Determination of molecular weight distribution for polypropylene by column fractionation and gel-permeation chromatography

Investigations on the molecular weight distribution of cyrstalline polypropylene were carried out by column fractionation and gel-permeation chromatography for the purpose of comparing the determination methods. Average molecular weight, standard deviation, skewness, and kurtosis were calculated as statistical parameters from the distribution curve of each, and the number-average and weight-average molecular weights were determined by osmometry and light scattering. The molecular weight distribution curves obtained from column fractionation were found to be narrower than those from gel-permeation chromatography, and it was confirmed that the molecular weight distribution curve obtained from GPC was more accurate and reliable than that from column fractionation by the fact that the average molecular weight from GPC agreed fairly well with that from the absolute methods. On the other hand, no clear difference between these determination methods was observed with respect to skewness and kurtosis of the data, while the standard deviation from GPC seemed to be greater than that from column fractionation.     

Determination of the width of a narrow molecular weight distribution by gel-permeation chromatography

A recycle gel-permeation chromatography (GPC) method for extrapolating to infinite resolution is proposed. From the GPC calibration curve, the extrapolated elution peak width volume can be converted into peak width in decades of molecular weight. For an essentially Gaussian distribution, the weight-average/number-average molecular weight ratio can be determined from a calculated conversion table.     

Molecular weight determinations by gel-permeation chromatography and viscometry

The hydrodynamic volume concept can be used effectively with gel-permeation chromatographic (GPC) and viscosity data to estimate the molecular weight of a variety of polymers. Agreement is within ±5–10% of the absolute values and thus is satisfactory for many purposes. An iterative computer technique and a method developed by Funt and Hornof for analyzing GPC–viscosity data were found to be equivalent with respect to estimating the molecular weights for the five cases studied. The latter is easily employed but restricted to the case where the sample of interest and the GPC calibration standards have approximately equal Mark-Houwink parameters. Since GPC measurements are commonly performed in thermodynamically good solvents, the general applicability of the method is not impaired. Using the unperturbed dimensions of the polymer chain to estimate the molecular weight of a variety of polymers was not as satisfactory as the above techniques. This approach generally gave biased molecular weight values (consistently low or consistently high). Agreement with the absolute values ranged from 10 to 30%. We therefore believe that either of the techniques based on the hydrodynamic volume concept can be used more effectively to estimate the molecular weight of a series of polymers than the treatment based on the unperturbed dimension.     

Application of gel-permeation chromatography to studies of the functionality distribution of carboxy- and hydroxy-polybutadienes

Separations of carboxy-polybutadienes and hydroxy-polybutadienes according to functionality have been effected using stepwise elution from silica gel. Recoveries in the 95–100% range have been achieved. Subjection of the fractions obtained from the silica gel separation to analysis via gel-permeation chromatography and infrared or near-infrared spectroscopy yields not only functionality distribution data, but also provides the relationship between molecular weight distribution and functional type. Analytical techniques and interpretation of data are discussed.     

Low-density polyethylene: Variation of branching frequency with molecular weight and its influence on molecular weight as determined by gel-permeation chromatography

A method for determining long-chain branching frequency and molecular weight averages for unfractionated low-density polyethylene (LDPE) by the combined use of gel-permeation chromatography (GPC) and intrinsic viscosity data has been reported (GPC–IV method). The method assumes that the number of long branches λ per unit molecular weight is a constant independent of molecular weight. Recent data reported on λ as a function of molecular weight M in commercial LDPE indicate that this assumption is not generally valid, and concern has been expressed as to the size of the errors in molecular weights calculated using this assumption. The errors associated with assuming that λ is constant were evaluated in this study by first determining the way in which λ varies with M for two typical commerical LDPE resins by fractionation and application of the GPC–IV method to representative fractions. The experimentally determined relations between λ and M were then employed in the calculation of molecular weight and molecular size averages from GPC–IV data on the original unfractonated samples. Although it was found that λ increases with molecular weight for both samples, the results indicate that the error involved in assuming that λ is a constant is no greater than the precision with which molecular weight averages can be evaluated by GPC.     

