Solvolysis–hydrolysis of N-bearing alkoxysilanes: Reactions studied with 29Si NMR

The hydrolysis reactions of 8 different N-bearing alkoxy-silane coupling agents, namely: 3-cyanopropyl triethoxy silane (CPES), triethoxy-3-(2-imidazolin-1-yl) propyl silane (IZPES), and amino silanes, 3-aminopropyl triethoxy silane (APES), 3-aminopropyl trimethoxy silane (APMS), 3-(2-aminoethylamino)propyl trimethoxysilane (DAMS), 3-[2-(2-aminoethylamino)ethylamino] propyl trimethoxysilane (TAMS), 4-amino-3,3-dibutyl trimethoxy silane (ADBMS) and trimethoxy [3-(phenylamino)propyl] silane (PAPMS) were carried out in ethanol/water 80/20 (w/w) solutions in acidic media and followed in situ by ¹H-, ¹³C- and ²⁹Si-NMR spectroscopy. Acidic conditions were selected in order to enhance the formation of silanol and to slow down the self condensation reactions of the hydrolyzed functions. ²⁹Si NMR spectroscopy revealed the formation of intermediate species, particularly the solvolysis of γ-amino silanes by reaction exchange with the alcoholic solvent.     

Hydrolysis-Condensation Kinetics of Different Silane Coupling Agents

Abstract

The hydrolysis kinetics of 14 alkoxy silane coupling agents were carried out in an ethanol:water 80:20 (w/w) solution under acidic conditions and were monitored by ¹H, ¹³C, and ²⁹Si NMR spectroscopy. Acidic conditions were selected in order to enhance the silanol formation and to slow down the self-condensation between the resulting hydrolysed silanol groups. In situ ²⁹Si NMR spectroscopy allowed the determination of the intermediate species as a function of the reaction time. Thus, the following silane coupling agents were studied: 3-methacryloxypropyl trimethoxy silane (MPMS), 3-mercaptopropyl trimethoxy silane (MRPMS), 3-cyanopropyl triethoxy silane (CPES), triethoxy vinyl silane (VES), trimethoxy (2-phenylethyl) silane (PEMS), octyl triethoxy silane (OES), trimethoxy (7-octen-1-yl) silane (OEMS), 3-aminopropyl triethoxy silane (APES), 3-aminopropyl trimethoxy silane (APMS), 3-(2-aminoethylamino)propyl trimethoxy silane, (DAMS), 3-[2-(2-aminoethylamino)-ethylamino]propyl trimethoxy silane (TAMS), 4-amino-3,3-dibutyl trimethoxy silane (ADBMS), trimethoxy [3-(phenylamino)propyl] silane (PAPMS), and triethoxy-3-(2-imidazolin-1-yl) propyl silane (IZPES). A parameter quantifying the grafting potentiality of each silane coupling agent towards OH-rich solid substrates (such as cellulose) was established as a function of the nature of the alkoxy groups (methoxy or ethoxy), as well as that of the fourth substituent (vinyl, aminopropyl, etc.) of the silane studied.

GRAPHICAL ABSTRACT     

Studies of interactions between silane coupling agents and cellulose fibers with liquid and solid-state NMR

The hydrolysis of three alkoxy-silane coupling agents, gamma-methacryloxypropyl trimethoxy silane (MPS), gamma-aminopropyl triethoxy silane (APS), and gamma-diethylenetriaminopropyl trimethoxy silane (TAS), was carried out in ethanol/water solutions (80/20 w/w) at different pH values and followed by 1H, 13C and 29Si NMR spectroscopy. Acidic media were found to stabilize the hydrolyzed forms. As expected, the formation of silanol groups was followed by their self-condensation to generate oligomeric structures, yielding, ultimately, solid homopolycondensated structures, as analyzed by 29Si and 13C high-resolution solid-state NMR. Hydrolyzed MPS in acidic media was then successfully adsorbed onto a cellulose surface and the ensuing substrates submitted to thermal treatment at 110-120 degrees C under reduced pressure, in order to create covalent bonds between cellulose and the coupling agent.     

