29Si NMR Study of Distribution of Oligomers in Polycondensation of Tetraethoxysilane

Abstract

29Si NMR spectrometry has been used to determine the number molecular weight distribution of polymerization degrees of a system of oligomers formed in the sol-gel polycondensation of tetraethoxysilane (TEOS). On the basis of identification of the oligomers, monitoring of the concentration changes during the initial phase of the sol-gel polycondensation, and from the deviations observed between the experimentally determined number molecular weight distribution of polymerization degrees and the Flory-Stockmayer model it has been proved that cyclization represents a significant process in the acid-catalyzed polycondensation with substoichiometric amount of water (ratio of amounts TEOS/H2O = 1 : 1). The cyclization becomes important at the moment when the reaction mixture contains increased amounts of the linear tetramer and pentamer. The cyclization products form a basis for formation of polycyclic, highly condensed building units whose gradual aggregation produces the gel.

29Si NMR and UV−Raman Investigation of Initial Oligomerization Reaction Pathways in Acid-Catalyzed Silica Sol−Gel Chemistry

“…Several authors adhere to a model of linear chain growth, 8,14,19 while others reported cyclization reactions competing with chain extension. 57,58 Theoretical predictions of gelation times were in better agreement with experimental data when cyclization was accounted for. 57,23 Depending on the alkoxide, the contribution of cyclization was found to increase with increasing r-value.…”

“…The latter were assigned to linear trimer (δ = -96.70 ppm) and tetramer (δ = -96.87 ppm). 10,58 The signals of the Si atoms terminating these chains, Q 1 , were observed at δ = -89.5 ppm, clearly different from the Q 1 signal of the dimer. This interpretation of the 29 Si NMR spectroscopic signature is in full accordance with the literature and confirmed by molecular modeling (Table 2).…”

Hyperbranched Polyalkoxysiloxanes via AB3‐Type Monomers

“…According to ref. [23,25] these peaks can be assigned to silicon atoms in cyclic subunits. Corresponding signals can also be detected in the spectra of the polymers obtained via the acetoxy route (Figure 3b and 3c).…”

“…[21,22] Even after prolonged reaction time the products still contain silanol groups. [23][24][25] These silanol groups can undergo further condensation and thus cause aging and changes in the properties upon storage. [26] The fraction of silanol groups can eventually be reduced if the water needed for the reaction is slowly formed in homogenous solution.…”

Effects of a Protic Ionic Liquid on the Reaction Pathway during Non-Aqueous Sol–Gel Synthesis of Silica: A Raman Spectroscopic Investigation

“…In particular, we analyze in detail the frequency evolution of the spectral feature at ~490 cm −1 , and acquire information on the network -Si–O–Si- bond angles and on the size of the silica rings [21]. Along with revealing the effects of adding a protic ionic liquid in the sol-gel synthesis, this work also contributes to the ongoing debate whether the reaction pathway under acidic conditions and sub-stoichiometric water contents follows the model of linear chain growth [22,23], or that of cyclization reactions competing with chain extension [24,25], as also discussed by Depla et al [26]. Finally, this work also aims to highlight the potential of Raman spectroscopy as a multi-faceted technique to characterize ionic liquid derived materials.…”