Discrimination of block structures in liquid adsorption chromatography of polymers. Simulation and experiment.

Di- and triblocks can be separated from each other under adsorption conditions for the outer block and critical or exclusion conditions for the inner block. If the center block is uniform, even a separation according to symmetry can be achieved. This behavior is studied in much detail by theory and simulations as well as in the chromatographic experiments with block copolymers of ethylene oxide and ɛ-caprolactone.

Separation of telechelic oligomers according to architecture by liquid chromatography.

Telechelic oligomers based on diethylene glycol or lower polyethylene glycols and caprolactone or butene oxide can be separated according to their architecture by means of liquid chromatography under adsorption conditions for the hydrophobic block and critical or exclusion conditions for PEG. This behavior, which is predicted by the theory, can be verified by the experiment for block copolymers of ethylene oxide with ɛ-caprolactone and butene oxide. The individual peaks were identified by matrix-assisted time-of-flight mass spectrometry (MALDI-TOF-MS) of fractions obtained by semipreparative liquid chromatography at critical conditions of center block (PEG) and adsorption conditions for outer blocks (PCL and PBO).

Separation of polysorbates by liquid chromatography on a HILIC column and identification of peaks by MALDI-TOF MS.

Polysorbates can be separated according to their functionality and architecture by liquid chromatography on a hydrophilic interaction chromatography (HILIC) column in acetone-water mobile phases containing 90-97% acetone. The different polymer homologous series are separated according to the number of terminal hydroxy groups and elute as narrow peaks. The hydrophilic part (ethoxylates of sorbitan, isosorbide, and poly(ethyleneglycol)) and amphiphilic funtionalites (ethoxylated mono-, di-, tri-, and tetraesters) were separated by HILIC mode of high-performance liquid chromatography (HPLC). All these separated functionaities are identified and confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). This combination of HPLC and MALDI-TOF MS has been proven to be an excellent tool for the characterization of heterogenous complex samples.

Characterization of polyoxyethylenes according to the number of hydroxy end groups by hydrophilic interaction chromatography at critical conditions for polyethylene glycol.

Polyoxyethylenes with different functionality and architecture can be separated according to the number of terminal hydroxy groups on polar stationary phases in acetone water mobile phases containing 90-97% acetone. The best results were obtained on a HILIC column. Typical samples, which can be analyzed by this technique, are polyethylene glycols, their mono- and dialkyl ethers, macromonomers, the fatty esters of PEG, and ethoxylated glycerol.

Analysis of polyethyleneoxide macromonomers by liquid chromatography along the critical adsorption line.

Polyoxyethylene macromonomers are analyzed by one-dimensional liquid chromatography under different conditions, depending on the required information. These samples may contain polyethylene glycol (PEG) and the corresponding di(meth)acrylate besides the desired mono(meth)acrylate. The molar mass distribution (MMD) of the PEG and the monoester can be obtained by liquid adsorption chromatography (LAC) on a reversed-phase column in acetone-water with a gradient from 10% to 20% acetone. The MMD of the diesters can be obtained with isocratic elution by liquid chromatography at critical conditions (LCCC) on a reversed-phase column in 31% acetone, or using size-exclusion conditions for PEG and LAC conditions for the end groups, which is the case in 40-55% acetone. The absolute amount of the series with different functionality can be obtained by LCCC in ternary mobile phases consisting of acetone, methanol, and water along the critical adsorption line. Under such conditions, all series elute as narrow peaks (regardless their MMD), which can easily be integrated and quantified.

Separation of all oligomers in polyethylene glycols and their monomethyl ethers by one-dimensional liquid chromatography.

Different polymer homologous series having the same repeat unit, but different end groups, can be separated by one-dimensional LAC according to the number of repeat units and functionality, if a favorable combination of the interaction parameters of the repeat unit and the end groups can be found. As an example, polyethylene glycol (PEG) can be determined in PEG monomethyl ethers. The molar mass distribution of the minor component in such samples can be determined even at concentrations of a few percent.

Quantification in the analysis of polyethylene glycols and their monomethyl ethers by liquid adsorption chromatography with different detectors.

It is shown that the molar mass distribution of polyethylene glycols (PEGs) and their monomethyl ethers can be determined by liquid adsorption chromatography (LAC) on reversed phases using isocratic or gradient elution. In gradient LAC, the evaporative light scattering detector (ELSD) has to be used, which is, however, problematic with respect to quantification. The response factors of the individual oligomers depend strongly on the operating conditions, molar mass, and sample size. These problems do not arise with density and refractive index detection, which can, however, only be applied with isocratic elution. A comparison of the results obtained with these three detectors showed that calibration of the ELSD has to be performed very carefully.

