Polymerization of bis(triethoxysilyl)ethenes. The impact of substitution geometry on the formation of ethenyl- and vinylidene-bridged polysilsesquioxanes Page: 4 of 5
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siloxane bonds at any one silicon atom. These silicon atoms are considered to
be trifunctional with respect to the alkoxide ligands and are denoted as Tn
which is a modification of the General Electric Q notation.6 The reaction of 1
with 1 eq. of water from 1N HCI after 24 hour gave a 29Si NMR spectrum
which consisted of two main peaks of roughly equal intensity at -61.5 ppm
and -68.7 ppm. The chemical shift of -61.5 ppm is the same as that for
unreacted monomer and must arise from T0 silicon nuclei while the second
signal is shifted 7.2 ppm upfield and is thus assigned to T1 silicon nuclei. No
T2 or T3 silicons are observed under the controlled hydrolysis and
condensation reactions with 1 eq. of H20 indicating that oligomerization is
occurring by linear acyclic chain growth The reaction was also followed by
mass spectroscopy (Cl MS, ammonia) and the two major ions observed were
at m/z 852 [trimer+NH3-EtOH] and 648 [dimer+NH4] . These two ions
are assigned to acyclic timer and dimer which is consistent with acyclic
chain growth. This type of polymerization is expected under acidic
conditions.
The addition of 2 eq. of water from 1 N HCl to a solution of 2 results in
a 29Si NMR spectrum which has two signals of equal intensity at -65.9 and
-68.6 ppm which are upfield from the starting monomer (2, -61.9 ppm). The
mass spectrum of the reaction shows basically one major product which is
represented by the parent ions at m/z 500 [4+NH4] and 483 [4+H] . The
only structure that is consistent with the spectral data is the bicyclic dimer 4
shown in Scheme I. The observed chemical shifts for the T1 and T2 silicons
are not shifted as far upfield as is normally expected for such condensation
products (ca 7-8 ppm). This is due to the ring strain present in the bicyclic
structure which shifts the signals downfield from where they would normally
appear in an acyclic product. A small concentration of trimer (m/z = 704
+ +
[trimer+NH4] and 687 [trimer+H] ) and tetramer (m/z = 909 [tetramer
+NH4] and 846 [tetramer-OEt] ) are observed in the mass spectrum and
these masses are consistent with the additional cyclization products shown in
Scheme I.
The same reaction conditions employed for 1 and 2 were repeated for 3
using 1 eq. of H20. Initially, only the hydrolysis products of 3 were observed
in the 29Si NMR spectrum. After 24 hours, the dominant peak in the
spectrum appeared at 6Si = -66.1 ppm which is shifted 8.1 ppm upfield from
the starting monomer 3. The mass spectrum shows only the presence of
monomer and dimer. The masses of m/z 574 [5+NH4] and 557 [5+H] and
the 29Si NMR spectrum are consistent with the dimer of 3 having the cyclic
structure (5) shown in Scheme I. Apparently, this structure is the bottleneck
in the polymerization of 3 since there are no higher oligomers present after 24
hours even though hydrolysis products of 3 and 5 are observed in the mass
spectrum.
Characterization of Xerogels. To determine what effect the different
polymerizaton processes have on the final properties of the polymers, the
xerogels prepared from each monomer were studied by nitrogen sorption
porosimetry and scanning electron microscopy. Due to the difficulty in
polymerizing 3 to a highly condensed xerogel, most of the analytical data was
obtained on ethenylene-bridged polysilsesquioxanes prepared from
monomers 1 and 2. Nitrogen sorption porosimetry was utilized to evaluate the
pore structure and surface area of each xerogel. The surface areas were
determined by the multi-point BET method and are tabulated along with
mean pore diameters (BJH) and pore volumes in Table II. Most of the
ethenylene-bridged polysilsesquioxanes are characterized by a small average
pore diameter of 22-24 A. However, the xerogels prepared from 2 under
basic conditions, which precipitated rather than geled, had average pore
diameters of 46-47 A and xerogel 3B had an average pore diameter of 62 A.
A general trend in the ethenylene-bridged polysilsesquioxanes is that xerogels
prepared using a basic catalyst exhibit much higher surface areas (473-779
m2/g) than those prepared with an acid catalyst (307-447 m2/g).Table 2. Summary of Surface Areas and Pore Diameters Determined by
Nitrogen Sorption Porosimetry for Materials from 1, 2, and 3.
Polymer BET Surface Areas Av. Pore Diam. Pore Vol.
(m2/g) (A) (cc/g)
lA 352 22 0.190
1B 691 22 0.387
2A 447 22 0.248
2B 473 47 0.561
3B 631 62 0.973
lA-THF 379 22 0,206
1B-THIF 779 24 0.466
2A-THF 307 24 0.180
2B-THF 742 46 0.850
Pore size distributions derived from the BJH method were studied to
determine which pore sizes contribute to the pore volume and to what extent.
The polymers prepared from 1 and 2 under acidic conditions were found to be
almost exclusively microporous (pore diameter < 20 A) with small
contributions to the pore volume from the lower end of the mesopore range
(pore diameter 20-50 A). The materials prepared from 1 under basic
conditions exhibit a similar porosity to those prepared under acidic
conditions. However, the polymerization of 2 utilizing a basic catalyst
yielded materials with very broad distributions of pore sizes. When the
polymerization is performed in ethanol, precipitation occurs almost
immediately and a material (2B) is obtained that shows significant pore
volume throughout the mesopore regime with a spike occurring in pores with
a diameter around 90 A. The polymer prepared from 2 in THF shows a
similar mesoporosity. The pore size distributions account for the larger
average pore diameters observed in 2B and 2B-THF.
To provide some insight into the origin of the meso and macroporosity
in materials 2B, 2B-THF, and 3B the polymers were examined by scanning
electron microscopy. The electron micrographs shown in Figure 1 show
distinctly different morphologies for the polymers prepared from 1 and 2.
Xerogel 1B exhibits a very smooth morphology with very fine features
consistent with the microporous structure identified in the porosimetry studies.
However, 2B has a very rough surface composed of globular structures and
fused particulates with diameters of approximately 50-150 nm. The materials
prepared in THF show a similar pattern. The voids between the particles and
globules which make up xerogels 2B and 2B-THF are most certainly the
source of the meso and macroporosity observed in these materials.Although the porosimetry studies did not reveal any profound
differences between the acid-catalyzed gels of 1 and 2, SEM studies exposed
significantly different morphologies similar to those observed in the base-
catalyzed materials. Both 1A and 1A-THF have continuous textures with no
well defined particulates while 2A and 2A-THF are characterized by rough
particulate or globular structures that are much finer (< 50 nm in diameter)
than those produced from 2 under basic conditions. The formation of
mesoporous or macroporous domains in 2A and 2A-THF is precluded by the
smaller particulates from which the xerogels are comprised.
Conclusions
The sol-gel polymerization studies of monomers 1, 2, and 3 reveal
unique reactivity patterns for each monomer under acidic conditions. Acyclic
linear chains predominate in the polymerization of 1 while the hydrolysis and
condensation of 2 is characterized by the formation of seven-membered rings
to build up the polymer network. The polymerization of 3 is apparently
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Carpenter, J. P.; Yamanaka, S. A.; McClain, M. D.; Loy, D. A.; Greaves, J. & Shea, K. J. Polymerization of bis(triethoxysilyl)ethenes. The impact of substitution geometry on the formation of ethenyl- and vinylidene-bridged polysilsesquioxanes, article, July 1, 1998; United States. (https://digital.library.unt.edu/ark:/67531/metadc702270/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.