Metal silanolates

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate polymerization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. 

Laganis, E. D. Chenard, B. L. Metal Silanolates Organic Soluble Equivalents for 0-2, Tetrahedron Lett. 1984, 25, 5831-5834. 

Base-Initiated Polymerization. Although the base-initiated mechanism for reaction 2 was once thought to proceed via free ionic intermediates 31), little evidence supported this mechanism. Conductivities of solutions of alkali metal silanolates in moderately polar solvents are essentially nil (32), and most workers now agree that reactive intermediates consist of ion pairs or charge-separated ion pairs 4-5). The key intermediate is believed to involve coordination of the countercation of the ion pair at the chain end, for example potassium, with the cyclosiloxane in a manner analogous to the crown ethers or cryptates (33), as shown by structure 1. 

The catalyst in these studies was a tetramethylammonium silanolate. [SiOH]o represents the initial trimethylsilanol concentration, and ky, and kc are rate constants. Use of alkali metal silanolates complicated the kinetics because of the self-association outlined in reaction 10. A reaction scheme consistent with the rate equation is given by equations 25-27. 

The use of alkali metal hydroxides to catalyse the polymerization of cyclodiorganopolysiloxanes was first disclosed by Hyde13 in 1949. He observed that initially, the metal hydroxide reacts with the cyclosiloxane to form a metal silanolate which he found to be capable of catalysing the polymerization. Grubb and Osthoff2 demonstrated that the rates of polymerization of octamethylcyclotetrasiloxane (D4) catalysed by equimolar amounts of potassium hydroxide and potassium silanolate were... 

In solution the primary aggregates will exist in dynamic equilibrium with smaller aggregates and we may consider the following sequence of equilibria to be representative of the metal silanolate (AM) in non-polar solvents ... 

Several of the metal silanolate species involved in these equilibria have the potential to catalyse the polymerization of cyclosiloxanes. However, it is probable that the primary catalytic species is the ion pair but this is not firmly established. If the rate of reaction of the ion pair with the cyclosiloxane... 

The reactivity of the metal silanolate catalyst is dependent on the nature of the metal counter-ion, the larger metal ions giving rise to more active catalysts. For example, in the metal silanolate series the order of catalyst activity23 is Liquaternary ammonium and quaternary phosphonium silanolates having the same order of activity as caesium silanolate. Lithium and sodium silanolates are not very powerful catalysts for cyclosiloxane polymerization unless used in conjunction with an activating solvent such as tetrahydrofuran (THF) or dimethyl sulphoxide (DMSO). 

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. 

The stepwise synthesis of Si-0 ehains and rings is feasible from alkali metal silanolates and fluorosilanes. The first heptasiloxane was prepared in this way (Eq. 

The structures of ether complexes alkali metal silanolates are associated, HMPA ones tetrameric while [PhMe2SiOK(CgHg)]4 shows CgH to K, KBa3(OSiHi3)5(DME)2 occurs as a bridged metal triangle, (Ph3SiO)3Bi(THF)3 is cis and (Ph3 iO)3Bi a polymer . Siloxy derivatives have been made for Ti and Zr, V, Nb and Ta, Mo and W, Fe and Co, Ni, Cu and Zn, and Y and the rare earths . ... 

Alekhin NN (1975) Investigation of polymerization of 1,1-dimethyl-1-silacyclobutane in the presence of alkali-metal silanolates, alkoholates, and alkatis. Ph D Thesis Academy of Sciences of the SSSR Topchiev Institute of Petrochemical Synthesis, Moscow... 

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