Separation of nonionic surfactants according to functionality by hydrophilic interaction chromatography and comprehensive two-dimensional liquid chromatography.

It is shown, that amphiphilic polymers--such as polysorbates and fatty esters of polyethylene glycol can be separated by comprehensive two-dimensional liquid chromatography using a reversed phase column (under critical conditions for the polyoxyethylene chain) and a HILIC column, which may arranged in different order. The mobile phases in both dimensions can be 93-97 wt% acetone water. As the retention of higher esters on the reversed phase column is very strong, this column should be used as the first dimension. On the HILIC column all fractions elute within a reasonably short time (at a flow rate of 2.5 ml/min within 2 min). With a flow rate of 0.1 ml/min in the first dimension, a full separation can be achieved in 90 min.

Liquid chromatography at critical conditions in ternary mobile phases: gradient elution along the critical line.

In ternary mobile phases consisting of acetone, methanol, and water, the retention of PEG on reversed-phase columns is independent on molar mass at certain compositions of the mobile phase. Along this critical adsorption line, the retention of polypropylene glycol varies quite strongly, which can be utilized in the separation of block copolymers. Gradient elution along the critical line allows a baseline separation of all oligomers in polypropylene glycol up to approximately 25 propylene oxide units. The same resolution can be achieved in the separation of ethylene oxide-propylene oxide block copolymers, regardless of the length of the ethylene oxide block.

Characterization of PEG in monomethyl ethers of PEG by 2-D LC.

PEGs present in samples of the corresponding monomethyl ethers are analyzed by 2-D LC with LC at critical conditions as the first and liquid adsorption chromatography as the second dimension. The fractions from the first dimension are transferred to the second one by heart-cutting using a ten-port valve. The efficiency of the separation is dramatically improved by adding water to the eluate from the first dimension before the switching valve in order to have a weaker eluent in the fraction than that of the second dimension. By this procedure, the fractions are focussed and reconcentrated. The molar mass distribution of the PEG fraction can be determined with good accuracy even at very low amounts of PEG in the PEG-monomethyl ethers (down to 0.5%).

Adsorption interaction parameter of polyethers in ternary mobile phases: the critical adsorption line.

It is shown that in LC of polymers, the interaction parameter in ternary mobile phases can be described by a plane, which is determined by the dependencies in binary mobile phases. Instead of a critical adsorption point, critical conditions are observed along a straight line of composition between the two critical points in binary mobile phases. Consequently, a separation of block copolymers under critical conditions for one block by an adsorption mechanism for the other block can be achieved in ternary mobile phases of different compositions, which allows an adjustment of the retention of the adsorbing block.

Retention of functional polymers in liquid adsorption chromatography: effect of the end groups in PEGs and their methyl ethers in different mobile phases.

It is shown that the end group parameter, which describes the influence of the end group on retention, can be determined in RP chromatography from two chromatograms of a nonfunctional and a monofunctional sample, if a sufficient number of peaks with the same number of repeat units in each sample can be reasonably resolved and identified. The same procedure can also be applied for pairs of di- and monofunctional polymers. End group parameters have been determined in three different mobile phases: acetonitrile-water, acetone-water and methanol-water of different compositions. The temperature dependence of the interaction parameter of the repeat unit and the end group parameter has been found to be different, which allows a fine-tuning of retention.

Determination of thermodynamic parameters in reversed phase chromatography for polyethylene glycols and their methyl ethers in different mobile phases.

The thermodynamic parameters (entropy and enthalpy change) and their increment per repeat unit have been determined in RP chromatography of PEG and its mono- and dimethyl ethers in different mobile phases. The different approaches for their determination and the problems related to the characteristic volumes (void volume, interstitial volume, pore volume) of the column are discussed. Very different dependences or the thermodynamic parameters were observed in aqueous mobile phases containing ACN, acetone, or methanol as organic modifier. In the first two mobile phases a linear dependence on the number of repeat units n is found, and the additional entropy and enthalpy changes per repeat unit are positive. In methanol-water, the enthalpy change is negative with a non-linear dependence on n, and the entropy change is independent on n and close to zero. The contribution of the end groups is almost the same in the first two mobile phases, but much smaller in methanol-water.

