Separation into molecular weight fractions

Separation of polymers into molecular weight fractions, especially lower molecular weight polymers (oligomers) (Chapter 8)... 

TEC is a useful technique for separating polymers into molecular weight fractions on a fairly small scale. It has been used to fractionate PET [135, 136], styrene-butadiene copolymers [137], styrene acrylonitrile copolymers [138], polyoxypropylene glycols [136], Nylon-styrene graft copolymers and PMMA [139-141], styrene-methacrylate copolymers [142], poly-a-methylstyrene [143], polyvinyl acetate-styrene copolymers, and polyvinyl alcohol-styrene copolymers [144]. 

Nonionic surfactants, including EO-PO block copolymers, may be readily separated from anionic surfactants by a simple batch ion exchange method [21] analytical separation of EO-PO copolymers from other nonionic surfactants is possible by thin-layer chromatography (TLC) [22,23] and paper chromatography [24], and EO-PO copolymers may themselves be separated into narrow molecular weight fractions on a preparative scale by gel permeation chromatography (GPC) [25]. 

Figure 9 shows the result of injecting 10 gA of the total low molecular weight fraction from GPC 1 (Column Code A2) into GPC 2 (Column Code Bl). With this column code, GPC 2 is performing as a High Performance Liquid Chromatograph (HPLC). Separation is based upon solubility (i.e. composition differences) rather than upon molecular size. Methyl methacrylate monomer was used as a reference and added to the solution injected into GPC 1. Concentrations of n-butyl methacrylate, styrene and conversion are readily calculated from the peak areas and initial concentrations. 

Khoe Robins, 1989 Bottero et ak, 1991 1994, Tchoubar et ak, 1991 Rose et ak 1997 Doelsch et ak 2000). Johnston and Lewis (1986) separated the species in partly neutralized 0.1 M Fe(N03)3 solutions into different molecular weight fractions by ul-trafiltration (Amicon DiafQters) and examined each fraction with Mossbauer spectroscopy. In all fractions Fe was octahedrally coordinated. Monomers and dimers predominated in the <500 fraction when no base was added. As OH/Fe increased to 0.5, the proportion of polymer species (MW >500) increased from 12 to 44% and at OH/... 

The solubility of polymers decreases slightly as the molecular weight increases. This difference is sufficient to permit the separation of high-mo-lecular-wcight fractions of polymers by the addition of small amounts of a poor solvent to a polymer solution. It is also sufficient to permit preferential extraction of low-molecular-weight fractions of polymers. When a solution of a polymer is cooled, the first fraction to precipitate is the highest-molecular-weight fraction. It is customary to separate a solution of a polydisperse polymer into about five fractions and then to separate each of these fractions into three subtractions. 

The determination of the excess isocyanate is more problematic. In one method, toluene diisocyanate (TDI) is vaporized below the decomposition temperature of the prepolymer and analyzed by gas chromatography. A more precise method is useful if a liquid chromatograph is available. The NCO groups are reacted with methanol and the prepolymer is separated into its constituent parts in a size exclusion column. The methanol-capped isocyanates constitute the lowest molecular weight fraction. The isomers of TDI are differentiated by this technique. 

The amount of hexane in the top phase was adjusted to 7%, which resulted in a cloud point of 54°C. The solution was re-heated to 57°C to make it homogeneous, and after four days at 29°C the solution separated into two clear phases. The top phase containing the low molecular weight fraction of the polymer was siphoned off, and the bottom phase was given a third fractionation. 

The results indicate that the Sternberg alkylation reaction can be effectively used for the introduction of labeled reagents into coal and that the alkylation products can be separated and analyzed by NMR methods. The results of the NMR analyses establish that the different molecular weight fractions contain different amounts of C butylation and O butylation products. The distribution of ethereal functional groups also appears to depend upon the molecular weight. 

Viscosity (dynamic) 5-15 mPas (5-15 cP) at 135°C. Comments the USPNF 23 states that synthetic paraffin is synthesized by the Fischer-Tropsch process from carbon monoxide and hydrogen, which are catalytically converted to a mixture of paraffin hydrocarbons. The lower molecular weight fractions are removed hy distillation and the residue is hydrogenated and further treated by percolation through activated charcoal. This mixture may be fractionated into its components by a solvent-separation method. Synthetic paraffin may contain not more than 0.005% w/w of a suitable antioxidant. 

