Siloxanes, redistribution reactions

Typical initiators for living anionic polymerization of siloxanes include conventional organoalkali compounds and lithium siloxanolates22). Initiators containing lithium counterions are preferable to sodium or potassium counterions due to the lower catalytic activity of lithium in siloxane redistribution reactions. Living anionic polymeriza-... 

The reaction of chlorosilanes produces hydrolyzates consisting of cyclics and linears with hydroxy and/or chlorine ends, depending upon conditions. Silanol functional linears are easily obtained and can be end-capped by silylation. More pertinent to this discussion, however, is the nature of end blocks that result from siloxane redistribution reactions. Conversion of cyclosiloxanes to equilibrium ring-chain distributions affords chains with ends arising from the catalyst i.e., if KOH is used, the chains have ends bearing silanol and K silanolate functionality. Neutralization with CO2 and HjO then converts the silanolates to silanols. Alternatively, the equilibrates, as well as the above hydrolyzates, can be silylated to convert the silanols and silanolates to other kinds of ends. 

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. 

Molecular weight distributions and the nature of the redistribution reactions involving the siloxane chain. 1301... 

Chojnowski and Mazurek16 have studied the reaction of phenyldimethyl silanolates with cyclosiloxanes under conditions where siloxane bond redistribution reactions involving the polymer and the catalyst are completely suppressed. For the reaction of sodium phenyldimethyl silanolate (I) with 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (II) they find that the rate of disappearance of I in an excess of II follows first-order kinetics. The observed first-order rate constant k varies with the initial... 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

Aminosilanes of the general formula R Si(NH2)4- , R Si(NHR)4 , R Si(NR2)4- are prepared by the reaction of silicon hydrides or chlorosilanes with ammonia or other amines in the presence of an inert solvent. These hydrosilylation reactions may be carried out at room temperature, with the reaction products being isolated by simple phase separation (3,46-48). To obtain the MD D M type aminofunctional siloxanes typically employed in personal care formulations, aminofunctional silanes (generally aminoethylaminopropyltri-methoxysilane or y-aminopropyltrimethoxysilane) may be polymerized with linear hydrolysates or with octamethylcyclotetrasiloxane to form aminofunctional silicone fluids. Nucleophilic substitution and redistribution reactions have also been used to prepare one modified silicone from another. For example, aminofunctional siloxanes may be prepared by substitution as illustrated in Eq. (4). 

Siloxane resins are receiving attention as precursor to ceramics and specialty glasses. Although polysiloxane resins are thermally stable they undergo Si—C and C—H bond cleavage when heated at about 600-900°C and form a compound of the formula SiCxOy [82]. At 300-600°C a redistribution reaction occurs involving the exchange of Si—O bond [82]. 

The product mixture from a typical MCS reaction is subjected to several distillation and isolation steps. The product mixture can be roughly divided into monomers and residue. The monomers are separated from the residue stream by distillation the residue contains siloxanes and disilanes. Some monomers can be recovered by various redistribution reactions of the residue mixture (15). The individual monomers are separated by distillation where the separation of Di from Tri is difficult. With Di as an example, equation 3 shows the hydrolysis and condensation to form linear and cyclic polysiloxanes. Another useful material is hexamethyldisiloxane (MM) which forms from hydrolysis/condensation of Me3SiCl (mono), equation 4. 

Figures 37a and b show that after sufficiently long reaction times the species distributions resulting from redistribution reactions are nearly identical to the distributions resulting from the hydrolysis of TMOS with one-half equivalent of water r = 0,5) employing 0.05 M HCI or KOH as a catalyst. (See Fig, 34.) This indicates that for dimers, siloxane bond formation is reversible. Furthermore, since redistribution under acidic and basic conditions reproduces the same types of polysilicate distributions generated by hydrolysis-condensation, Klemperer and Ramamurthi concluded that...

There are several explanations for the large concentration of monomers present under neutral and basic conditions. From Fig, 21 we see that the rate of hydrolysis of siloxane bonds increases by over three orders of magnitude between pH 4 and 7. Because hydrolysis occurs preferentially at less highly condensed Q sites [1], monomers are the primary by-product of siloxane bond hydrolysis. In a related study, Klemperer and Ramamurthi [93] have shown that siloxane bonds are broken by redistribution reactions under basic conditions (Eq. 42) that produce unhydrolyzed monomers as a by-product. In addition, from the pH-dependence of the hydrolysis reaction (Fig. 9), we see that the hydrolysis rate is minimized at neutral pH. Because the rate constant of the alcohol-producing condensation reaction is less than that of the water-producing reaction [63,95], unhydrolyzed or partially hydrolyzed monomers may persist in solution past the gel point. Presumably these combined factors contribute to the large concentrations of monomers observed under neutral and basic conditions. 

Redistributions on a siloxane was first observed as a side reaction during the chloroplatinic acid-catalyzed hydrosilation of 2-hexene with the heptamethyltrisiloxane, R2MeSiH (R = Me3SiO) (64). In addition to the expected product [R2MeSi (n-hexyl)], SiMe2R (n-hexyl) and R3SiH were also obtained. 

The reaction of PHMS to yield PMMS-type comb polysiloxanes is essentially quantitative for methoxypoly(ethylene glycol)s of up to 500. Substitution yields diminish for longer glycols, as indicated by the presence of residual Si-H groups in the IR and NMR spectra. Si NMR spectra and GPC data revealed that PM MS-8 is contaminated with nearly 25% cyclic products and that it also contained a considerable number of branched trisiloxy units. Both cyclic products and trisiloxy units are the result of redistribution processes common in nucleophilic siloxane reactions. The hydrosilylation reaction yielding the PAGS polymers is also quantitative, but no cyclic products are formed. 

As indicated in Table 1, the disiloxanes in the feed stream are reduced in concentration during the distillation process. H. F. Stewart of Dow Coming first discovered the redistribution of SiH and SiOSi bonds at 70 - 120 °C [4]. This reaction in the distillation system results in the conversion of SiH-containing siloxanes to usable monomer and higher siloxanes. It has the net effect of reducing the overall SiH content in the DPR and the recovery of valuable trichlotosilane monomer. 

Although there are a variety of routes presently available for siloxane production, only two are of commercial importance. These include hydrolytic reactions of organohalosilanes or organoalkoxysilanes and redistribution type polymerizations of... 

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