Preparative GPC

Choice of Solvent. JJ-Methylpyrrolidone (NMP) was initially used as the mobile phase but proved to be unsatisfactory because of (i) high solution viscosities, (il) exceedingly small differences in refractive index between NMP and cellulose triacetate solutions, (ill) erratic base line. In view of this dichloromethane was employed. Some additional benefits derived from this mobile phase are (i) a decrease in elution volume due to low solution viscosities, (il) fast solvent recovery due to low boiling point of dichloromethane and (iii) ease of obtaining preparative GPC cuts of cellulose triacetate. 

Preparative GPC of Cellulose Triacetate Sample. A 1% (m/V) solution of cellulose triacetate (medium) prefiltered through porosity 3 glass sinter was fractionated by repeated injection through the column set described above. Seven cuts covering the entire elution curve were collected. The flow rate, injection time and the experimental conditions were identical to those stated above. 

Narrow Molecular Weight Triacetate Fractions. Narrow molecular weight cellulose triacetate fractions were obtained by both fractional precipitation and preparative GPC as described above. The number average molecular weight (1 ) of the various fractions and cuts was determined by high speed membrane osmometry. A linear dependence of GPC elution volume on log molecular weight for all cellulose triacetate fractions was found in both methylpyrroli-done and dichloromethane. 

Although the small particle size, high efficiency columns are used today for analytical GPC, the large particle size materials (Styragel) are still useful for larger scale preparative separations (.3, ). Preparative GPC is useful for... 

Low molecular weight lignin model compounds (synthetic phenyl-tetramers and Igepals ) were found to fit universal calibration. Fractions from preparative GPC, when analyzed by universal calibration, yield molecular weight distributions which add to a similar value to that found for the unfractionated parent sample. 

In order to study birch lignosulfonates, spent sulfite liquor, from which monosaccharides had been removed by ion exclusion chromatography, was fractionated on the basis of molecular size by preparative GPC (Fig. 1). 

On the other hand, fractions D and E, which elute later in preparative GPC (Fig. 1), show clearly separated peaks in both the hydrophobic and hydrophilic zones when fractionated by RPC (Figs. 6 and 7). 

II. Preparative GPC Fractionation and Characterization of Poly-(methyl methacrylate) for Calibration in 2, 2, 2-Trifluoroethanol... 

Solvents. Reagent grade THF (nD25 = 0.888, bp = 64-66°C) containing 0.025 wt-vol % di-ferf-butyl-p-cresol which served as an antioxidant was used for the preparative GPC fractionation. The solvent TFE (nD20 = 1.2907, d25 = 1.3823, bp = 76°C, ionization constant Ka — 4.3 X 10"13) was obtained from Halocarbon Products Corp., Hackensack, N. J., and was used for both analytical GPC and viscometry. The recovery and... 

Evaluation and Preparative GPC Fractionation of PMMA. The baseline-adjusted retention volume curves of the preparative GPC fractions are shown in Figure 3. The characterization data are shown in Table III. 

Intrinsic Viscosity—Molecular Weight Relationship for PMMA in TFE. The intrinsic viscosities of the PMMA preparative GPC fractions and whole polymers in TFE at 50 °C and in benzene at 30 °C are shown in Table III and plotted in Figure 4. A least-squares analysis of the data plotted in Figure 4 yields the relation... 

The authors acknowledge the assistance of Masao Ohta in the polymerization of the PMMA whole polymers and in the recovery of the preparative GPC fractions and McClinton Rayford for some of the solution property characterization work. Also acknowledged is James E. Kurz for many helpful discussions. 

Compound 1 (0.25 mmol) was ground to a powder and then sandwitched between two cover glasses (25x80 mm, thickness 1.5 mm) and dipped in a water bucket. Irradiation was carried out with a 400-W high-pressure mercury lamp for 24 h. Dimers 5 were separated from intramolecular cyclization products by preparative GPC runs. Isomeric products 2 and 3 were separated by preparative HPLC runs. 

The synthesis of cyclic polydimethylsiloxane was first achieved through ring-chain equilibration of siloxane oligomers in the presence of potassium silanolate, as shown in Fig. 51 [163-165]. Cyclics recovered from ring-chain equilibration reactions have been fractionated by preparative GPC, yielding... 

Dehydrocoupling of 1 catalyzed by <50 mol% (i.e., M-H/Si-H < 0.5) of Red-Al yielded 2 as light yellow powders. Polymers with molecular weights (Mw) of 4600 and 4100 were isolated in 86 and 78% yields when 15 mol% and 50 mol% of Red-Al were used, respectively. Polymer yields and molecular weights when 15 mol% of Red-Al was used were higher compared to polymers obtained when 50 mol% of Red-Al was used. Products from the reaction of 1 with 15 mol% 25 mol%, and 50 mol% Red-Al were separated by preparative GPC and were characterized by NMR spectroscopy. Shorter oligomers such as silole dimer or trimer were not found in products. However, when 100 mol% of Red-Al (i.e., M-H/Si-H = 1) was used, the formation of silole dianion 333 was observed without forming 2 [Eq. (6)]. 

Preparative GPC. The preparative GPC work was performed on the experimental setup shown in Figure 1. Four 1-in. i.d. glass columns were packed with Styragel (Waters Associates) with 1 ft 104 A porosity, 2 ft 500 A porosity, and 1 ft 100 A porosity. The Styragel porosites were chosen to give good resolution for the entire range of molecular sizes found in residua. Two separate column systems were used—one for maltenes, the other for asphaltenes. 

Figure 10. aliphatic/aromatic ratios of preparative GPC fractions of the... 

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