Ethoxysilane Groups Research Articles

Creating Well-Defined monolayers of phosphine linkers incorporating ethoxysilyl groups on silica surfaces for superior immobilized catalysts

Many applications of catalysts immobilized on solid supports like silica via bifunctional phosphine linkers are still hampered by their decomposition, leaching, agglomeration, and uncontrolled nanoparticle formation, all of which change their activities and selectivities. In general, the success of an immobilized catalyst is crucially dependent on the linker and its attachment to the oxide support. In this contribution, an improved method for covalently binding phosphine linkers to silica via ethoxysilane groups is described. This method leads to well-defined sub-monolayers of linkers on silica surfaces without cross-linking of the linkers, which typically leads to clogged pores and metal agglomeration during catalysis, thus entailing less active and selective catalysts. The novel immobilization method has been supported by multinuclear classical CP/MAS solid-state NMR spectroscopy, as well as suspension NMR of slurries. It has been demonstrated by TEM that nickel complexes coordinated by immobilized phosphine linkers in a well-defined sub-monolayer coverage do not form larger aggregates or nanoparticles during the catalytic cyclotrimerization of phenylacetylene under various conditions, in contrast to analogous complexes in homogeneous catalytic runs.

Aldol Condensation and Esterification over Ti-Substituted *BEA Zeolite: Mechanisms and Effects of Pore Hydrophobicity

Aldol condensation and esterification reactions provide paths to upgrade ethanol and acetaldehyde to higher-value molecules useful as fuels or intermediates for the synthesis of polymers. Transition-metal-substituted BEA zeolites (M-BEA) catalyze these reactions; however, the mechanisms for these processes in M-BEA and the effects of incidental or purposefully included silanol groups are not reported. Here, we combine kinetic and spectroscopic measurements obtained during catalytic reactions of acetaldehyde (CH3CHO), ethanol (C2H5OH), and hydrogen (H2) mixtures over a series of Ti-BEA catalysts that possess a known range of silanol group densities to examine the kinetic relevance of intervening steps and the impact of silanol groups on catalytic rates. Across all Ti-BEA, rates for aldol condensation and esterification increase with the pressure of CH3CHO; however, C2H5OH and H2O weakly inhibit the rates of these reactions. The substitution of CD3CDO for CH3CHO decreases aldol condensation rates slightly (∼10%) but leads to greater esterification rates (2- to 5-fold). The kinetic isotope effects together with the measured dependence of rates on reactant pressures suggest that aldol condensation and esterification occur on unoccupied Ti sites and involve multiple kinetically relevant steps. CH3CHO deprotonates irreversibly, and the kinetically relevant nucleophilic attack of the enolate to CH3CHO* (i.e., adsorbed CH3CHO on Ti sites) leads to aldol products, while the nucleophilic attack of the enolate to C2H5OH* gives esters. Selectivities toward aldol condensation increase with the ratio of CH3CHO to C2H5OH pressure and with increases in the silanol density of the as-synthesized Ti-BEA. During catalysis, in situ infrared spectroscopy demonstrates that these silanol groups react with C2H5OH to form ethoxysilane groups (i.e., SiOC2H5) that modify the polarity of the environment near Ti active sites. As initial silanol densities increase, steady-state turnover rates for aldol condensation and esterification increase by factors of 5 and 2, respectively. The changes in rates and selectivities among Ti-BEA catalysts likely reflect changes in excess free energies of transition states for enolization and nucleophilic attack of the enolate to adsorbed coreactants. The differences in excess stability report on the interactions among reactive intermediates at framework Ti atoms and the ethoxysilane and remaining silanol groups present. The in situ modification of these pore environments confers changes in the stability of reactive species in a manner that contradicts intuition when considering the initial state of the catalyst but can be reconciled after accounting for the formation of persistent alkoxy surface moieties in the pores.

