Cyclosiloxanes cationic polymerization

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. 

The very straightforward results concerning the mechanism of propagation and cyclization in the polymerization of cyclosiloxanes were obtained by studying the radiation-induced cationic polymerization of 6-, 8-, and 10-membered cyclic siloxanes (D3, D4, Ds) [252,253]. 

This kind of mechanism is observed in cationic polymerization of cyclosiloxanes ... 

Cationic polymerization of D2 at room temperature initiated by EtsSiH + PhsC B(C6Fs)4 proceeds fast and chemoselectively with exclusive siloxane bond cleavage and reformation. Taking into account the similarity of cationic polymerization of D2 to that of D4, this reaction is a good model for study of the role of cyclic trisilyloxonium ions in the cationic polymerization of cyclosiloxanes. 

Cationic polymerization of cyclosiloxanes is carried out with strong protonic or Lewis acids. Industrially important catalysts of this type are perfluoroalkanesulfonic acids and/or sulfuric acid (H2SO4). 

Ionic polymerization (f) Cationic polymerization of permethyl cyclosiloxanes with tetraalkylammonium chloride surfactants in alkaline aqueous solutions 133... 

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. 

Cationic polymerization has witnessed an intense development in the middle of the twentieth century after it could be successfully applied to polymerize certain ethylenic hydrocarbons such as isobutene, carbonyl monomers such as formaldehyde, or cyclic ethers such as oxiranes, tetrahydrofuran, and cyclosiloxanes. 

Olah GA, Li XY, Wang Q, Rasul G, Prakash GKS (1995) Trisilyloxonium ions preparation, NMR spectroscopy, ah initio/IGLO studies, and their role in cationic polymerization of cyclosiloxanes. J Am Chem Soc 117 8962... 

A large effective cation size should suppress the cation-siloxane coordination, favor the Iree ion pair 3 in reaction 10, and enhance charge separation. The anticipated effects would be values of n approaching 1 in the rate equation 9, greatly enhanced rates of polymerization, and suppressed formation of cyclosiloxanes. Evidence that these effects are achieved is indicated by the effects seen with R4N countercations (35), the lithium cryp-tates (25, 27), and the crown ether-potassium silanolate complexes (39, 40). Additional evidence for the influence of the countercation on the equilibria is seen in deviations of the amounts of oligomer produced in equilibrated poly(dimethylsiloxane) from the normal distribution caused by specific interactions between the potassium silanolate chain ends (37, 38). More de-... 

Mechanism of Equilibration. The generally accepted mechanism for the base-catalyzed ring-opening polymerization of cyclosiloxanes involves attack of the basic catalyst at the silicon atom (15). It has been proposed, and generally accepted, that the active species is a partially dissociated siloxanolate anion (13). In the results presented in this chapter, significant differences in reaction rates were observed as the corresponding cation of the siloxanolate species was varied. The more rapid disappearance of D4 and aminopropyldisiloxane in the presence of these catalysts increased in the following order ... 

The structure of the active propagation center in the cationic ring-opening polymerization (ROP) of cyclosiloxanes is still controversial. Trisilyloxonium ions generated from hexamethyl-cyclotrisiloxane, D3, and octamethylcyclotetrasiloxane, D4, were observed at low temperature by Olah et al. [I] and their participation as intermediates in cationic ROP of cyclosiloxanes was postulated. However, the main objection against this mechanistic concept is a relatively low rate of polymerization when an initiator able to form persistent tertiary oxonium ions is used [2]. 

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]. 

Polymerization of cyclosiloxane oligomers by equilibration both anionically (with bases) and cationically (acids)... 

Cyclosiloxanes, cationic ring-opening polymerization of 85MI16. Cyclosiloxanes, structures of 80UK518. 

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