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Handbook of Size Exclusion Chromatography and Related ...

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constituents were not discriminated by SEC. On the other h<strong>and</strong>,some authors who<br />

have characterized complex polymers with coupled or two-dimensional HPLC<br />

procedures failed to compare their results with the data from simple, conventional<br />

SEC <strong>and</strong> with data calculated from polymerization kinetics. Further, molar mass<br />

valuesobtainedfromcoupledor2D-HPLCare<strong>of</strong>tencalculatedagainfromthepeak<br />

retention volumes using polystyrene calibration while peak widths/broadening are<br />

neglected.Thisallcastssomedoubtontheadvantages<strong>and</strong>evenonthenecessity<strong>of</strong><br />

complicatedHPLCmeasurements.Webelievethatitisveryimportant,ifacoupled<br />

or two-dimensional HPLC at least reveals the presence <strong>of</strong> macromolecular<br />

admixtures in the polymer characterized. In no way can 2D-HPLC <strong>and</strong> coupled<br />

HPLCproceduresbesubstitutedbysimpleSECmeasurementsifacomplexpolymer<br />

system contains two or ore constituents differing in their composition <strong>and</strong>/or<br />

architecture but possessing similar molecular sizes.<br />

In 2D-HPLC it is very important to choose an appropriate first dimension<br />

separation procedure <strong>and</strong> retention mechanism. Enthalpic partition with nonpolar<br />

column fillings <strong>and</strong> phase separation are preferred retention mechanisms for<br />

noncrystalline nonpolar polymers, while the adsorption retention mechanism is<br />

more attractive for medium-to-highly polar polymeric analytes (Sec. 3.2). A<br />

hybrid retention mechanism such as adsorption <strong>and</strong> partition <strong>of</strong> macromolecules<br />

within silica C18 column packings seems presently to be the most universal<br />

approach for many complex polymer systems.<br />

12.1 Oligomers<br />

Coupled HPLC procedures for separation <strong>of</strong> oligomers have been studied rather<br />

intensively for more than two decades. Very important results were obtained with<br />

HPLC under critical conditions (Sec. 5.1) by Entelis et al. (59), Pasch <strong>and</strong><br />

Trathnigg (4), <strong>and</strong> Kruger et al. (101,102). LC CC enables the separation <strong>of</strong><br />

oligomers according to the type <strong>and</strong> number <strong>of</strong> functional groups. The danger <strong>of</strong><br />

reduced sample recovery is much less pronounced with oligomers than with high<br />

polymers. In the second dimension separation system (SEC, eluent gradient or<br />

isocratic interaction liquid chromatography, or supercritical chromatography) the<br />

oligomer species in fractions from column #1 are separated according to molar<br />

mass <strong>of</strong> the main chain. If an oligomer sample contains two different kinds <strong>of</strong><br />

chains, for example, two blocks, LC CC can be used for separation exclusively<br />

according to the length <strong>of</strong> one block in the first dimension <strong>and</strong> the fractions are<br />

further separated according to the length <strong>of</strong> the second block in the second<br />

dimension separation system. The full retention–elution approach can also be<br />

applied for some oligomers, especially if efficient retention promoting liquid is<br />

continuously added to the column #1 effluent (95).<br />

Detection problems are mitigated by the fact that the initial sample<br />

concentration may be rather high. On the other h<strong>and</strong>, responses <strong>of</strong> all common<br />

© 2004 by Marcel Dekker, Inc.

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