Characterization of polyoxyalkylene block copolymers by combination of different chromatographic techniques and MALDI-TOF-MS.

Polyoxyalkylene diblock copolymers (consisting of PEO as hydrophilic block and PBO or PHO as hydrophobic block) are characterized by combination of two dimensional liquid chromatography and MALDI-TOF-MS. Liquid chromatography under critical conditions (LCCC) is used as first dimension and fractions are collected, mobile phase evaporated and diluted in the mobile phase used in the second dimension (SEC, LCCC or LAC). This two-dimensional chromatography in combination of MALDI-TOF-MS gives information about purity of reaction products, presence of the byproducts, chemical composition and molar mass distribution of all the products.

Calibration of chromatographic systems for quantitative prediction of chromatography of homopolymers.

A method is proposed for obtaining from the experimental data the main parameters, which determine the chromatographic behavior of homopolymers: a dependence of the adsorption interaction parameter on the mobile phase composition, and the pore volume and pore size values. The interaction parameter can be determined by this method in both adsorption, critical, and SEC-type regimes of chromatography. The method is tested in 'mathematical experiments' with theoretical data that mimic experiments, and is applied to calibrate real chromatographic systems. With the obtained calibrations quantitative modeling of chromatograms of PEG samples on two typical ODS columns with different pore size in methanol/water and acetone/water mixed solvents is performed; the simulation results are compared with real chromatograms.

Characterization of ethylene oxide-propylene oxide block copolymers by combination of different chromatographic techniques and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy.

Block copolymers of ethylene oxide (EO) and propylene oxide (PO) are characterized by combination of two-dimensional chromatography and MALDI-TOF-MS. Liquid chromatography under critical conditions (LCCC) is used as first dimension and fractions are collected, mobile phase evaporated and diluted in the mobile phase used in second dimension (SEC or LAC). This two-dimensional chromatography in combination of MALDI-TOF-MS gives information about purity of reaction products, presence of the byproducts, chemical composition and molar mass distribution of all the products.

A data base for polymer chromatography: temperature dependence of interaction parameters in liquid adsorption chromatography.

The temperature dependence of retention behaviour of polyethylene glycol (PEG) and its mono- and dimethyl ethers was studied on various RP columns in different mobile phases. The accessible volumes and the interaction parameters were determined from slope and intercept in a plot of the elution volumes of the oligomers of a polymer homologous series as a function of the difference of the elution volumes of consecutive oligomers. A quite different dependence of the interaction parameters was observed in the different mobile phases. While in methanol-water the interaction parameter decreases with increasing temperature, the opposite effect is observed in acetonitrile (ACN)-water. In acetone-water, the temperature dependence is almost negligible.

Full separation of oligomers in block copolymers of ethylene oxide and propylene oxide.

In this study di- and triblock copolymers of ethylene oxide (EO) and propylene oxide (PO) are analyzed by using CAP for one block and adsorption for the other block. This gives a complete picture of EO- and PO-based block copolymer with respect to the oligomers of both blocks. A full resolution of the individual oligomers can be achieved for both blocks up to an average molecular weight of 1000-1500 of each block.

Characterization of poly(ethylene glycol)-b-poly(epsilon-caprolactone) by two-dimensional liquid chromatography.

Block copolymers of ethylene oxide and epsilon-caprolactone were synthesized by microwave-assisted polymerization of epsilon-caprolactone with polyethylene glycol monomethyl ethers as initiator. The samples thus obtained were characterized by two-dimensional liquid chromatography with liquid chromatography at critical conditions as the first and liquid exclusion adsorption chromatography as the second dimension. A full baseline separation of all oligomers could be achieved in both dimensions.

Liquid chromatography under critical conditions: practical applications in the analysis of amphiphilic polymers.

Liquid chromatography under critical conditions (LCCC) allows the separation of block copolymers from the corresponding homopolymers as well as the separation of homopolymers according to their functionality. At the transition of exclusion and adsorption mode, the polymer chain becomes "chromatographically invisible," and thus a separation according to other structural units can be achieved. In the case of block copolymers this situation can be utilized to detect and determine unwanted homopolymers. At critical conditions for the repeat unit, the other block may be eluted in the exclusion or the adsorption regime. In the first case, the block copolymer is eluted before the homopolymer, and its molar mass can be determined as in size-exclusion chromatography. In the second case, it is eluted later than the homopolymer, and the separation of the individual oligomers can be achieved. Advantages and limitations of different approaches in LCCC are discussed.