Molecular size and gel formation in branched poly(methyl methacrylate) copolymers

Branched copolymers of methyl methacrylate with ethylene dimethacrylate or pentaerythritol tetramethacrylate with high weight average molecular weights M_w were prepared by free radical polymerization. The double bond conversion of these polymers (β) was measured dilatometrically and the gel point (β_c) was located by extrapolation of bulk viscosity measurements. The polymers were studied by light-scattering, ¹H NMR and viscometry. Hence it was possible to investigate the relationship between the molecular weight M_w and the reduced conversion quantity (1 ? β/β_c)^?1 and also the molecular weight dependence of the z-average mean square radius of gyration 〈S²〉_z. Considerable deviations from the classical Stockmayer theory but also from percolation theory were observed. ¹H NMR showed a relative small extent of ring formation (10 to 24%) and a reduced apparent reactivity of the pendant double bonds, while the delay in the gel formation appeared to be caused by a much higher ring formation (80 to 90%). This apparent inconsistency is explained by a size-dependent ring formation, where the extent of ring formation is increased with the molecular dimensions; simultaneously, the pendant double bonds become progressively less accessible to cross-linking by growing radicals. The molecular weight dependence of 〈S²〉_z is found to be in agreement with this heterogeneous reaction model.     

Molecular weights and molecular weight distributions of poly(trimethylhexamethyleneterephthalamide) (trogamid T®)

Molecular weights of Trogamid T® were determined by ultracentrifugation, light-scattering, osmometry and viscometry, molecular weight distributions by gel permeation chromatography (GPC) and five different fractionation methods. The separation efficiency of GPC with N,N-dimethylformamide as solvent was tested with three different types of gels, crosslinked polystyrene, porous glass and crosslinked poly(vinyl acetate), the last one yielding the best results. Solution fractionations show better results than precipitation methods. All fractionations gave far too small apparent non-uniformities. After correction according to the non-uniformities of the fractions the results corresponded to those of the other methods. All methods resulted in U = M?_w/M?_n ? 1 = 0,7 for the technical product Trogamid T®. The discrepancy to U = 1 of a Schulz-Flory distribution can be explained by the influence of the path of reaction.     

Baker-Williams fractionation of high molecular weight polyisobutylene and its evaluation by gel permeation chromatography

A sample from commercially available polyisobutylene with a viscosity average molecular weight of M?_v = 320 000 is fractioned according to Baker-Williams. 9 Fractions are recovered and are characterized by viscometry and gel permeation chromatography (GPC). Molecularly very homogeneous fractions with an inhomogeneity U = (M?_w/M?_n)?1 ≈ 0,05 can be obtained in the range of molecular weights up to M?_v = 600 000. Inversion of molecular weight occurs in the last three fractions. A 3 wt.-% portion with M?_v>10⁶ is lost during the fractionation.     

Molecular weight distributions of poly(methyl methacrylate) produced at high viscosities

The molecular weight distribution of poly(methyl methacrylate), which is produced by the free-radical polymerisation of methyl methacrylate, is influenced by the presence of poly(methyl methacrylate) which is dissolved in the monomer prior to the polymerisation. An experimental technique has been devised which permits a distinction to be made between polymer which is present at the start of the reaction and the polymer which is formed by the reaction. Mathematical analyses of alternative reaction schemes have been made. These predicted changes that would occur in the molecular weight distribution of the new polymer when the solution viscosity increased. Comparison with experimental measurements of molecular weights makes possible the distinction between the alternative reaction schemes. The results are compatible with a reaction mechanism in which the mobilities of the growing radicals are not influenced equally by the solution viscosity. The results are not those which would be expected by the simple inclusion of primary radical termination in the reaction scheme. In some cases, analysis by gel permeation chromatography showed that the molecular weight distribution of the poly(methyl methacrylate) formed by the reaction is bimodal. This is consistent with predictions from the mathematical analysis.     

