Polymerization of bis(triethoxysilyl) ethenes. Impact of substitution geometry on the formation of ethenylene- and vinylidene-bridged polysilsesquioxanes
Da. Loy et al., Polymerization of bis(triethoxysilyl) ethenes. Impact of substitution geometry on the formation of ethenylene- and vinylidene-bridged polysilsesquioxanes, CHEM MATER, 10(12), 1998, pp. 4129-4140
In this study, we utilized the substitution geometry of triethoxysilyl grou
ps about an organic bridging group to control the outcome of the sol-gel po
lymerization process. The substitution geometry of two triethoxysilyl group
s about a carbon-carbon double bond was determined to have a profound effec
t on sol-gel polymerizations of the E (1) and Z (2) ethenylene-bridged mono
mers and vinylidene-bridged monomer (3) and on the porosity in the resultin
g xerogels. Si-29 NMR and chemical ionization mass spectrometry were used t
o elucidate the early sol-gel chemistry in the acid-catalyzed polymerizatio
ns of 1-3. Trans substitution about the ethenylene-bridging group in 1 led
to acyclic and monocyclic dimers and trimers as condensation products under
acidic conditions and only microporous gels under both acidic and basic co
nditions. In contrast, cyclization reactions dominated the sol-gel chemistr
y of 2 beginning with intramolecular cyclization to give the cyclic disilse
s-quioxane (4) and continued with the formation of cyclic oligomers, includ
ing a bicyclic dimer. The cyclization of 2 slowed the rate of gelation comp
ared to 1 and afforded microporous xerogels under acidic conditions and mes
oporous gels under basic conditions. The sol-gel chemistry of the vinyliden
e monomer (3) was strongly retarded by the formation of a cyclic dimer (5).
Only mesoporous gels were formed under basic conditions after 9 months; no
gels were obtained under acidic conditions.