Preparation of sol�Cgel hybrid materials from ��-methacryloxypropyltrimethoxysilane and tetramethyl orthosilicate: study of the hydrolysis and condensation reactions

Organic?Cinorganic hybrid materials suitable for the development of sol?Cgel coatings for metallic surfaces were prepared by hydrolysis and condensation of ??-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethyl orthosilicate (TMOS). The hydrolysis of MAPTMS/TMOS was carried out in an ethanol/water solution. The prehydrolysis stage of MAPTMS/TMOS system was monitored by Fourier transform infrared spectroscopy (FTIR) and liquid-state ²⁹Si and ¹³C nuclear magnetic resonance (²⁹Si and ¹³C NMR). FTIR analysis indicated that the hydrolysis of MAPTMS/TMOS was accomplished as far as the (SiOMe) band corresponding to unhydrolyzed silane disappeared. The concentration of the alkoxy groups and the extent of self-condensation of mono-, di-, and trisubstituted siloxanes (T species) in the sol were estimated by using liquid-state ²⁹Si NMR spectroscopy. The hydrolysis of the prepared sol was also evaluated by liquid-state ¹³C NMR spectroscopy. The results indicated that under the adopted synthesis strategy conditions, the hydrolysis process requires 4?h to be completed.     

Novel Nafion®/ORMOSIL hybrids via in situ sol-gel reactions: 2. Probe of ORMOSIL phase nanostructure by 29Si solid state NMR spectroscopy

Perfluorosulfonic Acid (PFSA)]/[Organically-Modified Silicon Oxide (ORMOSIL)] hybrids were formulated via sol-gel reactions of mixtures of tetraethoxysilane (TEOS) and diethoxydimethylsilane (DEDMS) in the nanophase-separated morphology of a PFSA membrane (Nafion^®). The molecular structures of the ORMOSIL phases were investigated by ²⁹Si solid state NMR spectroscopy and the spectra compared with those of corresponding bulk ORMOSILs. PFSA-in situ ORMOSIL composition can be tailored by manipulating TEOS/DEDMS source solution composition and quantified by ²⁹Si solid state NMR spectroscopy. Copolymerization of TEOS and DEDMS monomers, rather than block formation, occurs within the PFSA as well as in the bulk. As DEDMS feed concentration increases, ORMOSIL nanostructures are more hydrophobic and more flexible. Dimethylsiloxane rings are dominant in PFSAs in which hydrolysis + condensation of pure DEDMS occurs.     

Chemistry of mixtures of bis-[trimethoxysilylpropyl]amine and vinyltriacetoxysilane: an NMR analysis

¹³C, ²⁹Si, and ¹H NMR were used to investigate the interaction of bis-[trimethoxysilylpropyl]amine (silane A) with vinyltriacetoxysilane (silane V). The reaction was found to generate a hydroxylamine, an amide and a silanol. A series of hydrolysis and condensation reactions were found to occur concurrently with the formation of the major reaction products. Upon dilution with acidified water, the amide decomposes, initiating hydrolysis of unreacted silanes. Hydrogen bonding between the N–H and SiOH groups is likely to be responsible for the stability of the dilute solutions.     

Study of the addition of diazoles to vinylsilanes

The reaction of trimethoxy(vinyl)silane with diazoles in the presence of metal lithium was studied. The structure of the reaction products was assigned by NMR spectroscopy (¹H, ¹³C, ²⁹Si, COSY, NOESY, HMBC, and HSQC). An explanation for the different reactivities of reagents (aliphatic amines, diazoles, allylalkylsilanes, vinylalkylsilanes, and vinylalkoxysilanes) was provided.     

Reactions in aminosilane-epoxy prepolymer systems. II. Reactions of alkoxysilane groups with or without the presence of water

The hydrolysis and reactions of alkoxy silane groups have been studied on a model compound (TA) prepared from 2 mol of phenyl glycidyl ether and 1 mol of aminopropyl triethoxy silane. At low (40°C) and high (140°C) temperatures, the monomer conversion and the evolution of the molecular mass are followed by size exclusion chromatography (SEC). During the same reaction time, the evolution of the functional groups, hydroxyl CH? OH, ethoxy ? O? C₂H₅, and siloxane Si? O? Si, is observed by FTIR spectroscopy. Without the presence of water, reactions between hydroxyl and ethoxy silane lead to gelation at the end of the reaction. A by-product, probably a cyclic tetramer is also formed. After the hydrolysis, the reaction of the model compound is quite different. The product of reaction is always soluble, even after a treatment at high temperatures, and the evolution of the molecular mass versus the reaction time seems to correspond to the condensation giving a dead cyclic tetramer. From this study it is evident that the curing cycle has a great influence on the properties of the interface of a composite based on a epoxy matrix.     