Molecular weight analysis of pitches and polymeric pitches by gel-permeation chromatography

The technique of gel-permeation chromatography (GPC) has been developed as a method for measuring molecular weight distribution in pitch materials. Molecular weight calibration data were obtained from measurements made on GPC fractions collected from a standard pitch. By solubilization of the polymeric portion of pitch through a reduction with lithium in ethylenediamine, the molecular weight range for analysis was extended to in excess of 3000. Mass spectroscopy has been used to further analyze some of the GPC fractions. The GPC calibration data can be employed, with the aid of computer analysis, to determine quantitatively number-average molecular weights M?_n weight-average molecular weights M?_w, and molecular weight distribution D (= M?_w/M?_n) in pitch materials.     

Determination of the molecular masses of tetrafluoroethylene telomers by gel-permeation chromatography

Gel-permeation chromatography has been employed to study the molecular-mass distribution of tetrafluoroethylene telomers prepared through γ irradiation of 0.05–0.56 mol/l monomer solutions in acetone. The molecular-mass-retention-volume V _R calibration dependence has been plotted from chromatograms of perfluoroenanthic and perfluoropelargonic acids (F(CF₂-CF₂) _n COOH; n = 3 or 4, respectively). The measured V _R-log n curve is located parallel to that plotted earlier for oligo(oxyethylene glycol)s (OH-(CH₂-CH₂O) _n -OH) and is shifted along the volume axis. The shift value is determined by the ratio between logarithmic volumes of chain units of these oligomers. The shift agrees with the assumption that the retention volumes of oligomers with the same spatial chain structure are proportional to the logarithm of the ratio between their van der Waals volumes. This assumption is supported by the quantum-chemical calculation of the molecular volumes of oligomers composed of (CH₂-CH₂) _n , (CH₂-CH₂O) _n , and (CF₂-CF₂) _n fragments. As the concentration of tetrafluoroethylene in the initial solution is increased, the average length of (CF₂-CF₂) _n chains increases from \(\bar n\) ≈ 3 to \(\bar n\) ≥ 8. The maximum values of n = 12–15 are determined by the solubility limit of telomers in THF.     

Molecular weight determination of vinylidene chloride copolymers by gel-permeation chromatography and viscometry

This is the first report on the characterization of several pairs of random copolymers of vinylidene chloride by gel-permeation chromatography in conjunction with viscometry. Such copolymers have not previously Received much attention, so we first determined the individual viscosity–molecular weight relationship. The K–a values were compared with known values for the parent homopolymers. We then derived calibration curves (different for each composition) and the calculation of the average molecular weight. These results are compared to those obtained by direct methods, without fractionation, and to kinetic observations. This method avoids misinterpretation of viscosity data and elution volume. Chromatograms may indicate heterogeneity of the compounds as related to polymerization conditions.     

Analysis of resoles by gel-permeation chromatography

Resoles, the complex, heat-sensitive product mixture from the alkali-catalyzed reactions of phenol with formaldehyde were investigated by gel-permeation chromatography (GPC). The low molecular weight species of these resins which consisted of mono-and dinuclear methylol phenols were resolved into multiple peaks. Model compounds were used to identify the peaks of the specific methylol phenols or methylene etherbridged diphenols. Differences in the refractive index of individual species restricted the quantitative analysis of low molecular weight components in the resole. The effects of sodium and barium hydroxides and hexamethylene-tetramine as catalysts, reaction temperature, and time on the total composition of a resole are demonstrated with the gel-permeation chromatographic spectrum and with the aid of NMR. Formation of a “secondary” resole by methylolation of the bisphenol of formaldehyde was also monitored by GPC.     

Examination of dye-protein interaction by gel-permeation chromatography

The interaction of Cibacron blue F3G-A with two therapeutic proteins, recombinant human growth hormone and recombinant human interferon-alpha2b, has been examined by applying gel-permeation chromatography in combination with the absorption difference spectroscopy. The complexes of these proteins with Cibacron blue F3G-A have been isolated, and their absorbance spectra have been registered. The influence of Cibacron blue F3G-A on the oligomeric state of proteins has been investigated. It was found that Cibacron blue F3G-A promotes the generation of interferon-alpha2b dimers at pH 5.0.     

Superheated water chromatography of low molecular weight polyethylene glycols with ultraviolet detection.

