Silanolate anion

The condensation catalyzed by a strong base is first order with respect to substrate and catalyst (74,75). Because of the high acidity of silanol, all the alkah metal base (MtOH) is usually transformed into the silanolate anion. In the rate-determining step, the sdanolate anion attacks the siHcon atom in the silanol end group (eq. 12 and 13). 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

In the reaction of lithiated alkylbenzothiazoles with BTSP, the formation of methylated products always competes with the formation of silyl ethers. The demethylation of BTSP can tentatively be conceived as being derived from the nucleophilic attack of alkylben-zothiazole anion at the methyl group of BTSP with displacement of silanone and silanolate anion, while silyl ethers would form by nucleophilic attack of the alkylbenzothiazole anion at the oxygen of BTSP (Scheme 8/. ... 

Figure 5.5 Scheme illustrating potential reorientation of bonded imidazolium ligands in response to deprotonation of residual silanols. Anion is not shown for clarity. (Adapted from Wang, Q., Baker, G. A., Baker, S. N., and Colon, L. A., Analyst, 131,1000-1005, 2006.)... 

Spliting in any of the indicated ways leads to chain propagation. In non-polar media, free ions only exist for a short time during bond rearrangement. Thus the silanolate group, rather than a silanolate anion, should be regarded as the active centre [320]. Therefore we have proposed a scheme respecting this circumstance ... 

A similar case was later found in the base-catalyzed ringopening polymerization of cyclic siloxanes. In this case, on the basis of kinetic measurements, it was shown (7, that the active species is the free silanolate anion (-Si-0 ) and no true... 

If both the ion pair and the silanolate anion are active catalytic species in the polymerization, then the rate of polymerization of D4 is given by... 

The polymerization is accelerated in solvents of high dielectric constant which do not have the capability to solvate the ion pair. Under these conditions which would favour the dissociation of the ion pair into the free ions it is perhaps reasonable to conclude that the polymerization is catalysed by both the ion pair and the free silanolate anion. 

The behavior of oligosiloxanediols in the presence of strong bases is different. The contribution to the overall process of the disproportionation reaction, involving a migration of the ultimate siloxane unit between siloxane molecules, is much greater and may even completely dominate the polycondensation reaction (80). The reactivity enhancement of the siloxane bond adjacent to the silanolate anion can be understood in terms of n(0) — (7 (SiO) conjugation. 

Bis(dimethylamino)cyclodisilazane catalyzed by a silanolate anion forms two products ... 

Linear polysiloxanes are prepared by anionic and cationic polymerization of cyclosiloxanes (362), such as hexamethylcyclotrisiloxane (D3) and octamethyl-cylotetrasiloxane (D4). Anionic polymerization is initiated by hydroxides, al-coholates, phenolates, silanolates, and siloxanolates. The active species is the silanolate anion. 

According to Pohl and Osterholtz [50], deuteroxide (hydroxyl) anion reversibly reacts with silanetriol in a rapid first step leading to an equilibrium concentration of sHanolate anion, SifOHljO". Silanolate anion reacts with neutral triol in a slower rate-determining step resulting in dialkyl-tetra-hydroxydisiloxane and regeneration of hydroxyl anion. Consistent with this mechanism, the condensation rate is observed to be first-order in deuteroxide anion and second-order in triol, -d[silanetriol]/dl = (ki/ 2// i)[RSl(OD)3] [OD ]. Further condensation of the disiloxane was not observed at short reaction times, presumably due to steric effects. 

It is generally agreed that both hydrolysis and condensation occur by adder base-catalyzed bimolecular nucleophilic substitution reactions involving, e.g., Sf Z-Si, S 2 -Si, or S 2 -Si transition states or intermediates. The acid-catalyzed mechanisms are preceded by rapid protonation of the OR or OH substituents bonded to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Statistical and steric effects are probably most important in influencing the kinetics however. Inductive effects are certainly evident in the hydrolysis of organoalkoxysilanes. 

Phosphazene superbases (stmctures 10 and 11) have recently been explored as extremely effective initiators of the ROP of cyclosiloxanes. " Neutral phosphazene bases (10, 11) require a proton donor, such as methanol, to form the tme initiator, phosphazenium alkoxide (eqns [17] and [18]). Bulky phosphazenium cations are able to very effectively stabilize a positive charge by the resonance effect. The existence of the bare silanolate anion in such systems is very probable. They are also well soluble in the polymerization system. 

Linear polysiloxane can be synthesized by both anionic and cationic polymerizations of cyclic siloxanes such as hexamethylcyclotrisiloxane (n = 3) and octamethyl cyclotetrasiloxane (n = 4). Anionic polymerization is initiated by hydroxide, alkoxides, phenolates, silanolates and siloxoano-lates. The active species in the polymerization is the silanolate anion. Cationic polymerization is initiated by strong protonic acids such as sulfuric acid, trifluoromethane sulfonic acid and trifluoro-acetic acid (equation 53). Both the anionic and the cationic species undergo backbiting reactions during polymerization, such that an equilibrium exists between linear chains and rings. ... 

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