A Designed 5‐Fluorouracil‐Based Bridged Silsesquioxane as an Autonomous Acid‐Triggered Drug‐Delivery System

Two new prodrugs, bearing two and three 5‐fluorouracil (5‐FU) units, respectively, have been synthesized and were shown to efficiently treat human breast cancer cells. In addition to 5‐FU, they were intended to form complexes through H‐bonds to an organo‐bridged silane prior to hydrolysis‐condensation through sol–gel processes to construct acid‐responsive bridged silsesquioxanes (BS). Whereas 5‐FU itself and the prodrug bearing two 5‐FU units completely leached out from the corresponding materials, the prodrug bearing three 5‐FU units was successfully maintained in the resulting BS. Solid‐state NMR (²⁹Si and ¹³C) spectroscopy show that the organic fragments of the organo‐bridged silane are retained in the hybrid through covalent bonding and the ¹H NMR spectroscopic analysis provides evidence for the hydrogen‐bonding interactions between the prodrug bearing three 5‐FU units and the triazine‐based hybrid matrix. The complex in the BS is not affected under neutral medium and operates under acidic conditions even under pH as high as 5 to deliver the drug as demonstrated by HPLC analysis and confirmed by FTIR and ¹³C NMR spectroscopic studies. Such functional BS are promising materials as carriers to avoid the side effects of the anticancer drug 5‐FU thanks to a controlled and targeted drug delivery.     

The structure of phosphate and borosilicate glasses and their structural evolution at high temperatures as studied with solid state NMR spectroscopy: Phase separation,crystallisation and dynamic species exchange

In this contribution we present an in-depth study of the network structure of different phosphate based and borosilicate glasses and its evolution at high temperatures. Employing a range of advanced solid state NMR methodologies, complemented by the results of XPS, the structural motifs on short and intermediate length scales are identified. For the phosphate based glasses, at temperatures above the glass transition temperature T_g, structural relaxation processes and the devitrification of the glasses were monitored in situ employing MAS NMR spectroscopy and X-ray diffraction. Dynamic species exchange involving rapid P–O–P and P–O–Al bond breaking and reforming was observed employing in situ ²⁷Al and ³¹P MAS NMR spectroscopy and could be linked to viscous flow. For the borosilicate glasses, an atomic scale investigation of the phase separation processes was possible in a combined effort of ex situ NMR studies on glass samples with different thermal histories and in situ NMR studies using high temperature MAS NMR spectroscopy including ¹¹B MAS, ²⁹Si MAS and in situ ²⁹Si{¹¹B} REAPDOR NMR spectroscopy.     

Ammonia Catalyzed Hydrolysis-Condensation Kinetics of Tetraethoxysilane/Dimethyldiethoxysilane Mixtures Studied by <Superscript><Emphasis Type="Bold">29</Emphasis></Superscript>Si NMR and SAXS

In-situ ²⁹Si liquid-state nuclear magnetic resonance (NMR) was used to investigate the ammonia catalyzed hydrolysis and condensation of the mixed systems of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS) dissolved in methanol. With ammonia catalysis, the hydrolysis reaction orders for TEOS and DDS in the mixed systems remained first order, which is similar to that observed for their corresponding single silane component precursor systems. The hydrolysis rate constant for TEOS in the mixed systems was larger than that of TEOS in the single silane component precursor systems. Meanwhile, the hydrolysis rate constants of DDS in the mixed precursor systems were smaller than those of DDS in the single silane component precursor systems. The hydrolysis and condensation kinetics showed more compatible hydrolysis-condensation relative rates between TEOS and DDS, which remarkably affected the final microstructure of the resulting silica particles. Small angle X-ray scattering (SAXS) experiments showed a typical double fractal structure in the particulate networks.     

Structure of hybrid organic–inorganic sols for the preparation of hydrothermally stable membranes

A procedure for the preparation of hybrid sols for the synthesis of organic–inorganic microporous materials and thin film membranes is reported. We describe silane reactivity and sol structure for acid-catalysed colloidal sols from mixtures of either tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES), or bis(triethoxysilyl)ethane (BTESE) and MTES. Early-stage hydrolysis and condensation rates of the individual silane precursors were followed with ²⁹Si liquid NMR and structural characteristics of more developed sols were studied with Dynamic Light Scattering. Condensation was found to proceed at more or less similar rates for the different precursors. Homogeneously mixed hybrid colloids can therefore be formed from precursor mixtures. The conditions of preparation under which clear sols with low viscosity could be formed from BTESE/MTES were determined. These sols were synthesised at moderate water/silane and acid/silane ratios and could be applied for the coating of defect-free microporous membranes for molecular separations under hydrothermal conditions.     

