A Kinetic Analysis of the Initial Stages of the Sol−Gel Reactions of Methyltriethoxysilane (MTES) and a Mixed MTES/Tetraethoxysilane System by High-Resolution ²⁹Si NMR Spectroscopy

Abstract

29Si solution NMR is used to follow the kinetics of the methyltriethoxysilane (MTES) homopolymerization and that of the MTES/tetraethoxysilane (TEOS) copolymerization. Individual kinetic rate constants for the hydrolysis and dimerization reactions were obtained by fitting the concentration curves as functions of time for the different intermediate species. It was found that the individual hydrolysis kinetic constants for the MTES homopolymerization are greater than those for the TEOS homopolymerization. Using the homodimerization and codimerization kinetic rate constants for the MTES/TEOS copolymerization, reactivity ratios were calculated. These reactivity ratios suggest that the two monomers react to form a random copolymer.

Characterisation of hybrid xerogels synthesised in acid media using methyltriethoxysilane (MTEOS) and tetraethoxysilane (TEOS) as precursors

“…In HCl-catalysed reactions, hydrolysis is faster than condensation (Devreux et al 1990;Meixner and Dyer 1999), and alkyl-substituted trialkoxysilanes hydrolyse faster than the corresponding tetraalkoxysilanes (Himmel et al 1990). In particular, MTEOS hydrolyses faster than the TEOS monomer during each sequential hydrolysis reaction (Fyfe and Aroca 1997;Jitianu et al 2003). The replacement of the acidic conditions of an alkoxy by non-hydrolysable radicals leads to higher hydrolysis rates because the inductive effect of these radicals lowers the positive charge on the silicon and increases the negative charge on the oxygen atoms around the silicon atom as compared with TEOS (Devreux et al 1990;Jitianu et al 2003).…”

“…The decrease in skeletal density can be attributed to the incorporation of methyl groups into the xerogel and to the reduction of crosslinking. MTEOS/TEOS copolymerisation tends to approximate a random copolymerisation, producing a matrix with a randomly distributed methyl functionality (Fyfe and Aroca 1997). Figure 2 shows the X-ray diffraction patterns for samples synthesised from 0%, 30%, 70%, and 100% MTEOS molar ratios.…”

Comparative study on the hydrolysis kinetics of substituted ethoxysilanes by liquid-state 29Si NMR

“…The exponents a, b and c are the order of the reaction with respect to ethoxysilanes, NH 3 and H 2 O, respectively. Since NH 3 was used as the catalyst during the process of the hydrolysis and condensation, its concentration remained; the amount of water was in excess of the amount needed to completely hydrolyze monomer, and its largest change was less than 5% of the initial concentration [6], so it was also considered invariable. If we define (2) can be simplified to…”

“…In the last two decades, 29 Si NMR was widely used to investigate the hydrolysis of non-substituted tetraethoxysilane (TEOS) [5][6][7], methyl-substituted ethoxysilanes such as methyltriethoxysilane (MTES) [5][6][7][8][9] and dimethyldiethoxysilane (DDS) [5,10]. However, these works mostly focused on the acidic conditions, based on a 0022-3093/$ -see front matter Ó 2004 Elsevier B.V. All rights reserved.…”

Multi-nuclear liquid state NMR investigation of the effects of pH and addition of polyethyleneglycol on the long-term hydrolysis and condensation of tetraethoxysilane

“…The hydrolysis and condensation reactions of the sol-gel process are characterized by 1 H, 13 C and 29 Si multi-nuclear liquid NMR spectroscopy. In most of the previous studies, liquid 29 Si NMR spectroscopy was used to measure the hydrolysis and condensation reaction of the sol-gel process of TEOS [18,20,21]. However, the sensitivity of the NMR spectroscopy of the nucleus with low natural abundance is low (the natural abundances of 1 H, 13 C, and 29 Si are 99.8%, 1.11%, and 13%, respectively.)…”