Effect of the direction of ester linkage on molecular shape selectivity through multiple carbonyl–π interaction with octadecyl chain branched polymers as organic phases in reversed-phase high-performance liquid chromatography

Poly(vinyl octadecanoate)-grafted porous silica (Sil-VOD_n, n = 23) was newly prepared to investigate the efficiencies of the carbonyl groups in the polymer chain for recognition of polycyclic aromatic hydrocarbons (PAHs) in RP-HPLC. In Sil-VOD₂₃, the octadecyl side chains were connected to the polymer main chain through ester linkage in opposite direction to that in poly(octadecylacrylate)-grafted silica (Sil-ODA_n, n = 25) which has been reported by us. Sil-ODA_n performs enhanced molecular shape selectivity of PAHs in RP-HPLC through multiple carbonyl–π interaction of aligned carbonyl groups which are induced by the formation of highly oriented structure of side chains. Differential scanning calorimetry of VOD₂₃ demonstrated that octadecyl alkyl chains showed crystalline to isotropic phase transition with endothermic peak at 48.7 °C which was similar to ODA₂₅ (at 47.8 °C). After grafting of both polymers, phase transition phenomenon was completely disappeared in Sil-VOD₂₃ whereas Sil-ODA₂₅ still exhibits phase transition although at lower endothermic peak top temperature (38.5 °C). This indicates that the slight structural change in Sil-VOD_n and Sil-ODA_n influence the ordered structure of side alkyl chains. Moreover, solid-state ¹³C NMR revealed that the long alkyl chain in Sil-VOD₂₃ is highly disordered as compared with that of Sil-ODA₂₅. Sil-VOD₂₃ was applied to RP-HPLC stationary phase using PAHs as π-electron containing elutes, and compared with Sil-ODA₂₅ and conventional monomeric octadecylated silica (ODS). Results confirmed that Sil-VOD₂₃ showed much higher selectivity for PAH isomers than ODS, but lower than Sil-ODA₂₅. For example, the separation factors for trans-/cis-stilbene were 1.47 (Sil-VOD₂₃), 1.70 (Sil-ODA₂₅) and 1.07 (ODS), respectively. These results indicate that carbonyl groups in Sil-VOD₂₃ are effective for molecular shape recognition of PAHs through carbonyl–π interactions even in the disordered state.     

Recognition of critical pairs of polycyclic aromatics on crystalline, liquid-crystalline and isotropic regions of silica-supported polymer in HPLC

Summary The silica-supported comb-shaped polymer (Sil-ODA₁₈) with a “crystalline-to-isotropic” phase transition showed the characteristic behaviors of both polymeric and monomeric ODS phases. This was examined using a SRM 869 test mixture. Furthermore, Sil-ODA₁₈ exhibited a specific selectivity towards the critical pairs of polycyclic aromatic hydrocarbons (PAHs), such as acenaphthene and fluorene, or benz(a)anthracene and chrysene, or benzo(ghi)perylene, indeno(1,2,3-cd)pyrene and dibenz(a,h)anthracene. The separation of these solutes was controlled by selecting temperature and solvent systems. The selectivity on Sil-ODA₁₈ was also compared with a conventional ODS (octadecylsilyl silica gel) column. The selectivity on a Sil-ODA₁₈ matrix is attributable to an orientation change of the long-chain alkyl substituents on the surface of the silica base material on transition from crystalline to isotropic state.     

Tuning of the molecular packing structure of comb-shaped polymer-grafted silica by using surface-initiated ATRP to enhance the molecular-shape selectivity towards polycyclic aromatic hydrocarbons

