Silanol ions

The kinetics of this process is strongly affected by an association phenomenon. It has been known that the active center is the silanolate ion pair, which is in equUibrium with dormant ion pair complexes (99,100). The polymerization of cyclosiloxanes in the presence of potassium silanolate shows the kinetic order 0.5 with respect to the initiator, which suggests the principal role of dimer complexes (101). 

The hydrogen-bond donor and acceptor properties of a packing are another dimension of column selectivity. They are independent of the silanol ion-exchange activity described above, and affect the selectivity of a packing in a separate way. We have measured these properties by the relative retention between butylparaben, a hydrogen-bond donor, and dipropylphthalate. The same property, or at least a very similar property, has been called hydrogen bond basicity by Snyder.In our case, it has been defined as follows ... 

The validity of this expression indicated that the reaction between silanol and methanol was essentially complete. Kt decreased in going from MeOH to EtOH to Jt-PrOH (i. e.( relatively more -SiOSi- is present at equilibrium) and the equilibrium was reached more slowly if bulky organic groups were attached to the silicon. A mechanism consistent with the observed facts is back-side displacement of the alkoxy group by silanol or by silanolate ion. 

Under certain conditions (for instance, high [TES] or low water concentrations), the reaction between not fully hydrolyzed species must also be considered. In this case the condensation and hydrolysis reactions can occur in one step After attack of a silanolate ion, the transition state can subsequently lose an alkoxy group. Alkoxy groups that end up on a highly condensed unit are then much more difficult to hydrolyze because of steric constraints. 

The mechanism proposed by Grubb and Osthoflf2 for the bulk polymerization of D4 catalysed by KOH is based on the assumption that the potassium silanolate dissociates into silanolate ions which are the active catalytic species in the polymerization. 

The anionic polymerization of cyclosiloxanes is a complex process. For the alkali metal silanolate catalysts the weight of experimental evidence supports a mechanism based on growth from the metal silanolate ion pair. The ion pair is in dynamic equilibrium with ion-pair dimers which, for the smaller alkali metal ions like lithium and sodium, are themselves in dynamic equilibrium with ion-pair dimer aggregates. The fractional order in catalyst which is observed is a direct result of the equilibria between ion pairs, ion-pair dimers and ion-pair dimer aggregates. Polar solvents break down the aggregates and increase the concentration of ion-pair dimers and hence the concentration of ion pairs. Species like crown ethers and the [2.1.1] cryptate which form strong complexes with the metal cation increase the dissociation of ion-pair dimers into ion pairs. In the case of the lithium [2.1.1] cryptate dissociation into ion pairs is complete and the order in catalyst is unity. 

---