Syntheses, structures, and immobilization of ruthenium(II) complexes with alkoxysilane groups functionalized N,N′-diamine and phosphine ligands

Treatment of [RuCl2(PPh3)3] with N-(β-aminoethyl)-γ-aminopropylmethylbimethoxysilane (L1) in tetrahydrofuran at room temperature gave a mononuclear ruthenium(II) complex [Ru(PPh3)2Cl2 (κ2-N,N-L1)] (1). Condensation of γ-aminopropyltriethoxysilane and potassium diphenylphosphine in the molar ratio of 1:1 in tetrahydrofuran afforded phosphine (PPh2)(CH2)3Si(OEt)3 (L2) with ethoxysilane groups. Reaction of [Ru(η6-p-cymene)Cl2]2 with L2 in refluxing tetrahydrofuran afforded a ruthenium(II) phosphine complex [Ru(η6-p-cymene)Cl2(κ-P-L2)] (2). Complexes 1 and 2 were systematically characterized by microanalyses, FT-IR, and NMR spectroscopies. Their structures have been unambiguously established by single-crystal X-ray diffraction. Immobilization of 1 and 2 on SBA-15 and characterization of these hybrid heterogeneous catalysts were studied by transmission electron microscopy (TEM), FT-IR, and low-pressure N2 adsorption/desorption measurements. Transfer hydrogenation of acetophenone was also briefly investigated with the heterogeneous catalysts.

Synthesis and characterization of tridentate phosphine ligands incorporating long methylene chains and ethoxysilane groups for immobilizing molecular rhodium catalysts

The new tridentate phosphine ligands EtOSi((CH2)nPPh2)3 and O[Si((CH2)nPPh2)3]2 (n = 4, 7, 11), as well as their precursor ethoxysilanes EtOSi((CH2)mCH = CH2)3 and disiloxanes O[Si((CH2)mCH = CH2)3]2 (m = 2, 5, 9) and EtOSi((CH2)11PPh2RhClCOD)3 (COD = cyclooctadiene) have been synthesized and fully characterized. The ethoxysilane- and disiloxane-containing phosphine ligands have been immobilized on silica via covalent siloxane bonds. The immobilized linkers have been characterized by 31P and 29Si CP/MAS and HRMAS NMR spectroscopy. The covalent siloxane bonds between the linkers and the silica surface prevent translational mobility of the phosphines. Immobilized Wilkinson-type Rh hydrogenation catalysts have been obtained by ligand exchange of ClRh(PPh3)3 and ClRhpyCOD (py = pyridine) with the surface-bound ligands. The single crystal X-ray structure of ClRhpyCOD has been reported. The activities and lifetimes have been studied for the hydrogenation of 1-dodecene. The new catalysts are highly active and they can be recycled up to 15 times without major loss of activity. Within the first hours of the catalytic reaction the initially molecular complexes form Rh nanoparticles with a narrow size distribution on the surface. The nanoparticles do not leach into the supernatant solution and are not air-sensitive.

Ruthenium(III)-carbonyl and ruthenium(II)-nitrosyl complexes of nitrogen-containing ligands with ethoxysilane groups

Reactions of γ-aminopropyltriethoxysilane and 4-(diethylamino)salicylaldehyde in ethanol afforded a Schiff base L1H, which reacted with [Ru(CO)2Cl2]n in the presence of Et3N in THF giving a ruthenium(III) carbonyl complex RuCl(CO)2(η1-O-L1)(η2-O,N-L1) (1). Treatment of γ-aminopropyltriethoxysilane with 4-pyridinecarboxaldehyde gave the Schiff base L2. Interactions of L2, γ-aminopropyltriethoxysilane, and Ru(NO)Cl3⋅H2O in THF led to the formation of a ruthenium(II) nitrosyl complex RuCl3(NO)(L2)[H2N(CH2)3Si(OEt)3] (2) with linear N≡O ligand. Complexes 1 and 2 were characterized by microanalyses and IR and MS spectroscopies and confirmed by single-crystal X-ray diffraction.