Molecular size estimates of the human phosphoglucomutase isozymes by gel filtration chromatography

1. Molecular sizes of the three sets of the human phosphoglucomutase isozymes attributable to the loci PGM₁, PGM₂ and PGM₃ have been studied by gel nitration. 2. The PGM₂ isozymes (M.W. C. 61,000) appear to be significantly larger than the PGM₁ and PGM₃ isozymes and the PGM₃ isozymes (M.W. C. 53,000) appear to be slightly larger than the PGM₁ isozymes (M.W. C. 51,000). 3. No inter-allelic or intra-allelic differences in molecular size were detected. 4. The estimated molecular size for rabbit muscle PGM by the same method is similar to that obtained for the human PGM₁ isozymes. This is somewhat lower than previously reported estimates obtained by ultra centrifugation.     

Rapid molecular weight estimation and separation of selected immunoglobulin chains by high speed gel filtration

Rapid and reliable molecular weight estimations of reduced and alkylated immunoglobulin heavy or light chains were performed by high speed gel filtration in 6 M guanidinium chloride using a short (30 cm X 7.5 mm) TSK 3000 SW type column. Molecular weight estimations based on Kav values of eluted polypeptides and glycopolypeptides were generally unaffected by protein bound carbohydrate. Rapid separation of immunoglobulin H and L chains was also achieved during high speed gel filtration.     

Molecular weight determination of isotactic polypropylene by high-sensitivity ebulliometry

The use of a new differential ebulliometer and the determination of the number average molecular weight of unfractionated isotactic polypropylenes is described; the results were reproducible and of satisfactory accuracy. Application of a thermopile with 150 junctions and of suitable devices for the regularity of boiling allowed measurements of number average molecular weights M?_n up to 100000 in good agreement with results obtained by other methods. In this way it is possible to measure M?_n directly, e.g. for reliable determinations of M?_w/M?_n also for polymer samples with low molecular weights and/or wide molecular weight distributions.     

Optimization of solution fractionation of poly(vinyl chloride)

The optimization of solution fractionation of poly(vinyl chloride) (PVC) is described with regard to the variation of several parameters. The best separation efficiency is obtained by a single-step discontinuous solution fractionation at the following conditions: solvent: cyclohexanone (CHN); nonsolvent: ethylene glycol (EG); temperature: 130°C, isothermal; contact-time: 2 h per fraction; amount of polymer: 2 mg/g support (glass beads). An essential improvement with respect to the separation of high molecular weight species can be achieved, applying a re-deposition method of the polymer remaining in the column after half of the fractionation steps. This diminishes the unfavorable formation of a molecular weight- and concentration gradient along the column, occurring during fractionation. The optimally controlled single-stage discontinuous solution fractionation of PVC turns out to be superior to multistage processes. This is attributed to the very critical problem of formation of insoluble microcrystalline aggregates with increasing duration of stay of the polymer in the column. The efficiency of the fractionation technique can be established by the characterization of fractions (center fractions: M?_w/M?_n < 1.1) and by confirmation of small differences in molecular weight distributions, shown by other methods.     

Molecular weight determination of macromolecules by vapor pressure osmometry

A new vapor pressure osmometry (VPO) apparatus having a quite good sensitivity was constructed on the basis of a Hitachi 115 molecular weight measurement apparatus. In order to match a pair of thermistors, a conventional Wheatstone bridge circuit was modified by introducing a matching resistor, which enabled us to detect the temperature difference of ca. 6. 10^?6°C. By employing this apparatus the measurement of the temperature difference ΔT_s between a solution drop and a solvent drop at steady state was carried out as a function of concentration with solutions of the three normal alkanes (C₁₈, C₂₈ and C₃₆) dissolved in the normal alkanes (C₆, C₈ and C₁₀), and of eight atactic polystyrenes (the number-average molecular weights ranging from M?_n = 2030 to 4,1.10⁴) in benzene over a wide temperature range. The ratio of the calibration parameter K_sto the corresponding constant K_e(≡(RTV₀)/ΔH, R = gas constant, T₀ = measuring temperature in K, V₀ = molar volume of solvent, ΔH = partial molar heat of condensation), K_s/K_e, which represents the efficiency of measurement was found to be about 0,5, almost independent of the molecular weight of the solute, ranging from 200 to 20.10⁴. The maximum number-average molecular weight measurable by this apparatus with a precision of ±10% was estimated to be ca. 1·10⁵. The time necessary to reach a steady state was about 9 min regardless of the molecular weight.     

