Cyclization Phenomena in the Sol−Gel Polymerization of α,ω-Bis(triethoxysilyl)alkanes and Incorporation of the Cyclic Structures into Network Silsesquioxane Polymers

Abstract

Intramolecular cyclizations during acid-catalyzed sol−gel polymerizations of α,ω-bis(triethoxysilyl)alkanes substantially lengthen gel times for monomers with ethylene (1), propylene (2), and butylene (3) bridging groups. These cyclization reactions were found, using mass spectrometry and 29Si NMR spectroscopy, to lead preferentially to monomeric and dimeric products based on six- and seven-membered disilsesquioxane rings. 1,2-Bis(triethoxysilyl)ethane (1) reacts under acidic conditions to give a bicyclic dimer (5) that is composed of two annelated seven-membered rings. Under the same conditions, 1,3-bis(triethoxysilyl)propane (2), 1,4-bis(triethoxysilyl)butane (3), and Z-1,4-bis(triethoxysilyl)but-2-ene (10) undergo an intramolecular condensation reaction to give the six- and seven-membered cyclic disilsesquioxanes 6, 7, and 11. Subsequently, these cyclic monomers slowly react to form the tricyclic dimers 8, 9, and 12. With NaOH as polymerization catalyst, these cyclic silsesquioxanes readily reacted to afford gels that were shown by CP MAS 29Si NMR and infrared spectroscopies to retain some cyclic structures. Comparison of the porosity and microstructure of xerogels prepared from the cyclic monomers 6 and 7 with those of gels prepared directly from their acyclic precursors 2 and 3 indicates that the final pore structure of the xerogels is markedly dependent on the nature of the precursor. In addition, despite the fact that the monomeric cyclic disilsesquioxane species cannot be isolated from 1−3 under basic conditions due to their rapid rate of gelation, spectroscopic techniques also detected the presence of the cyclic structures in the resulting polymeric gels.

Evaluation of hybrid silica sols for stable microporous membranes using high-throughput screening

“…This is in line with a study by Loy et al, who showed that propane-bridged ethoxysilanes react to a six-membered intramolecular condensation product which further dimerizes to a bicyclic product [23]. Because of the much lower network forming ability of this product, the growth rate of the hybrid silica particles is slowed down.…”

Structure–property tuning in hydrothermally stable sol–gel-processed hybrid organosilica molecular sieving membranes

“…In early studies on hybrid organosilica membranes based on BTESE, it was attempted to avoid the formation of condensed bicyclic carbosiloxanes [7,19]. Such oligomers have been reported to slow down or even inhibit gelation and may not become incorporated in a continuous network structure [22,23]. To lower the statistical chance of forming inert cyclic oligomers, BTESE was mixed with MTES in a 1:1 molar ratio.…”

“…To avoid a wide distribution of sizes in the sol, the hydrolysis is often carried out in two steps so that water-lean conditions are maintained throughout the process [24]. It is noted that in later studies hybrid organosilica membranes were mostly prepared from a single type of alkoxide precursor such as BTESE or bis(triethoxysilyl)methane (BTESM) [12,25,26], although condensation of monomers into relatively inert cyclic dimers that do not contribute to gel formation may have led to a decreased yield in these cases [22,27].…”

Discovery, integration, and interrogation of biotic/abiotic materials and systems.

“…1000-1030 cm -1 upon cyclization). 34 As siloxane network formation proceeds, requiring the opening of the cyclic species, the energy of the v(Si-O) band again increases 34 to ca. 1040-1080 cm , within the region in which cyclized species are expected to occur.…”

“…A degree of condensation figure is difficult to calculate for BTESE materials, as cyclic T 2 and acyclic T 1 signals overlap 34 (analogous to the nomenclature for TEOSbased materials, T n is the fraction of the different T species with connectivity n; the subscript c indicates the signal from a cyclized product). Examination of the NMR data, however, clearly shows that CTAB-templated BTESE contains a higher relative contribution from T 0 and T 1 (cyclized) species than lipid-templated BTESE, consistent with an increase in condensation reactivity when lipid is present.…”