The mechanism of chemical modification of artificial fibers based on cellulose derivatives

A fabric based on cellulose derivatives has been hydrophobized via coating with oligochloromethylethoxysiloxane and then treated with butyl alcohol, benzyl alcohol, and higher fatty alcohols. With the use of X-ray photoelectron spectroscopy, it has been shown that the reaction of alcohols with the siloxane coating proceeds through the exchange of ethoxysilane groups for higher alkoxy groups, whereas the expected reaction of chloromethyl groups with alcohols (the Williamson reaction) does not occur under the chosen conditions.

Preparation of Poly(MA-alt-α-olefin-C6, 8, 12, 18)/Silica Nanohybrids via in situ generated nanofillers for use as a dual function organonanofiller

Four types of copolymer-silica nanocomposites have been prepared via ring-opening grafting of γ-aminopropyltrimethoxysilane (APTS) as reactive coupling agent onto preformed copolymers of maleic anhydride (MA) with 1-hexene, 1-octene, 1-dodecene and 1-octadecene and in situ hydrolysis (polycondensation) of side-chain ethoxysilane groups and tetraethoxysilane as a precursor in the presence of HCl catalyst. The copolymers of MA with 1-hexene, 1-octene and 1-dodecene were synthesized by free radical polymerization and another MA copolymer with 1-octadecene was supplied commercially as matrix copolymer. Chemical/physical structures, thermal behavior and morphology investigations of the generated hybrids were performed by FTIR, 13C, 29Si-NMR, TGA, SEM and TEM analysis methods. Nano-level hybridization through covalent bonding (amidization) between the anhydride unit of copolymers and amine group of APTS was observed, and nano-silica networks (hydrolysis) were obtained through acid catalyzed co-polycondensation of TEOS and ethoxysilane fragments from both coupling agent and precursor. Agreeing with 29Si-NMR and TGA quantitative analysis results, the degree of hydrolysis of ethoxysilane groups varied from 51.0 to 60.9%, and the content of in situ generated silica particles was found to be around 70.7-75.7%. Thermal properties and thermal stability of the obtained hybrids were found to be enhanced with silica content. SEM analysis confirmed the formation of nanostructural hybrids with relatively fine distributed nanoparticles. TEM analyses of all the nanohybrids indicate the formation of spherical morphologies. These novel copolymer hybrids are expected to be a promising and efficient organonanofiller for the preparation of polymer nanocomposites with both dual functionality and compatibilizer effects. Poly(MA-alt-α-olefin)/silica nanohybrids were obtained by an in situ sol–gel process of TEOS via nano-level hybridization as promising and effective organo nano-fillers.

Dioxomolybdenum(VI) and dioxotungsten(VI) complexes: efficient catalytic activity for crosslinking reaction in ethylene‐vinyl acetate copolymer/alkoxysilane composites

The catalytic performances of several bis(acetylacetonato)metal complexes [Cu(acac)2, Zn(acac)2, TiO(acac)2, VO(acac)2, MoO2(acac)2, and WO2(acac)2] were investigated for the crosslinking reaction via transesterifications in the ethylene‐vinyl acetate copolymer/tetraethoxysilane (EVA/TEOS) composite system by means of dynamic attenuated total reflectance Fourier transform infrared, solvent swelling, and solid‐state 29Si cross polarization/magic angle spinning nuclear magnetic resonance techniques. Results of the kinetic examination revealed that MoO2(acac)2 and WO2(acac)2 exhibited a higher catalytic activity than di‐n‐butyltin(IV) oxide, which is a catalyst most commonly used for the transesterification process in polymer system, but has a toxic effect on the environmental health. And furthermore, the crosslink density and final siloxane network structure of crosslinked EVA/TEOS composites are found to be greatly correlated with the catalyst used. On the basis of the SN2‐Si pathway, a plausible catalytic mechanism of MoO2(acac)2 and WO2(acac)2 was proposed for the crosslinking reaction via transesterifications of the vinyl acetate moieties in EVA backbone with the ethoxysilane groups in one TEOS molecule. The findings in this study may fill the blank in the high performance and environmentally friendly catalyst in the field of the crosslinking reactions in polymer system and provide useful clue for other transesterifications. Copyright © 2015 John Wiley & Sons, Ltd.