Poly(octadecyl acrylate)-grafted silicas were prepared by surface-initiated atom transfer radical polymerization (ATRP). Initially, undecyl ester and allyl ester-based ATRP initiators were synthesized and then immobilized on silica. The surface-initiated ATRP of octadecyl acrylate was carried out from the initiator-grafted silicas using copper(I) bromide and N,N,N′,N′,N′′-pentamethyldiethylenetriamine as catalyst precursors to produce poly(octadecyl acrylate)-grafted silicas, Sil-C₁₁-ODA_n (obtained from undecyl ester) and Sil-C₃-ODA_n (originated from allyl ester), respectively. Both Sil-C₁₁-ODA_n and Sil-C₃-ODA_n were characterized by DRIFT, suspension-state ¹H NMR, solid-state ¹³C CP/MAS NMR spectroscopies, thermogravimetric analysis (TGA), elemental analysis and differential scanning calorimetry (DSC) measurements. Suspension-state ¹H NMR, solid-state ¹³C CP/MAS NMR and DSC analyses suggest that Sil-C₁₁-ODA_n demonstrated more ordered structure than Sil-C₃-ODA_n. In this paper, it is also described that for ordering of the polymer phase is accompanied by the selectivity increase for the separation of poly cyclic aromatic hydrocarbons (PAHs) in RP-HPLC.     

Selectivity enhancement of diastereomer separation in RPLC using crystalline-organic phase-bonded silica

Summary Poly(octadecyl acrylate) exists in a crystalline state at room temperature. This crystalline state was formed even after being grafted onto silica through the terminal group of the polymer. When this polymer-grafted silica (Sil-ODA_n) was used as a reversed phase liquid chromatography packing material, better selectivity for diastereomerized phenylethylamines derivatized by (S)-(−)-(2,3-naphthalenedicarboximidyl)propionyl fluoride was observed at room temperature than was observed for simple octadecylated silica (ODS). On the other hand, increasing temperature reduced the selectivity to be close to that of ODS. Additional experiments showed the better selectivity derived from Sil-ODA_n was due to both the highly-oriented structure of the polymer and the carbonyl-π interaction with the diastereomer. A theoretical investigation of this carbonyl-π interaction is also described briefly.     

Facile synthesis of high-density poly(octadecyl acrylate)-grafted silica for reversed-phase high-performance liquid chromatography by surface-initiated atom transfer radical polymerization

To introduce high-density polymeric organic phase onto silica, initiator-modified silica was prepared and then surface-initiated atom transfer radical polymerization (ATRP) ("grafting-from" method) was carried out with octadecyl acrylate. The resultant polymer-grafted silica was characterized by diffuse reflectance infrared Fourier transform, suspension-state (1)H NMR, solid-state (13)C cross-polarization magic angle spinning nuclear magnetic resonance (CP-MAS-NMR), solid-state (29)Si-CP-MAS-NMR spectroscopies, elemental analysis and differential scanning calorimetry measurements. ATRP-based poly(octadecyl acrylate)-grafted silica (Sil-ODA(n)-1), was used as a stationary phase in high-performance liquid chromatography (HPLC) and the chromatographic behavior was evaluated by the retention studies of polycyclic aromatic hydrocarbons (PAHs) and aromatic positional isomers. Compared with previous poly(octadecyl acrylate)-grafted silica (Sil-ODA(n)), which was prepared by the "grafting-to" method, we have observed longer retention and greater selectivity for Sil-ODA(n)-1 towards PAHs event at higher temperature.     

Temperature effect in separation of fullerene by high-performance liquid chromatography

Summary The effect of column temperature, especially at low temperatures, on the separation of fullerenes on monomeric and polymeric octadecyl silica (ODS) bonded phases has been studied. Decreasing the column temperature induces an increase in selectivity. The best temperature for the separation of fullerenes was determined for both types of ODS phase with n-hexane eluent. The selectivity for higher fullerenes on monomeric phases becomes similar to that on polymeric phases to low temperature. It has been found that as the carbon content of monomeric phases is increased, the selectivity also becomes similar to polymeric phases.     

