Siloxanes redistribution

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. 

General route for the Preparation of a,oo-Organofunctionally-Tenninated Siloxane Oligomers by Equilibration (Redistribution) Reactions8... 

No evidence of a redistribution of Me and H groups [Eq. (50)], was observed with these siloxanes. Unsaturated adducts from Eq. (53) showed up see Table X. 

During the cationic polymerisation, e.g. with sulfuric acid, the process is the following at the initial stage of initiation, when organocyclosiloxanes interact with sulfuric acid, the acid proton attacks the oxygen atom of the siloxane cycle. As a result of the redistribution of the electron density, the =Si-0 bond breaks, opening the cycle and forming an active centre at the end of the chain ... 

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 redistributions of pentamethyldisiloxane EE and hexamethyltri-siloxane E DE were reinvestigated using our computerized data system. The purpose of these experiments was to gain some insight into the... 

With one exception (the Me/Cl exchange in Table I), all redistributions of siloxanes catalyzed by transition metals observed to date occur only when at least one silicon-hydrogen bond is present in the molecule. We have established that hexamethyldisiloxane (EE) and hexamethylcyclotri-siloxane (D3) are completely inert under the same conditions that cause extensive redistribution of hydridosiloxanes. 

Si NMR of siloxane systems has been used in studying organosilicone containing block copolymers to determine block length and chemical redistribution during polymerization. (85) Block copolymers of bisphenol A polycarbonate (BPAP) and polydimethyl-siloxane (PDMS) [20] were studied by both and Si NMR to determine a variety of structural parameters. 

The redistribution of siloxane units between chains via equilibrium 3... 

Otherwise, the method can be very misleading it failed badly when published data from certain copolymerizations were scrutinized by a specially developed analytical method for copolymerization data (52, 53). For these reasons, any published data on copolymerization reactivity ratios must be approached with caution, and the reader should be assured that the criteria just mentioned have been properly met. These precautions are especially true for siloxane copolymerizations, because reversibility can be established quickly, and rapid redistribution of comonomer units between rings and chains via equilibria 2 and 3 will confound any meaningful study of comonomer reactivity ratios. 

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... 

As predicted by theory, the position of the ring/chain equilibrium was found to be independent of the nature of the redistribution catalyst employed (acid or base) (4,13,24- ) and of the specific inert solvent used (26). Russian authors (4,27) equilibrated mixtures of eyelosiloxanes comprised of dimethylsiloxane (75 mole %) and either trifluoropropylmethyl, cyanoethylmethyl, or cyanopropylmethyl siloxane (25 mole %) in acetone at a siloxane repeating unit concentration of 0.833 moles/A. They measured the dipole moments of the respective cyclosiloxanes, [(CH3)9SiO]3[Si(CH3)R0j>, to be 2.76 for R = trifluoropropyl, 3.45 for R cyanoethyl, and 3.58 in the case of R cyanopropyl. The equilibrium weight fraction of rings,... 

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