Phosphorus based organic–inorganic hybrid materials prepared by reactive processing for EVA fire retardancy

A new ethylene vinyl acetate (EVA)-based hybrid material containing silicon and phosphorus has been developed in order to improve fire retardancy of EVA. The preparation of the hybrid material was based on exchange reaction between acetoxy groups from EVA and ethoxysilane groups from diethylphosphatoethyltriethoxysilane (SiP). The exchange reaction was characterized by NMR spectroscopy, showing the presence of silicon in the crosslinking bridges bonded to unreacted phosphonate groups. The formation of the crosslinked network was characterized by rheology and TGA–GC–MS. The influence of silicon and phosphorus on the thermal behaviour and the flame retardancy of EVA was investigated by thermogravimetric analysis (TGA) and cone calorimeter. The results showed a synergistic effect between silicon and phosphorus on the fire properties for low charge contents (1.3 wt% of silicon and 1.4 wt% of phosphorus). The peak heat release rate (PHRR) measured in a cone calorimeter decreased by 35% for EVA hybrid materials compared to pure EVA due to the formation of a compact charred layer. The charred residue was analysed by NMR spectroscopy and showed the presence of silicophosphorated complexes.

Effect of EtOH/H2O Ratio and pH on Bis-Sulfur Silane Solutions for Electrogalvanized Steel Joints Based on Anaerobic Adhesives

A minimum hydrolysis time is required to get an adequate crosslinking between a silane film and a metallic substrate and that depends on the contents of silane, ethanol, and water in the silane solution. The objectives for this work are: 1) to study the effect of different ratios of ethanol/water on the hydrolysis time for a 1% bis-sulfur silane solution at pH 6, by attenuated total reflectance infrared spectroscopy. The different solutions studied correspond to different ratios of silane/ethanol/water by volume. The study was done following appearance of the siloxane and silanol groups and the disappearance of the ethoxysilane groups. 2) Also studied was the adhesion and corrosion behaviour of bis-sulfur silane (bis[3-(bis[3-(triethoxysilyl)propyl]disulfide, DS) in solutions of ethanol/water at pH 4 and 6 and γ-methacryloxypropyltrimethoxysilane (MPS) coatings obtained by a two-step process on galvanized steel samples. This coated surface was analysed by reflection absorption infrared spectroscopy (FTIR-RA), scanning electron miscroscopy (SEM), and polarization curves. Results obtained at pH 6 were compared with the ones for pH 4. Single lap shear tests were used to contrast the behaviour of anaerobic adhesives on electrogalvanized steel silanized samples. It was observed that higher hydrolysis time was necessary to get good adhesion behaviour if the solution was prepared at pH 4, while at pH 6 the best behaviour was observed for short hydrolysis times.

Ethoxysilyl-modified hyperbranched polyesters as mulitfunctional coupling agents for epoxy-silica hybrid coatings

Partially ethoxysilyl-modified hyperbranched aliphatic–aromatic polyesters (HBPs) were effectively used as toughening as well as multi-site coupling agents in the preparation of organic–inorganic UV-thermal dual-cured epoxy/TEOS coatings. Through chain transfer reaction of the phenolic terminal units of the HBPs effective incorporation in the epoxy resin is achieved in the photo-initiated cationic polymerization whereas the ethoxysilane groups allow effective formation of a strongly interconnected inorganic–organics network during the in-situ sol-gel process with TEOS by binding the organic to the inorganic phase. Under those conditions, the addition of the inorganic precursor to the epoxy/HPB (20 wt%) system induced an increase of the storage modulus and more important, an improvement of the viscoelastic properties by extending the performance of the elastic modulus to higher temperatures. Thus, highly transparent hybrid coatings with enhanced thermal-mechanical and surface hardness properties resulted by the use of the partially ethoxysilyl-modified HBPs as multifunctional coupling agents.