Molecular motion of the main chain for a series of Poly(alkyl l-glutamate)s as studied by 2H NMR

²H NMR measurements were carried out for a series of poly(alkyl l-glutamate)s (PALG) in which the ¹H of the amide group in the main chain is replaced by ²H in order to investigate the mobility and motional mode of the main chain. At low temperature, the ²H spectra were typical powder patterns, which have three principal values. The temperature dependencies for the ²H NMR spectra varied with the side chain length. For PALG with a short side chain length, Δv₁, Δv₂, and Δv₃ are almost constant in all temperature ranges. As the side chain length increases, the difference between the peaks and shoulders decreased with temperature. For PG-12-N-D, the peaks and shoulders are fused at high temperature in a liquid crystalline state. The mobility and molecular motion of the main chain is discussed based on the obtained ²H NMR spectra.     

Polymer complexes XXXIV. Potentiometric and thermodynamic studies of monomeric and polymeric complexes containing 2-acrylamidosulphadiazine

2-Acrylamidosulphadiazine (ASD) was synthesized and characterized by elemental analyses, IR and 1H NMR spectra. Proton-monomeric ligand dissociation and metal-monomeric ligand stability constants of ASD with some metal ions were determined potentiometrically in 0.1 M KCl and 40% (v/v) ethanol-water mixture. In the presence of 2,2^′-azobisisobutyronitrile as initiator, the dissociation and stability constants of ASD were determined in polymeric form (PASD). The influence of temperature on the dissociation of ASD and the stability constants of their complexes in monomeric and polymeric forms were critically studied. The pK₂^H value of PASD was found to be higher than ASD, this means that the vinyl group in the monomeric form decreases the electron density and hence reduces the N-H bond strength. The stability constants of the metal complexes in polymeric form are higher than those of the monomeric form. This is quite reasonable because the ligand in a polymeric form is considered as a better complexing agent.     

Highly hydrophilic and nonionic poly(2-vinyloxazoline)-grafted silica: a novel organic phase for high-selectivity hydrophilic interaction chromatography

A new hydrophilic and nonionic poly(2-vinyloxazoline)-grafted silica (Sil-VOX _n ) phase was synthesized and applied for the separation of nucleosides and nucleobases in hydrophilic interaction chromatography (HILIC). Polymerization and immobilization onto silica were confirmed by using characterization techniques including ¹H NMR spectroscopy, elemental analysis, and diffuse reflectance infrared Fourier transform spectroscopy. The hydrophilicity or wettability of Sil-VOX _n was observed by measuring the contact angle (59.9°). The chromatographic results were compared with those obtained with a conventional HILIC silica column. The Sil-VOX _n phase showed much better separation of polar test analytes than the silica column, and the elution order was different. Differences in selectivity between these two columns indicate that the stationary phase cannot function merely as an inert support for a water layer into which the solutes are partitioned from the bulk mobile phase. To elucidate the interaction mechanism, the separation of dihydroxybenzene isomers was performed on both columns in normal-phase liquid chromatography. Sil-VOX _n was very sensitive to the dipole moments of the positional isomers of polycyclic aromatic compounds in normal-phase liquid chromatography. The interaction mechanism for Sil-VOX _n in HILIC separation is also described.

Surface Chemistry of Nanohybrids with Fumed Silica Functionalized by Polydimethylsiloxane/Dimethyl Carbonate Studied Using 1H, 13C,and 29Si Solid-State NMR Spectroscopy

The investigation of molecular interactions between a silica surface and organic/inorganic polymers is crucial for deeper understanding of the dominant mechanisms of surface functionalization. In this work, attachment of various depolymerized polydimethylsiloxanes (PDMS) of different chain lengths, affected by dimethyl carbonate (DMC), to silica nanoparticles pretreated at different temperatures has been studied using ²⁹Si, ¹H, and ¹³C solid-state NMR spectroscopy. The results show that grafting of different modifier blends onto a preheated silica surface depends strongly on the specific surface area (SSA) linked to the silica nanoparticle size distributions affecting all textural characteristics. The pretreatment at 400 °C results in a greater degree of the modification of (i) A-150 (SSA = 150 m²/g) by PDMS-10/DMC and PDMS-1000/DMC blends; (ii) A-200 by PDMS-10/DMC and PDMS-100/DMC blends; and (iii) A-300 by PDMS-100/DMC and PDMS-1000/DMC blends. The spectral features observed using solid-state NMR spectroscopy suggest that the main surface products of the reactions of various depolymerized PDMS with pretreated nanosilica particles are the (CH₃)₃SiO-[(CH₃)₂SiO-]_x fragments. The reactions occur with the siloxane bond breakage by DMC and replacing surface hydroxyls. Changes in the chemical shifts and line widths, as shown by solid-state NMR, provide novel information on the whole structure of functionalized nanosilica particles. This study highlights the major role of solid-state NMR spectroscopy for comprehensive characterization of functionalized solid surfaces.     

