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.
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