Synthesis of poly(arylate-siloxane)s in supercritical carbon dioxide

The reaction between poly(4,4′-isopropylidene-2,2′-diphenylene (tere)isophthalate) copolymer and 3-aminopropyltriethoxysilane was investigated in supercritical carbon dioxide at 150 atm and 100° C. It was found that ester bonds in the copolyarylate undergo aminolysis under the above conditions to give rise to the formation of siloxane-containing amides, whose interaction with phenol groups of the macromolecules and moisture affords an insoluble poly(arylate-co-siloxane) fraction. Under the action of moisture, three-dimensional structures containing polyarylate and polysiloxane fragments are formed in the soluble fraction of the copolymer via the combined hydrolysis of ethoxysilane groups. The formation of the siloxane network affects the properties of the polymers. In particular, thermomechanical and thermal tests showed that sample deformation at 250°C decreases from 80 to 15%, while the carbon residue at 725°C increases twofold as compared to that of an initial polyarylate sample.

Preparation of a Novel Poly(Methyl Methacrylate)-Dimethyl Diethoxysilane Nano-Composite

The poly(methyl methacrylate)/silica nano-composite made from trimethoxysilyl functionalized poly(methyl methacrylate) and dimethyl diethoxysilane was newly prepared and its apatite-forming ability and mechanical properties were evaluated comparing to poly(methyl methacrylate)/silica nano-composite made from trimethoxysilyl functionalized poly(methyl methacrylate) and tetraethyl orthosilicate. Its apatite-forming ability was similar to that of poly(methyl methacrylate)/silica nano-composite using tetraethyl orthosilicate but its fracture toughness was much improved. Its high fracture toughness might come from the less quantity of siloxane linkages in its structure because dimethyl diethoxysilane had only two ethoxysilane groups while tetraethyl orthosilicate had four ethoxysilane groups. From the results, it can be concluded that it has a possibility to be used as bioactive bone cement.

Synthesis and characterization of novel (amide–imide)-silica composites by the sol–gel process

Novel (amide–imide)-silica composites that can be used to enhance the mechanical properties of high performance polymers have successfully been synthesized by a sol–gel process. In a first step, a dimer containing two chemically pre-formed imide rings has been synthesized and fully characterized. The dimer was then used to synthesize an intermediate with amide linkages and terminal ethoxy-silane groups which were finally reacted with varying amounts of tetraorthosilicate (TMOS) in a sol–gel reaction in order to generate the (amide–imide)-silica composites. The amount of TMOS added to the reaction was found to influence the amount of silica incorporated in the composites as well as their microstructure. It has been observed that the addition of 25% of TMOS with respect to the ethoxy-silane content of the intermediate leads to the formation of a highly homogeneous microstructure with well-defined grains as opposed to an inhomogeneous distribution of pure silica resulting from excess of TMOS in the composite to which 100% of that component was added.

Effect of the anchorage of Rh(III) ions by imidazoline type ligands on the location of Rh 0 particles within the calcined form of the SBA-15 support

Two series of mesoporous SBA-15 silica materials with N-propyl-4,5-dihydroimidazole groups have been prepared. It is shown that up to 15% of the silicon atoms can be provided by the organosilane precursor, N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (Si-Imi), either by one-pot (SBA-OP x%) or post-synthesis (SBA-GR x%) procedures. The hybrid solids were characterised by elemental analysis, XRD, TEM, nitrogen sorption isotherms, 29Si or 13C NMR, FTIR and UV–Vis spectroscopy. Higher apparent yields of Si-Imi incorporation were obtained in the grafting mode. However, a large number of ethoxysilane groups remain unaltered in comparison to those of SBA-OP x% solids. Furthermore, post-synthesis conditions favour the deposition of large fractions of the organic function on the external surface. Well structured SBA-15 type solids with a large proportion of imidazoline groups inside the pores were obtained by the co-condensation method provided that TEOS was pre-hydrolysed. Complexes of rhodium(III) with imidazoline ligands supported by SBA-GR or OP 5% were successfully tested as precursors of small rhodium(0) nanoparticles. Their location in the two solids (Rh 0-SBA-Gr 5% and Rh 0-SBA-OP 5%), obtained by a two-step calcination/reduction process, appears to reflect that of the ligands. Particles generated in the reference solids prepared by the impregnation of SBA-15 with rhodium(III) in the presence of NH 3 are not included in the pores. The different materials were tested in the room-temperature hydrogenation of styrene and anisole in 1 atm H 2. Their catalytic activities vary in the following order: Rh 0-SBA-NH 3 ≪ Rh 0-SBA-GR 5% < Rh 0-SBA-OP 5%.