Silica-Grafted Tris(neopentyl)aluminum: A Monomeric Aluminum Solid Co-catalyst for Efficient Nickel-Catalyzed Ethene Dimerization

A silica-supported monomeric alkylaluminum co-catalyst was prepared via surface organometallic chemistry by contacting tris(neopentyl)aluminum and partially dehydroxylated silica. This system, fully characterized by solid-state ²⁷Al NMR spectroscopy augmented by computational studies, efficiently activates (ⁿBu₃P)₂NiCl₂ towards dimerization of ethene, demonstrating comparable activity to previously reported dimeric diethylaluminum chloride supported on silica. Three types of aluminum surface species have been identified: monografted tetracoordinated Al species as well as two types of bisgrafted Al species—tetra- and pentacoordinated. Of them, only the monografted Al species is proposed to be able to activate the (ⁿBu₃P)₂NiCl₂ complex and generate the active cationic species.     

Preparation and Characterization of a New Quercetin‐bonded Stationary Phase for High Performance Liquid Chromatography

A quercetin‐bonded silica gel stationary phase (QUSP) containing natural flavonoid ligand was first prepared via γ‐glycidoxypropyltrimethoxysilane (KH‐560) as a coupling reagent for high‐performance liquid chromatography. Its chemical structure was characterized by Fourier infrared spectroscopy, elemental analysis, thermal thermogravimetry and ¹³C cross polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR). The chromatographic property of QUSP was systematically evaluated by using neutral, basic and acidic aromatic compounds as probes. In order to clarify its retention mechanism, a comparative study of QUSP with conventional octadecylsilyl‐bonded stationary phase (ODS) was also carried out under the same conditions. The results showed that the new quercetin‐bonded phase exhibited an excellent reversed‐phase chromatographic property with relatively weak hydrophobicity. However, it has an advantage over ODS in the fast separation of polar aromatic compounds because the quercetin ligand could provide various sites besides hydrophobicity, such as hydrogen bonding, dipole‐dipole, π‐π staking and charge transfer interactions. QUSP was performed in the baseline separations of ionized polar basic or acidic compounds, including pyridines, anilines, pyrimidines, purines and phenols with symmetric peak shape in common mobile phases without buffer salt within relatively short time. The natural ligands from herbs are readily available and contain a variety of active sites, which facilitate the exploration of industrial chromatographic separation materials for green products.     

The effect of silica nanoparticles on the morphology, mechanical properties and thermal degradation kinetics of PMMA

Silica-PMMA nanocomposites with different silica quantities were prepared by a melt compounding method. The effect of silica amount, in the range 1-5 wt.%, on the morphology, mechanical properties and thermal degradation kinetics of PMMA was investigated by means of transmission electron microscopy (TEM), X-ray diffractometry (XRD), dynamic mechanical analysis (DMA), thermogravimetric analyses (TGA), Fourier-transform infrared spectroscopy (FTIR), ¹³C cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy (¹³C{¹H} CP-MAS NMR) and measures of proton spin-lattice relaxation time in the rotating frame (T_1ρ(H)), in the laboratory frame (T₁(H)) and cross-polarization times (T_CH). Results showed that silica nanoparticles are well dispersed in the polymeric matrix whose structure remains amorphous. The degradation of the polymer occurs at higher temperature in the presence of silica because of the interaction between the two components.     

