Anionic polymerization silanolates

Anionic Polymerization of Cyclic Siloxanes. The anionic polymerization of cyclosiloxanes can be performed in the presence of a wide variety of strong bases such as hydroxides, alcoholates, or silanolates of alkaH metals (59,68). Commercially, the most important catalyst is potassium silanolate. The activity of the alkaH metal hydroxides increases in the foUowing sequence LiOH < NaOH < KOH < CsOH, which is also the order in which the degree of ionization of thein hydroxides increases (90). Another important class of catalysts is tetraalkyl ammonium, phosphonium hydroxides, and silanolates (91—93). These catalysts undergo thermal degradation when the polymer is heated above the temperature requited (typically >150°C) to decompose the catalyst, giving volatile products and the neutral, thermally stable polymer. 

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). 

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. 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

The anionic polymerization of cyclic siloxanes can be initiated by alkali metal hydroxides, alkyls, and alkoxides, silanolates such as potassium trimethylsilanoate, (CH3)3SiOK, and other bases. Both initiation... 

Alkoxide-Type Initiators. Using the guide that an appropriate initiator should have approximately the same structure and reactivity as the propagating anionic species (see Table 1), alkoxide, thioalkoxide, carboxylate, and silanolate salts would be expected to be useful initiators for the anionic polymerization of epoxides, thiiranes, lactones, and siloxanes, respectively (106—108). Thus low molecular weight poly(ethylene oxide) can be prepared... 

The complex Me3SiCH2SiMe20Li (2) (7Li shift 0.74 ppm) is the only product identified in the reaction of 1 (7Li shift 2.5 ppm) with D3 or D4 at 20 °C in toluene even in the presence of excess siloxane, as shown in equation 1. Addition of the cryptand [211] shifted the Li resonance to —0.93 ppm in agreement with other lithium cryptand [211] complexes. This lithium silanolate was then shown to initiate polymerization of D3, D4, Dg and functional cyclics such as (SiMe(HC=CH2)0)4 and (SiMe(CH2CH2CF3)0)3. Kinetic measurements using this initiator show a reactivity order of D3 D4 >Ds >Dg and the results are in good agreement with those previously reported for anionic polymerization under similar conditions. Co-polymerization reaction involving vinyl dimethyl cyclics... 

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 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 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 kinetically controlled living anionic polymerizations of first D/ and then D3 were carried out in two steps under argon at room temperature. Purified D4 was first allowed to react with the initiator in THF for 3 h, in order to quantitatively afford lithium silanolates. Then, the activating agent, [12]crown-4, was added to the stirred mixture. After five days, when up to 80% of the D/ (Mw/Mn 1.40) had been consumed, a solution of Dj in THF was introduced (Eq. 8). 

In contrast to the polymerization of D4, the anionic polymerization of hexamethyl cyclotrisiloxane (D3) with lithium as counterion is a living polymerization which produces polydimethylsiloxanes with well-defined structures. Useful initiators include lithium silanolates or the product from the reaction of 3 mol of butyllithium with D3 in a hydrocarbon solvent as shown in Scheme 7.19. It is noteworthy that no polymerization occurs in the absence of a Lewis base promoter such as THF, glymes, DMSO, or HMPA. 

Anionic polymerization can be carried out by metal hydroxides or even by lithium salts of silanols. This method of polymerization affords high-molecular-weight polymers. 

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. 

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