Synthesis of UV-curable organic–inorganic hybrid urethane acrylates and properties of cured films

An organic–inorganic hybrid urethane acrylate hydrolytic condensate (HUA-HC) was prepared through sol–gel method with an acid catalyst of low concentration from a hybrid urethane acrylate prepolymer (HUA), which is a disfunctional silane containing hydrolysable ethoxysilane groups (Si–OEt). The synthesis was monitored by Fourier-transformed infrared spectroscopy, 1H nuclear magnetic resonance (NMR), 13C NMR and 29Si NMR. The photopolymerization kinetics studied by photo differential scanning calorimetry showed that HUA-HC had a faster apparent photopolymerizaton rate and a higher conversion of double bonds than HUA under 72 °C due to the vicinity of double bonds in HUA-HC. However, HUA-HC showed a slower apparent photopolymerizaton rate and a lower conversion of double bonds than HUA at higher temperatures above 72 °C, which indirectly indicates that more extensive condensation between ethoxysilane (Si–OEt) and silanol (Si–OH) groups in HUA-HC than that in HUA. Both 100-μm-thick films of HUA-HC and HUA cured by ultraviolet (UV) light showed high pencil hardness (> 4 H) and excellent abrasion resistance (< 40 mg after 600 cycles). 2-mm-thick specimen of HUA-HC and HUA cured by UV light showed high tensile strength (> 30 MPa). The cured HUA-HC film showed better performance than the cured HUA film in the investigated aspects due to more inorganic Si–O–Si linkages in HUA-HC, which is also the reason that HUA-HC film had higher decomposition temperatures and more residue than HUA in thermal gravity analysis.

Smart Hydrogels for in Situ Generated Implants

The objective of this study was to explore the use of reverse thermo-responsive (RTG) polymers for generating implants at their site of performance, following minimally invasive surgical procedures. Aiming at combining syringability and enhanced mechanical properties, a new family of injectable RTG-displaying polymers that exhibit improved mechanical properties was created, following two different strategies: (1) to synthesize high-molecular-weight polymers by covalenty joining poly(ethylene glycol) and poly(propylene glycol) chains using phosgene as the coupling molecule and (2) to cross-link poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-PEO triblocks after end-capping them with triethoxysilane or methacrylate reactive groups. While the methacrylates cross-linked rapidly, the triethoxysilane groups enabled the system to cross-link gradually over time. The chain-extended PEO/PPO copolymers had molecular weights in the 39 000-54 000 interval and exhibited improved mechanical properties. Reverse thermo-responsive systems displaying gradually increasing mechanical properties were generated by cross-linking triethoxysilane-capped (EO)(99)-(PO)(67)-(EO)(99) (F127) triblocks. Over time, the ethoxysilane groups hydrolyzed and created silanol moieties that subsequently condensated. With the aim of further improving their mechanical behavior, F127 triblocks were reacted with methacryloyl chloride and the resulting dimethacrylate was subsequently cross-linked in an aqueous solution at 37 degrees C. The effect of the concentration of the F127 dimethacrylate on the mechanical properties and the porous structure of the cross-linked matrixes produced was assessed. Rheometric studies revealed that the cross-linked hydrogels attained remarkable mechanical properties and allowed the engineering of robust macroscopic constructs, such as large tubular structures. The microporosity of the matrixes produced was studied by scanning electron microscopy. Monolayered conduits as well as structures comprising two and three layers were engineered in vitro, and their compliance and burst strength were determined.