The interaction of dibutyltin dilaureate with tetraethyl orthosilicate in sol-gel systems

Sols composed of dibutyltin dilaureate (DTL) and tetraethyl orthosilicate (TEOS) were prepared using a mixture of methyl ethyl ketone (MEK) and acetone as the solvent in order to study the interaction between the oligomeric Sn and Si species. The hydrolysis molar ratio r (r=nH₂O/nM (M: Si, Sn or Si+Sn) was 2. The use of an acid or basic catalyst was avoided, as the sols are intended to be used in the formulation of potential stone consolidants. The sols were studied by several spectroscopic techniques including Small Angle X-ray Scattering (SAXS), ²⁹Si and ¹¹⁹Sn NMR, Fourier Transform Infrared (FTIR) spectroscopy and X-ray diffraction (XRD). According to the spectroscopic results the lauric acid produced by the hydrolysis of DTL modifies the condensation path of the Si species, leading to the formation of two types of oligomeric chains: linear swollen and multiparticle diffusion-limited aggregates, depending on the molar ratio Sn/Si. The ²⁹Si NMR results indicated that the hydrolysis of DTL catalizes the condensation of the Si species, giving as a result higher condensation extents of the Si species in the Sn-Si sols compared to a pure Si sol. Based on the Radial Distribution functions (RDF) and the FTIR results, heterocondensation occurred.     

Cleavage of the C-Si bond in Trifluoro(phenyl)silane with Aliphatic Alcohols

Trifluoro(phenyl)silane reacts with aliphatic alcohols under reflux. The reaction involves not only Si-F bond cleavage to form ethoxyfluoro(phenyl)silanes, but also C-Si bond cleavage to form benzene and alkoxyfluoro- and tetraalkoxysilanes. The formation of the latter products was proved by ¹⁹F and ²⁹Si NMR spectroscopy and also by model disproportionation reactions of trifluoro(phenyl)silane with trimethoxy-(phenyl)-, tetramethoxy-, or tetraethoxysilanes.     

29Si NMR研究TEOS/DDS双前驱体体系的水解动力学

原位引入有机组分对氧化硅体系改性是合成有机-无机杂化硅材料的重要方法. 利用原位的²⁹Si液体核磁, 研究了甲醇为溶剂、氨水催化条件下的四乙氧基硅烷(TEOS)和二甲基二乙氧基硅烷(DDS)原位共水解的动力学过程. 通过改变反应体系中氨和水的浓度, 拟合出单体及中间产物浓度随时间的变化曲线, 得到了TEOS和DDS各自的水解速率常数以及相应各反应物的反应级数. 与单前驱体水解一致的是, 在双前驱体系中TEOS和DDS自身的反应级数仍保持一级, 但是氨和水的反应级数都有不同程度的增大. 与单前驱体水解速率方程相比, 混合体系中TEOS的水解速率常数增大. 同时, DDS在双前驱体中比单前驱体中的水解速率常数有很大程度的减少. 水解动力学表明, TEOS和DDS在双前驱体体系中显示出更平行的水解速率. 利用固体²⁹Si MAS NMR, XPS及小角X射线散射(SAXS)手段对双前驱体体系研究得到的信息显示, 碱催化条件下原位的TEOS水解中间物与DDS中间产物的原位共缩聚程度很弱.     

NMR Study of the Hydrolysis and Oligomerization of Alkyltrichlorosilanes in Silanizing Solutions Used to Prepare Alkylsiloxane Self-Assembled Monolayers

²⁹Si NMR spectroscopy is not suited to following a number of fast occurring processes involving silicon centres due to long accumulation times resulting from low detector sensitivity factors and poor natural abundance. By observing subtle changes in signals due to the methylene protons adjacent to the silicon centre, the hydrolysis of alkyltrichlorosilanes in tetrahydrofuran and acetone-d₆ solutions, and subsequent polycondensation reactions, were studied using ¹H and ¹³C NMR spectroscopy, to gain an understanding of the processes that lead to formation of oligomers in silanizing solutions. The hydroxysilanes formed by hydrolysis of alkyltrichlorosilanes were characterized by ¹H and ¹³C NMR studies at low temperatures. Formation of oligomeric and polymeric species from these hydroxysilanes at higher temperatures was monitored by ¹H NMR studies. The data for oligomer formation were fitted to the kinetic model of an acid-catalyzed stepwise condensation. The implications of these results for the problem of oligomer formation competing with self-assembly processes in formation of alkylsiloxane monolayers are discussed.     

