Silica-alumina catalysts for polymerization of cyclic siloxanes

Abstract

Silica-alumina catalysts of varying acidity were used for polymerization of hexamethyl cyclotrisiloxane and octamethyl cyclotetrasiloxane monomers (cyclic trimer and tetramer, respectively). Acidic sites of silica-alumina are responsible for the polymerization and variation in the acidity of the catalyst were shown to influence the polymerization significantly. Mordenite type zeolites with low silica-to-alumina ratio (Ç 5) gave low yields of polymer from tetramer and zeolite ZSM-5 with a much higher silica-to-alumina ratio ( Ç 40) was found to be a very efficient catalyst for polymerization. Studies on the effect of water on the polymerization suggested that the polymerization (rate and yield) depended to some extent on the amount of water present in the system. The products were characterized using FTIR,Si-NMR, and GPC techniques. A plausible mechanism for polymerization of cyclic siloxane monomers on silica-alumina catalysts was proposed.

Hydrophobization of Inorganic Oxide Surfaces via Ring-Opening Polymerization of Cyclic Siloxane Vapor

“…On the other hand, surface bound water is known to increase the acidity/basicity of inorganic oxide surfaces. 54,55 Since cyclic siloxanes are well-known to polymerize in the presence of acids or bases, direct silanolysis or metalolysis is also a probable outcome, wherein water would serve as the "catalyst" to activate the acid/base surface groups. The exact role that water plays in this mechanism is a bit academic as the ultimate result remains the same: covalent attachment of siloxane chains.…”

Properties of Dicationic Disiloxane Ionic Liquids

“…The disproportionation reaction of symmetrical disiloxanes proceeds by the mechanism of hydrolytic condensation in the presence of water adsorbed on the surface of strongly acidic cation exchange resin in the H + form ( Scheme 3 ). Purolite CT175 resin, like some other solid catalysts, is most effective when it contains 10 wt % of physically adsorbed water [ 20 , 21 , 22 , 23 ].…”

“…The disproportionation reaction of symmetrical disiloxanes proceeds by the mechanism of hydrolytic condensation in the presence of water adsorbed on the surface of strongly acidic cation exchange resin in the H + form (Scheme 3). Purolite CT175 resin, like some other solid catalysts, is most effective when it contains 10 wt % of physically adsorbed water [20][21][22][23]. The target product-asymmetric disiloxane-is formed as a result of condensation of two different silanols in the bulk of the reaction mixture after splitting of symmetrical disiloxane by the active center of an acid catalyst (Scheme 3).…”

Polymerization of Vinyl Ether by Use of Yttrium Silicate Gel as Solid Acid

“…[9][10][11][12] In these ring opening reactions, the silicate structure is recognized to provide an appropriate reaction environment with Brønsted and Lewis acidity. In addition, the solid catalyst, which can be removed readily from the reaction system by a simple filtration, should provide a convenient and practical procedure in polymer synthesis.…”

“…[3][4][5][6][7] Furthermore, the solid silica derivatives containing the metals such as aluminum, chromium, zirconium, and vanadium have been employed for the well defined ring opening polymerization of 1,2-epoxide 8 and lactones. [9][10][11][12] In these ring opening reactions, the silicate structure is recognized to provide an appropriate reaction environment with Brønsted and Lewis acidity. In addition, the solid catalyst, which can be removed readily from the reaction system by a simple filtration, should provide a convenient and practical procedure in polymer synthesis.…”