A new imidazolium-embedded C18 stationary phase with enhanced performance in reversed-phase liquid chromatography

In this paper, a new imidazolium-embedded C₁₈ stationary phase (SiImC₁₈) for reversed-phase high-performance liquid chromatography is described. 1-Allyl-3-octadecylimidazolium bromide ionic liquid compound having a long alkyl chain and reactive groups was newly prepared and grafted onto 3-mercaptopropyltrimethoxysilane-modified silica via a surface-initiated radical-chain transfer addition reaction. The SiImC₁₈ obtained was characterized by elemental analysis, infrared spectroscopy, thermogravimetric analysis, diffuse reflectance infrared Fourier transform, and solid-state ¹³C and ²⁹Si cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy. The selectivity toward polycyclic aromatic hydrocarbons relative to that toward alkylbenzenes exhibited by SiImC₁₈ was higher than the corresponding selectivity exhibited by a conventional octadecyl silica (ODS) column, which could be explained by electrostatic π–π interaction cationic imidazolium and electron-rich aromatic rings. On the other hand, SiImC₁₈ also showed high selectivity for polar compounds, which was based on the multiple interaction and retention mechanisms of this phase with different analytes. 1,6-Dinitropyrene and 1,8-dinitropyrene, which form a positional isomer pair of dipolar compounds, were separated successfully with the SiImC₁₈ phase. Seven nucleosides and bases (i.e. cytidine, uracil, uridine, thymine, guanosine, xanthosine, and adenosine) were separated using only water as the mobile phase within 8 min, which is difficult to achieve when using conventional hydrophobic columns such as ODS. The combination of electrostatic and hydrophobic interactions is important for the effective separation of such basic compounds without the use of any organic additive as the eluent in the octadecylimidazolium column.     

Synthesis and liquid crystal properties of phthalocyanine bearing a star polytetrahydrofuran moiety

A new polymeric ligand, 6-(3,4-dicyanophenylthio)-hexyl-2-polytetrahydrofuranacetate (2) and its liquid crystalline polymeric phthalocyanine, 2,9,16,23-tetrakis-{6-(polytetrahydrofuran-2-carboxylate)-hexylthio-phthalocyaninatocobalt(II) (3), {Co[Pc(S-C₆H₁₃OCO-poly-THF)₄]}(CoPcLC) have been synthesized. The ligand and the phthalocyanine, bearing polytetrahydrofuran moieties (poly-THF), were characterized using elemental analysis, FTIR, ¹H and ¹³C NMR, and UV–Vis techniques. CoPcLC (3) promotes a greater interaction between the mesogens, resulting in solution aggregations together with a red-shift in the Q-band in the presence of the soft Ag⁺ ion. The dielectric anisotropy and phase transition temperature values of CoPcLC doped with 4-cyano-4′-n-pentylbiphenyl (5CB) were found to be 7.17 and 40.5 °C, respectively. The dielectrical anisotropy behaviour of the liquid crystals changes from the positive to the negative type. The current–voltage characteristics of the liquid crystals show a non-linear behaviour.     

Structures and chromatographic characteristics of capsule-type silica gels coated with hydrophobic polymers in reversed-phase liquid chromatography

Summary Silicone polymer-coated silica gels modified with octadecyl and octyl groups (S/S-C₁₈, S/S-C₈), or “capsule-type silica gels” were developed as packing materials for reversed-phase liquid chromatography. They were obtained by coating the surface of totally porous silica gel with a homogeneous silicone polymer film, and thereafter modifying the coating polymer with octadecyl or octyl groups. Retaining the advantages of silica-based packings, they show strong resistance of alkali-like organic porous polymeric materials.     