Hydrogen‐bonding‐aided synthesis of novel ladderlike organobridged polysiloxane containing side‐chain naphthyl groups

With a hydrogen‐bonding template, a novel soluble aryl amide‐bridged ladderlike polysiloxane, containing naphthyl as the side‐chain group, has been successfully synthesized via a stepwise coupling polymerization. It is proposed that the monomer, N,N′‐di(3‐naphthyldiethoxylsilyl‐propyl)‐[4,4′‐oxybis(benzyl amide)], prepared by Grignard and hydrosilylation reactions, undergoes self‐assembly first via amido hydrogen bonding and then via hydrolysis, followed by condensation under controlled reaction conditions to yield a high molecular weight, soluble, dark yellow polymer. The analytical results (Fourier transform infrared, ¹H NMR, ²⁹Si NMR, X‐ray diffraction, differential scanning calorimetry, and vapor pressure osmometry) show that the polymer possesses an ordered ladderlike architecture. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 636–644, 2003     

29Si MAS-NMR Study of Silica Gels and Xerogels: Influence of the Catalyst

Four silica gels were prepared by hydrolysis of tetraethoxysilane (TEOS) in ethanol, using different catalysts: HCl, NaOH, NH₃, and NBu₄F. Nitrogen adsorption-desorption isotherms indicated that the HCl-catalyzed xerogel was purely microporous, whereas the other samples exhibited a very broad distribution of mesopores and a variable amount of micropores. ²⁹Si MAS NMR spectroscopy of the wet gels (before drying) pointed to a low degree of condensation for the HCl-catalyzed gel, and to the presence of unhydrolyzed TEOS monomer in the NaOH-catalyzed gel. Comparison with the ²⁹Si MAS NMR spectra of the xerogels indicated a significant increase of the degree of condensation during the drying procedure (3 hrs at 120°C under vacuum) for the HCl-catalyzed gel.     

Synthesis,Characterization and Properties of (Vinyl Triethoxy Silane-grafted PP)/Silica Nanocomposites

A new route has been developed to produce PP/silica nanocomposites starting from porous PP reactor powder and making use of sol-gel chemistry. Silica-like, nano-sized particles were prepared in the pores of the PP reactor powder with a controlled degree of adhesion between PP and silica. Magic-angle spinning (MAS) ²⁹Si NMR spectra showed that the chemical building blocks of the silica-like clusters are of Q³ and Q⁴-type. For (vinyl triethoxy silane (VTES)-grafted PP)/silica nanocomposites, VTES was grafted via solid-state modification (SSM) in porous PP particles. Subsequently, silica particles were prepared by sol-gel technology in the VTES-grafted PP. MAS ²⁹Si NMR and FT-IR spectroscopy showed that the grafted VTES becomes part of the in-situ formed silica particles. The study on the mechanical properties of (VTES-grafted PP)/silica nanocomposites showed that the silica particles improved the impact toughness of PP by a factor of 2, when there is no chemical interaction between the particles and the matrix, while for (VTES-grafted PP)/silica nanocomposites the impact toughness decreased. This indicates that chemical bonding between the filler particles and the PP-matrix results in brittle failure and supports the hypothesis that debonding is necessary for improving the impact toughness of PP with inorganic fillers.     

Composition, structure, and properties of pharmacologically active silicon dimethylglycerolates

¹H and ²⁹Si NMR spectroscopy showed that the product of the reaction of dimethyldiethoxysilane with glycerol in the molar ratio 1: 2 is an equilibrium mixture of dimethyldiglyceroxysilane and low-molecular-weight condensation products. The change of viscosity of silicon dimethylglycerolates, synthesized using different excesses of glycerol, versus time was studied by viscosimetry. Composition of products of hydrolysis and condensation of silicon dimethylglycerolates is determined by their concentration in the starting aqueous solutions.