Superheated water chromatography (SWC) with ultraviolet detection was applied to the separation of low molecular weight polyethylene glycols (PEGs). PEG oligomers could be detected sensitively when the detection wavelength was set at 190 nm. The effect of column temperature on the separation of PEG oligomers was investigated. The elution time of all PEG oligomers decreased with increase in the column temperature; linear relationships were obtained between ln k and 1/T. A temperature-programmed SWC separation enabled the baseline separation of a PEG 200 sample within 50 min.     

Rapid on-line analysis of low molecular weight hydrocarbons using glass capillary gas chromatography

Rapid analysis is important for on-line chromatography. Gaseous or vaporized samples have been injected via heated gas sampling valves of less than 500 μl dead volume. The critical sampling and split problems could be solved by temperature programming. The general analysis described could be successfully used inter alia in scouting reactions.     

Parallel preparative high-performance liquid chromatography with on-line molecular mass characterization

High-throughput chemistry (HTC) is now an integral part of the lead discovery process in many pharmaceutical and related chemical companies. As this process becomes refined or improved, with the integration of systems with enhanced capabilities, and the requirement for quality compounds of high purity increases, purification is often considered a bottleneck. Although a wide range of purification techniques is available, high-performance liquid chromatography (HPLC) is usually the preferred method of purification to produce high-purity compounds. Parallel preparative HPLC with robust UV-guided fraction collection has been shown to reduce the bottleneck and complement the parallel synthesis systems. However, despite the success of this technique, post-purification analysis of fractions to identify the target compound adds an additional level of complexity. This paper describes the interfacing of the Biotage Parallex with the MUX interface on a single quadrupole mass spectrometer, thus combining robust UV-guided fractionation with on-line MS characterization.     

How to approach preparative liquid chromatography

Summary This work shows the influence of the injection volume on peak broadening owing to the increase of the sample concentration; use of the maximum injection volume is advocated. Once the chromatographic system is defined, a method for calculating the dimensions of the preparative column is developed. Applications of this method are shown.     

凝胶渗透色谱在高聚物分子量测定中的应用

本文综述了近年来凝胶渗透色谱在油溶性和水溶性高聚物分子量测定中的应用,着重分析测试条件对水溶性样品分子量的影响并归纳了解决的办法。同时,介绍了配备紫外可见和示差折光检测器、示差和光散射检测器后凝胶色谱的应用。此外,还概述了几种常见高聚物分子量的标定和校准方法,以便多种方法相互验证,得到较准确的分子量值。     

Determination of molecular-weight distribution and average molecular weights of block copolymers by gel-permeation chromatography

The problem of preparation of a block copolymer of precise molecular-weight distribution (MWD) and with heterogeneous composition on the basis of gel-permeation chromatography (GPC) data has been investigated. It has been shown that in MWD calculations the distribution f(p) of the composition p in individual GPC fractions should be taken into account. The type of the f(p) functions can be simultaneously established by an independent method, such as use of adsorption-column or thin-layer chromatography sensitive to the composition of the copolymer. It has also been shown that the actual f(p) may be replaced by a corresponding piecewise distribution, of simple form, without decrease in the precision of calculation of the MWD and average molecular weights of most known block copolymers.     

Characterization of diallyl phthalate prepolymers by gel-permeation chromatography

The polydispersity of diallyl phthalate prepolymers was determined by gel-permeation chromatography (GPC). The measurements were carried out in tetrahydrofuran and chloroform on Mexel 2000, Poragel, and Styragel polystyrene gels. The influence of the polymerization conditions, initiator, and type of transfer agent on the molecular weight distribution of the prepolymers is shown. Other correlations of interest, such as molecular weight distribution of Dapon 35 and Dapon 201, are also presented.     

Influence of preparative conditions on molecular weight and stereoregularity distributions of polypropylene

The influence of preparative conditions on the molecular weight and stereoregularity distributions of polypropylene was investigated. The stereoregularity distribution is narrowed by using a highly stereospecific catalyst, by decreasing the polymerization temperature, and for the three-component catalyst by keeping the mole proportion of the electron-donating third component at 0.5. The molecular weight distribution can be narrowed by using a highly stereospecific catalyst, a high monomer concentration, and a high polymerization temperature, and by having a lower conversion, particularly at low monomer concentration. The possibility of long-chain branching in polypropylene was indicated by data from the fractionation of tritium-labeled polymers.