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Handbook of Size Exclusion Chromatography and Related ...

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Unfortunately, applications <strong>of</strong> eluent gradient HPLC to block copolymers so<br />

far has not led to clearly positive conclusions. On the other h<strong>and</strong>, graft copolymers<br />

were successfully separated by EG HPLC (108).<br />

LC CC is the most important first dimension separation system for block<br />

copolymers in the present state <strong>of</strong> 2D-HPLC method development (5). However,<br />

numerous pitfalls <strong>of</strong> this method should be considered. The situation with graft<br />

copolymers <strong>and</strong> miktoarm copolymers is even more complicated. For example, LC<br />

CC results were reasonable for graft copolymers poly(styrene-graft-ethylene oxide)<br />

with short grafts but less satisfactory for the same copolymers with long grafts (109).<br />

12.5 Macromolecules with Complex Architectures<br />

The most frequent <strong>and</strong> practically important polymers with distributions in their<br />

architecture are long-chain branched species. They belong to the few examples for<br />

which a single exclusion mechanism can produce decisive molecular information.<br />

SEC <strong>of</strong> branched polymers is discussed in several chapters <strong>of</strong> this book.<br />

LC CC, LC LC, <strong>and</strong> eluent gradient HPLC can discriminate macromolecules<br />

according to their fine structural features, such as cis–trans isomerism (84) or<br />

stereoregularity (62,110). In the 2D-HPLC <strong>of</strong> stereoregular poly(ethyl<br />

methacrylate)s, macromolecules were first separated by conventional SEC according<br />

to their molar mass/size <strong>and</strong> LC CC was used as the second dimension separation<br />

system (61). An NMR detector confirmed good overall separation<br />

selectivity (62). LC CC also discriminates linear <strong>and</strong> cyclic macromolecules with<br />

similar molar masses (63–65).<br />

12.6 Unknown Samples<br />

In the preceding sections, we tried to assist polymer analysts in orientation among<br />

presently available HPLC procedures for molecular characterization <strong>of</strong> complex<br />

polymer systems The aim was to help in identifying appropriate steps in method<br />

development for solving a particular analytical problem, that is, when<br />

the basic information on the polymer type were available or it could be reasonably<br />

assessed. Unfortunately, owing to the existence <strong>of</strong> a large variety <strong>of</strong> complex<br />

polymers, so far no universal protocol for 2D-HPLC can be prepared. The<br />

situation is even more complicated when a completely unknown polymeric<br />

material appears, a “sample.” The following proposed actions <strong>and</strong> their sequence<br />

should be considered as tentative only:<br />

1. Determine the chemical composition <strong>of</strong> the unknown sample by<br />

applying all conventional solid-state bulk methods available, including<br />

(reflectance) infrared spectroscopy or pyrolysis gas chromatography.<br />

© 2004 by Marcel Dekker, Inc.

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