerizations, the reader is referred to Voronkov (O, Noll O), and Meals (10). In addition, Noll (7, Eaborn (11), and Arkles and Peterson ( 1 2) offer reviews of the general chemistry of silicon compounds. 

Various reactions, both of polymerization and of polymer degradation, can produce cyclic polymer molecules. A well-known process is the ring-chain equilibration reaction, which may be used to produce cyclic siloxanes and o er cyclic polymers. The linear chain reacts intramolecularly and yields a cyclic and a linear chain. In the initial stages, the molar fraction of cyclics increases at the expense of the linear chains. After some time, equilibrium conditions are achieved and the molar fraction of cyclics remains constant. In some cases, all the sites in the macromolecular backbone are equivalent and no peculiar bond exists which is preferentially attacked. This case is referred to as thermodynamically controlled cyclization. 

Poly(dimethylsiloxane) and the hydrosilylation polyaddition product were placed in a three-neck flask. After they has been heated to 60 °C 200 ppm of (PNChli was added and the mixture was heated to 100 °C. At a temperature of 80-100 °C poly(dimethylsiloxane-co-methylhydrosiloxane) was mixed with the other components. The equilibration process is accompanied by a visible homogenization and decrease in viscosity of the reaction mixture. The viscosity should reach a value below 180 mm Vs. The equilibration was complete after approximately 2 h and was stopped by neutralization with 1 % (w/w) MgO. After filtration and the removal of low molecular weight cyclic siloxanes in vacuum a fluid transparent oil was obtained and the SiH content was calculated by Si NMR spectroscopy. 

This mixture can be condensed and equilibrated by a number of different catalysts, or the cyclic siloxanes can be removed and polymerized separately. Catalysts for these equilibration reactions can be basic, such as the hydroxides of lithium, sodium, potassium, and cesium, and potassium amide." Some acidic catalyst are sulfuric and ethylsulfuric, chlorosulfonic phosphoric, py rophos-... 

In their earlier studies on the synthesis of a, ajrhydroxyl terminated siloxane oligomers Marvel and co-workers (21) have utilized sulfuric acid as the catalyst in the equilibration of cyclic tetramer (D ) and l,3-bis(4-hydroxylbutyl)tetramethyldisilox-ane (DSX). However the molecular weights of the oligomers obtained were several times higher than the expected values. They have... 

The synthesis of cyclic polydimethylsiloxane was first achieved through ring-chain equilibration of siloxane oligomers in the presence of potassium silanolate, as shown in Fig. 51 [163-165]. Cyclics recovered from ring-chain equilibration reactions have been fractionated by preparative GPC, yielding... 

Siloxane-containing block copolymers are often prepared by step-growth or condensation polymerization of preformed diftmctionalized siloxane oligomers with other diftmctionalized monomers or oligomers. Our current work (3, 4, 6-8) on siloxane chemistry includes the preparation of a number of functionalized oligomers, with emphasis on equilibration processes with the commonly available cyclic tetramer, octamethylcyclotetrasiloxane (D4), in the presence of a functionalized disiloxane or end blocker. 

The first application of the equilibrium cyclic concentration method to measure precisely the average conformations of chain molecules in a polymer was for poly-(dimethylsiloxane) 16,18-20, 36). In Fig. 4, Kx values for a bulk equilibrate at 383 K (19) are compared with the corresponding values for a solution equilibrate (in toluene at a siloxane concentration of 224 g dm ) [Refs. 18,20) and with theo-... 

Fig. 7. Weight fractions of cyclics (R(CH3)SiO)x in high molecular weight (p S1) polysiloxane equilibrates at 383 K (R = CH3, CH3CH2, CF3CH2CH2) and 273 K (R = H) as a function of the volume per cent siloxane in cydo-hexanone (R = CF3CH2CH2 and toluene (R = H, CH3, CH3CH2)...

This illustrative study of ring-chain equilibria in a paraffin-siloxane system demonstrates that the conformational characteristics of chain molecules can be investigated provided suitable labile groups are incorporated into the molecular structures. It should prove within the capabilities of chemists to synthesise a wide variety of molecules suitable for study by the equilibrium cyclic concentration method using the dimethylsiloxane (or other siloxane) linkage as a labile linkage, incorporated into the molecular structure, throu whicJi to effect ring-cJiain equilibration reactions. 

By contrast, the corresponding values for the large cyclics decrease along the same series. The total weight fraction (experimental) of cyclics in these bulk equilibrates are listed in Table 4. The effect of dilution with cyclohexanone in the syntheses of these same polymers is illustrated in Figure 3. In all cases, a critical dilution point was reached as predicted but it was reached at a lower diluent volume than the theoretical value. Molar cyclization equilibrium constants for undiluted poly(methylphenyl)siloxane were later measured (19) and found to be similar to those previously obtained in the case of poly(ethylmethyl)siloxanes. 

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