| Abstract
| - Effects of the structure of an alkoxysilane on the early reaction kinetics of sol−gel polymerization areinvestigated in basic and acidic conditions. Six silanes, including tetraethoxysilane (TEOS), tetramethoxysilane(TMOS), methyltrimethoxysilane (MTMS), bis(trimethoxysilyl)ethane (BTMSE), bis(trimethoxysilyl)hexane(BTMSH), and bis(trimethoxysilylpropyl)amine (BTMSPA), are selected to examine the effects of progressivechanges in the silane's structure. In basic conditions, reactivities of the silanes follow the pairwise trendsTEOS ≪ TMOS, TMOS > MTMS, MTMS < BTMSE, BTMSE ≫ BTMSH, and BTMSH ≪ BTMSPA.Hydrolysis rate coefficients of BTMSE, TMOS, MTMS, and BTMSPA are similar in value and are almostone order of magnitude larger than the coefficients of BTMSH and TEOS. In acidic conditions, the pairwisereactivity trends are TEOS ≈ TMOS, TMOS < MTMS, MTMS > BTMSE, BTMSE ≫ BTMSH, and BTMSH> BTMSPA. The condensation rate coefficients of BTMSH and BTMPA are much smaller than the others.Theses relationships can be explained mainly on the basis of the expected inductive and steric factors of thesilanes. Bulky organics attached to the silane lead to low reactivity in both acidic and basic conditions. Ahigh electron density at silicon leads to a low reactivity in basic conditions but a high reactivity in acidicconditions (pH ∼2.3). Only BTMSPA in basic conditions defies these trends, possibly because of the effectsof solvation near the reactive silicon site.
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