Polymer HPLC polydispersity

The relative concentration of the various butadiene microstructures, (1,4 cis, 1,4 trans, and 1,2 vinyl), were determined from the infrared spectra of solid films cast on KC1.(26) The 1,2 microstructure content of all the polymers considered in this paper were between 5-8 mole percent as determined from the IR spectra. Number average and the weight average molecular weight of the polymers were obtained via osmotic pressure and HPLC. The molecular weight of all polymers is around 200,000 g/mole while the polydispersities were about 1.1 thus, all of these polymers have a relatively narrow molecular weight distribution. Note, that both the precursor diene blocks and hydrogenated copolymers... 

Some studies have successfully demonstrated separations of linear copolymers using HPLC. However, it is important to realize that unlike in the HPLC of small molecules where a peak shows the concentration of only one type of molecule, the SEC chromatogram of a complex polymer is really an envelope covering possibly thousands of different components. Even with modem detectors it is often very difficult to ascertain that the desired fractionation has really been accomplished. Universal calibration in SEC is of practical utility because the same fractionation (i.e., a fractionation according to molecular size) occurs whether monodisperse, polydisperse or complex polymer molecules are involved. The fractionation is reliable. This is a very difficult requirement for HPLC methods because of the variety of complex molecules that can be present. 

Molecular weight and polydispersity of the acetone extracted fractions from thermally degraded samples were determined by SEC with a Jasco PU-1580 HPLC pump equipped with two Pigel mixed D columns (Polymer Laboratories UK) connected in series, and a Jasco 830RI refractive index detector. Sample elution was with THF at 1 ml/minute flow rate. The instrument was calibrated with standard polystyrene samples. 

A solution of the polydisperse polymer in the same solvent as was used to swell the resin is placed on the top of the column and eluted in the same manner as for standard column chromatography or high-pressure liquid chromatography (HPLC). In GPC, however, it is not the interaction of the dissolved analyte molecules with the stationary phase relevant for separation but the different (hydrodynamic) volumes of the polymers associated with their different molecular weights. Only solvent molecules and those macromolecules whose size is less than the prevailing pore size can diffuse into the pores of the swollen gel their separation succeeds because the polymer molecules get caught up in the holes in the beads, then come out, pass on down the tube a little way, then get caught in another pore, and so on. 

This technique coupled to HPLC has been used to determine the molecular weight of ter -octylphenol-ethoxylate surfactant polymers [235]. Schriemer and Li [236] used MALDI-MS to detect high molecular weight narrow polydisperse polymers up to 1.5 X 10 daltons. 

Masci and co-workers [9] characterised block copolymers of N-isopropyl acrylamide with dimethyl acrylamide or 3-sulfopropyl methacrylate by HPLC, gel permeation chromatography (GPC) and MALDI-ToF-MS. Data are provided on the reaction conditions, kinetic data, M and polydispersities for the polymers and copolymers. 

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