Cyclosiloxanes anionic polymerization

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). 

Cyclosilazanes are found to be reluctant to polymerize by the ring-opening process, probably for thermodynamic reasons. On the other hand, six- and eight-membered silazoxane rings are able to undergo anionic polymerization under similar conditions to those which have been widely used for cyclosiloxane polymerization provided there is no more than two silazane units in the cyclic monomer. They can also copolymerize with cyclosiloxanes however, the chain length of the linear polymer formed is substantially decreased with increasing proportion of silazane units. 

With the purpose of increasing the range of available block copolymers, comonomers other than methacrylates and acrylates can also be involved in sequential polymerization, provided that they are susceptible to anionic polymerization. Dienes, styrene derivatives, vinylpyridines , oxiranes and cyclosiloxanes are examples of such comonomers. The order of the sequential addition is, however, of critical importance for the synthesis to be successful. Indeed, the pX a of the conjugated acid of the living chain-end of the first block must be at least equal to or even larger than that of the second monomer. Translated to a nucleophilicity scale, this pK effect results in the following order of reactivity dienes styrenes > vinylpyridines > methacrylates and acrylates > oxiranes > siloxanes. 

Summary Phosfdiazene bases represent a new class of highly active non-ionic catalysts that rapidly polymerize cyclosiloxanes with equilibrium attained in very short reaction times at very low catalyst levels. To date, phosphazene base catalysts have been considered an academic curiosity because of the complicated and hazardous synthetic protocol used to prepare them. A facile synthetic process has been developed, which yields ionic phosphazene bases in three steps with an overall yield of qrproximately 75%. This is achieved through nucleophilic substitution of ionic phosphonitrilic chloride oligomers with secondary amines, followed by anion exchange. These ionic phosphazenes were found to exhibit similar reactivity in the ring-opening polymerization of cyclosiloxanes to that of the non-ionic phosphazene base. 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

Anionic Polymerization of Cyclosiloxanes with Cryptates as Counterions... 

The anionic polymerization of cyclosiloxanes was examined In benzene and toluene with lithium cryptates as counterions. Only one type of active species Is observed In the case of LI + [211] thus, the kinetics of the propagation and of the by-product cyclosiloxanes formation can be studied In detail for the first time. The reactivity of cryptated sllanolate Ion pairs toward the ring opening of D3 Is greatly enhanced compared to that of other systems. 

The anionic polymerization of cyclosiloxanes has been studied for a long time. However the knowledge of the mechanism of the process Is still unfortunately rather limited > This Is partially due to... 

Anionic polymerization of cyclosiloxane species of more than four siloxane... 

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 anionic polymerization is initiated by the reaction of the base with a cyclosiloxane such as [Me2SiO]4 to generate the open-chain compound with a Si-D K" end-group (See Eq. 6.18). 

Anionic polymerization of permethyl cyclosiloxanes with dodecylbenzene sulfonic acid as both catalyst and surfactants in acid aqueous solutions 134... 

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. 

In this chapter we consider the two general methods of ROP of cyclosiloxanes (i) the equilibrium polymerization, which is commonly used when the polymer yield at equilibrium is relatively high and (ii) the non-equilibrium polymerization, which is quenched before equilibrium is attained. Moreover, both anionic and cationic processes-the mechanisms of which are quite complex-can be used for the ROP of cyclosiloxanes, and are treated independently here. A special emphasis is placed on both cationic and anionic polymerizations of cyclosiloxanes in aqueous media throughout the chapter, as this represents an original and sustainable... 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyclosiloxanes. Aromatic radical anions... 

Cationic polymerization of cyclosiloxanes is well known but used much less frequently than anionic reactions. The most widely used catalysts include sulfuric acid and its derivatives, alkyl and aryl sulfonic acids and trifluoroacetic acid1 2,1221. Due to their ease of removal, in industrial applications acid catalysts are generally employed on supports such as bentonite clay or Fuller s earth. 

Rings Ha, b and c were found to polymerize in bulk with DMSO as an activator and various catalytic amounts of the anionic initiator, potassium poly(dimethylsiloxane)diolate (for polymerization of Ila and b, see (22)). The latter is well known as an efficient initiator of cyclosiloxane polymerization. Thus... 

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