Speciation of prehydrolyzed Al salt coagulants with electrospray ionization time-of-flight mass spectrometry and Al NMR spectroscopy

Based on the speciation results of the most two concerned coagulant component (i.e., monomer and Keggin-Al₁₃), Al species in polymeric Al salt coagulants were fully investigated with the combination of electrospray ionization time-of-flight mass spectrometry and ²⁷Al NMR spectroscopy. Keggin-Al₁₃⁷⁺ could transform into Al₁₃ⁿ⁺ (n = 1-3) by dehydrogen reaction without destroying the Keggin structure in mass spectrometer. There exist differences in the intensity and the observed sequence of the Al₁₃ⁿ⁺ (n = 1-3) species in the mass spectra of polymeric Al coagulants. Several other polymers (i.e., Al₁₉³⁺, Al₂₀³⁺ and Al₁₆ⁿ⁺, n = 1-3) might also be formed by the decomposition and repolymerization of Keggin-Al₁₃⁷⁺. Like monomeric Al salt coagulant, species in polymeric Al coagulants with low basicity were mainly detected as low polymers with mono-charge in mass spectrometry. With the increase of basicity, the dominant species often transform into high polymers with higher charges and fewer categories. The Al₁₃³⁺ species detected in monomeric Al coagulant should have octahedral structure and be formed by self hydrolysis, which is different with the species detected in purified Al₁₃ coagulant. On the whole, the detected species in mass spectrometry could roughly represent their dissolution status in original solutions and could also be used to explain the difference of their coagulation performance in water treatment process.     

Head-to-head and head-to-tail multilayer n-alkylsilsesquioxane films

Two lamellar organosilica films made up of stacks of head-to-head and head-to-tail alkyl bilayers were synthesized by a UV-driven polymerization of n-C₁₈H₃₇SiCl₃ and n-C₁₈H₃₇Si(OCH₃)₃, respectively. In addition to thermogravimetry and small-angle X-ray scattering, the characterization of these multilayer systems was focused on a variety of multinuclear solid-state NMR methods (¹H, ²⁹Si, ¹³C, 2D WISE). The chemical structure of the siloxane interlayer, the alkyl chain conformation and thermal stability were systematically investigated and compared. Through its marked sensitivity to molecular motion, solid-state NMR was used to investigate the effect of interdigitation on alkyl chain dynamics.     

Synthesis and structural characterization of monomeric and polymeric supramolecular organotin(IV) 4-chlorophenylethanoates

Four monomeric [n-Bu₂SnL₂ (1), Et₂SnL₂ (2), Me₂SnL₂ (3), and n-Oct₂SnL₂ (7)] and three polymeric {[n-Bu₃SnL]_n (4), [Me₃SnL]_n (5), and [Ph₃SnL]_n (6)} organotin(IV) carboxylates, where L?=?4-chlorophenylethanoate, were synthesized and characterized by elemental analysis, FT-IR, and multinuclear NMR (¹H, ¹³C, and ¹¹⁹Sn). Compounds 2 and 5 were also analyzed by X-ray single-crystal analysis showing monomeric and zigzag structures, respectively. Two types of O…H (2.641?Å) and Cl…H (2.943?Å) non-covalent interactions generate a 2-D supramolecular structure for 2. Layer-by-layer supramolecular structure was observed for 5 in which polymeric chains are connected via non-covalent interactions {Cl…H (2.869?Å), H…π (2.899?Å)}, and unconventional dihydrogen {H…H (2.381?Å)} interactions.     

Microstructure elucidation of historic silk (Bombyx mori) by nuclear magnetic resonance

¹H NMR cryoporometry and solid-state ¹³C cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy were used to characterize the microstructure of historic and fresh silk samples. Silk is a polymeric bicomponent material composed of fibroin and water located in micropores. According to the ¹H NMR cryoporometry method, the intensity of the water resonance as a function of the temperature was used to obtain the pore size distribution, which was strongly asymmetric with a well-defined maximum at 1.1 nm. Compared with the fresh silk samples, the volume of pores around 1.1 nm decreased distinctly in the historic silk, and more pores larger than 2 nm emerged accordingly. In addition, these results correlated well with solid-state ¹³C CP/MAS NMR spectroscopy as the percentage of random coil in the historic silk sample was much less than that in the fresh silk samples. Therefore, it is suggested that the water-filled microvoids grow larger as the random coil conformation fades away in the degradation process.