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Handbook of Size Exclusion Chromatography and Related ...

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Finally, the use <strong>of</strong> a light-scattering detector allows simultaneous monitoring<br />

<strong>of</strong> the evolution <strong>of</strong> molecular weight during the reaction. It is important to realize<br />

that Eqs (1), (3), <strong>and</strong> (4) cannot be used directly for determining Mw in the case <strong>of</strong><br />

copolymers, because the different values <strong>of</strong> @n=@c <strong>of</strong> each comonomer, together<br />

with compositional heterogeneity, lead to apparent values <strong>of</strong> Mw from which the<br />

true value <strong>of</strong> Mw could traditionally only be determined by running light-scattering<br />

experiments in three different solvents (45,46). Reference 42, however, gives a<br />

means <strong>of</strong> computing Mw on-line, by exploiting the continuous knowledge <strong>of</strong><br />

composition. The use <strong>of</strong> a viscometer furnishes an additional crosscheck on<br />

molecular weight evolution, <strong>and</strong> can potentially also be used to study differences<br />

in branching <strong>and</strong> copolymer viscosity characteristics.<br />

Hence, use <strong>of</strong> ACOMP, with no model-dependent assumptions, can provide<br />

average composition, <strong>and</strong> molecular mass distributions. These are typically found<br />

after copolymer production by laborious crossfractionation techniques (47,48). If<br />

models for mass, composition, <strong>and</strong> sequence length are evoked, then the average<br />

distributions from ACOMP can be folded with the appropriate instantaneous<br />

distribution forms to arrive at full distribution representations, including the<br />

composition/mass bivariate distribution (49).<br />

2.1.6 Current <strong>and</strong> Future Directions for ACOMP<br />

Research <strong>and</strong> development are currently under way both to improve the<br />

instrumentational base for ACOMP <strong>and</strong> to extend the method to more complex<br />

polymerization reactions. Instrumentational developments include improved<br />

front- end modules that can withdraw <strong>and</strong> mix from high-viscosity reactor liquids<br />

(over one million centipoise), <strong>and</strong> sample conditioning modules for “flashing<br />

monomer,” rapidly dissolving <strong>and</strong> treating slurries <strong>and</strong> grains, <strong>and</strong> removing<br />

polymer from emulsions. Ultimately, a fully ruggedized extraction/dilution/<br />

conditioning system should be available that will be suitable for use in full-scale<br />

industrial reactors. Extension to other detectors including electron spin resonance<br />

(ESR), near infrared, <strong>and</strong> evaporative light-scattering are also expected.<br />

New types <strong>of</strong> reaction scenarios include the use <strong>of</strong> CRP to produce more<br />

complex polymer architecture, atom transfer radical polymerization (50),<br />

hydrophobically modified copolymers (51,52), photopolymerization, polymerization<br />

in microemulsions (53), high-pressure polymerization, <strong>and</strong> fluidized bed<br />

reactions. Additional strategies for on-line characterization <strong>of</strong> branching <strong>and</strong><br />

crosslinking are also being developed.<br />

2.2 Degradation Reactions<br />

When a polymer is degraded by agents such as radiation, acids, bases, enzymes,<br />

heat, ultrasound, <strong>and</strong> so on, its mass decreases, <strong>and</strong> hence also the intensity <strong>of</strong><br />

scattered light at small angles. It is possible to find quantitative relationships<br />

© 2004 by Marcel Dekker, Inc.

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