In general, synthetic polymers exhibit a wide molecular weight distribution. Their separation Into narrower molecular weight fractions is a fairly difficult task. Distillation Is not an effective separation technique for these materials because of their low vapor pressures. The solubility of different fractions of a parent polymer In conventional liquid solvents are normally too high which makes the liquid extraction a nonselective technique for these materials. Fractionation of polymers by using supercritical fluids presents unique advantages and has attracted the attention of several investigators in the past few years (1-5). 

Lysyl hydroxylase (peptidyllysine,2-oxoglutarate oxygen 5-oxidore-ductase EC 1.14.11.4) has not been studied as extensively as prolyl hydroxylase, but it appears to have very similar properties, including requirement for the same co-factors as prolyl hydroxylase (III). Chick-embryo lysyl hydroxylase was separated into two active fractions of molecular weights ca. 200,000 and 550,000 (133). Naturally occurring compounds and synthetic substrates containing the sequence -X-Lys— Gly- are readily hydroxylated by lysyl hydroxylase (134). The Km values decreased as the length of chain of polymers (-X-Lys-Gly-) increased but Vmax remained constant. 

Three histone-specific acetyltransferases have been partially purified and characterized from rat thymus nuclei (225). The enzymes were extracted from rat thymus nuclei by sonication in the presence of 1M ammonium sulfate and separated into two active fractions (A and B) by DEAE-cellulose chromatography. Fraction B was further separated into two active fractions (Bi and B2) by gel filtration on Sephadex G-200. Each fraction was then purified further by chromatography on hydroxyapatite. The molecular weights, determined by Sephadex G-200 and by sucrose density gradient centrifugation, were 99,000, 110,000, and 92,000 for enzymes A, Bi, and B2, respectively. All three enzymes required acetyl CoA as acetate donor, and the activity of the enzymes was inhibited by p-chloromercuribenzoate. Acetyltransferase A preferentially acetylated histone I (FI) and also poly-L-lysine. Acetyltransferase Bi and B2 preferred histone H4 (other names IV, F2al) and did not acet-ylate poly-L-lysine and histone H3 (III, F3). In addition to c-N-acetyl-lysine, two other unidentified amino acid derivatives were obtained from a digest of histone H4 acetylated by the two B enzymes. 

When the polymerizations was carried out with a separately prepared ferric chloride-propylene oxide catalyst, the 1-monomer formed a high molecular weight polymer which could be separated into an amorphous fraction with low optical rotation and a crystalline form with the same optical rotation as had been obtained with potassium hydroxide initiation. 

The pyrolytic "wet" tar-sirup can be readily separated into two principal fractions by water extraction. The water-insoluble fraction is derived from lignin while the water-soluble fraction is carbohydrate in origin. Analytical results indicate large amounts of low molecular weight ( 100) lactones and aldehydes, and a significant fraction of these are multifunctional in nature. Four major classes of chemicals can be differentiated,... 

Casein is a naturally occurring macromolecule that accounts for approximately 80% of the protein content of cow s milk it is a phosphoprotein that can be separated into various electrophoretic fractions, such as aj-casein, /c-casein, fi-casein, and y-casein in which each constituent differs in primary, secondary, and tertiary structure, amino acid composition, and molecular weight (Ghosh et al., 2009 Audic et al., 2003 Barreto et ai., 2003]. It finds use in making adhesives and paper coatings. 

Photofrin, a mixture of porphyrin oligomers, is currently the most widely used photosensitizer in PDT. Despite numa ous studies wiffi Hiotofrin, little is known about its mode of action, principally due to its complex chemical composition. Thus, establishing the chemical nature of Photofrin has been difficult. The commercial method for preparation of Photofrin is depicted in Scheme 61 The aqueous solution of HpD is separated by size exclusion gel chromatography into two main fractions. The higher molecular-weight fraction contained the material which is active in vivo (the so-called Photofrin) and a lower molecular-weight fraction containing mainly the monomers, which are not retained in tumors. 

The material to be separated is placed as a thin layer on an inert carrier and then eluted. Metal foil or quartz sand, for example, are suitable inert carriers. The metal foil is dipped into the macromolecular solution and then dried. The surface film is then eluted at constant temperature with solvent-precipitant mixtures of increasing solvent power. Thus, the lower molecular weight fractions are removed first. 

The poloxamers are unsaturated to the extent of about 0.02 to 0.07 mEq/g as determined by titration with mercuric acetate. Analysis of poloxamers 407 and 188 indicate that the unsaturated poloxamer molecules separate overw helmingly into the lower molecular weight fraction during gel permeation chromatography. 

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