Optimization of solution fractionation of poly(vinyl acetate)

The theoretical assumptions to understand the elution mechanism of the discontinuous solution fractionation and complete elution method could be confirmed for the system poly(vinyl acetate)/butyl acetate/cyclohexane. In this case the process of complete elution, however, can only be carried out with an unfavorable high consumption of time and material. The complete elution method with all its advantages can be preserved, if the process is carried out as multistage process with a limited number of discontinuous steps. By means of such a multistage procedure improved separation results were achieved compared with the conventional single stage procedure. Furthermore some results are given which show a decrease of the separation effect of solution fractionation due to procedural limitations.     

Polymer fractionation by gel permeation chromatography

The molecular weight distributions of polystyrene, poly(vinyl chloride), polychloro-prene and polyethylene have been determined by gel permeation chromatography using silica gel as the stationary phase. The factors affecting fractionation efficiency, such as column dimensions and polymer loading are discussed. Fractionation efficiency for polystyrene, poly(vinyl chloride) and polychloroprene is comparable to that obtained from such established methods as fractional precipitation and fractional elution. For polyethylene, however, the gel permeation technique using silica gel as the stationary phase, does not appear to have the fractionation efficiency of the column elution method of FRANCIS, COOKE, and ELLIOTT.     

Molecular weight distribution of native poly(D-β-hydroxybutyric acid)

The molecular weight distribution of the biopolyester 1 from D -β-hydroxybutyric acid, isolated from Rhizobium meliloti by methods which preserved its native molecular properties, was studied. The weight average degree of polymerization is approximately DP _w=23000, 1 being therefore one of the highest biopolymers with respect to chain length. The distribution of the degree of polymerization of 1 is distinguished by a great homogeneity, which it still possesses even after 30 hours of synthesis. One can conclude, therefore, that biosynthesis of an endogeneous bacterial polymer like 1 takes place also by a mechanism which produces molecules of homogeneous chain length, as e.g. a Possion type mechanism, and where the mean time of synthesis coincides approximately with the mean time of generation.     

Molecular association in aqueous solutions of high molecular weight poly(N-vinyl-2-pyrrolidone)

A viscosimetric method was used to investigate the molecular association in aqueous solutions of poly(N-vinyl-2-pyrrolidone), (PVP), of molecular weight 700 000. Limiting viscosity numbers [η] and Huggins constants k_H of the polymer solutions were observed to decrease upon addition of denaturing agents such as thiourea and guanidinium sulfate. Guanidinium sulfate was found to be even more effective in its denaturing action as compared to urea and thiourea. The decrease in [η] and k_H values increased with increasing concentrations of the denaturing agents. This behaviour was explained on the basis of the rupture of bridges formed by water molecules among different PVP chains, hydrogen bonding being responsible for these intermolecular associations. While no change in the [η] values was observed for PVP in 2 M thiourea solutions upon increasing the temperature from 25°C to 35 and 45°C, for the same increase in temperature the [η] values of aqueous PVP solutions showed substantial decreases. This is also attributed to the breaking of hydrogen bonds existing between PVP molecules.     

Molecular weight estimation of immunoglobulin subunits on polyacrylamide gel

Molecular weights of immunoglobulin polypeptide chains can be accurately determined by dodecyl sulphate polyacrylamide gel electrophoresis. Non-alkylated immunoglobulin preparations can be used. Depending on the mobility of the polypeptide chain, a relatively narrow range of standard size should be used to calibrate the molecular weight plots; this is concluded from an evaluation of the molecular weight-mobility relationship of dodecyl sulphate-complexes of proteins in 10 per cent polyacrylamide gel.     

Conformational transitions in poly(methacrylic acid) gel/poly(ethylene glycol) complexes. Effect of the gel cross-linking density

We present results on the effect of the cross-linking density on the collapse of poly-(methacrylic acid) gels induced by the interaction with linear poly(ethylene glycol) and on their subsequent reswelling (reentrant transition). The abruptness and the amplitude of both transitions increase with decreasing cross-linking density of the gel. A special feature of the reswelling transition for the PMAA gel/PEG system is the existence of a wide range of PEG concentrations where both collapsed and swollen states of the gel are stable. The conditions for the existence of such a region are easier realized for slightly cross-linked gels. The corresponding theoretical explanation is given. The swelling degree of the collapsed gels increases with increasing cross-linking density, which is explained by the presence of a certain amount of hydrophilic PEG loops in highly cross-linked gels.