Infrared attenuated total reflection spectroscopy studies of aprotic condensation of (EtO)3Si[sbnd]R[sbnd]Si(OEt)3 and R[sbnd]Si(OEt)3 systems with carboxylic acids

Aprotic hydrolysis and condensation reactions of bis end-capped trialkoxysilanes ((EtO)3SiRSi(OEt)3), linked via the organic chain R containing urea groups chemically bonded to a poly(propyleneglycol) (PPG) chain, in the presence of carboxylic acids, i.e. acetic-, chlorodifluoroacetic- and trifluoroacetic acids, were studied using infrared attenuated total reflection (IR ATR) spectroscopy. IR and 29Si NMR spectral analysis revealed solvolysis reactions: the carboxylic acids interacted with ethoxysilane groups forming silyl esters leading to the formation of bridging SiOSi groups and carboxylic acid ester by-products. These results were compared with those obtained on simpler single capped methyltriethoxysilane (MeSi(OEt)3, MTEOS) condensed with trifluoroacetic acid. Gelation of (EtO)3SiRSi(OEt)3 (catalyzed with acetic acid) encapsulated between a transparent conductive oxide (TCO) glass and a conductive and IR transparent silicon wafer was followed with the help of IR reflection–absorption spectroscopy. The results revealed that aprotic solvolysis of the hybrid precursor with acetic acid led to the formation of non-aqueous gels with low silanol content, confirming the advantages of aprotic solvolysis of organic–inorganic hybrids used as redox electrolytes in hybrid electrochromic (EC) and dye-sensitized photoelectrochemical (DSPEC) cells. Some comments regarding the accuracy of IR ATR spectral measurements compared to IR transmission spectra are also given.

Immobilization of Phosphines on Silica: Identification of Byproducts via 31P CP/MAS Studies of Model Alkyl-, Aryl-, and Ethoxyphosphonium Salts

Immobilizing bifunctional phosphines with ethoxysilane groups on silica often leads predominantly, and sometimes quantitatively, to P(V) side products that occupy space on the surface but cannot bind metal complexes. Although this side reaction is well-known, and dreaded because it leads to leaching of adsorbed catalysts that are not bound covalently, the exact nature of the surface-bound side product was not yet known. With the help of polycrystalline model compounds of the types [R3PEt]+X- and [R3POEt]+X- (R=alkyl, aryl; X=Cl-, Br-, I-, BF4-) and their solid-state NMR characteristics [delta(31P), CSA], it is demonstrated that the side product is an ethylphosphonium salt bound to the surface by a siloxide anion, [R3PEt]+[Si-O]-.

Ethoxysilane-capped PEO–PPO–PEO triblocks: a new family of reverse thermo-responsive polymers

New reverse thermo-responsive polymers systems combining reverse thermal gelation behavior and a gradual increase in the mechanical properties, were created by crosslinking ethoxysilane-capped poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblocks in aqueous solutions at physiological conditions. Pluronic F127 (PEO 99–PPO 67–PEO 99) was functionalized with (3-isocyanatopropyl) triethoxysilane (IPTS) by reacting its terminal hydroxyl groups with the isocyanate. The silane-capped PEO–PPO–PEO triblock was characterized by 1H-NMR, GPC, FT-IR and DSC and the rheological behavior of its aqueous solutions were studied. The silane-containing triblock retained the reverse thermo-responsive characteristics displayed by the original Pluronic. Over time, the ethoxysilane groups hydrolysed and created silanol moieties that subsequently condensated, crosslinking the material and generating hydrogels that exhibited gradually increasing mechanical properties. It was found that the higher the pH, the faster the process and the higher the viscosity levels attained. Finally, the ability of these gels to perform as matrices for drug delivery was exemplified by releasing metronidazole and methylene blue. Findings showed that while a 30% F127 gel at 37°C delivered all the drug within less than 3 days, F127di-IPTS gels completed the process at a much slower rate (up to 15 days).