Handbook of Size Exclusion Chromatography and Related ...

Scheme 1
Usually, a nitroxide acts as the counter-radical X*. TEMPO (2,2,6,6tetramethylpiperidine
nitroxide) and SG1 (N-tertiobutyl-1-diethylphosphono-2,
2-dimethylpropyl nitroxide) are among the agents commonly used in CRP.
An extensive literature exists on the theoretical (35) and applied aspects of CRP
(36–39).
ACOMP was adapted to CRP monitoring for the SG1 controlled, bulk
polymerization of butyl acrylate (40). A high-pressure mixing scheme was used to
deal with the high reactor viscosities. The monomer conversion kinetics closely
resembled a first-order process, and Mw increased linearly with f , although the initial
Mw is finite, not zero, as often reported. GPC sampling during the reactions showed
that, as in anionic polymerization, the polydispersity decreases during the CRP.
2.1.5 Copolymerization
There are many ways of producing copolymers, including free and controlled
radical copolymerization (41). Average sequence lengths of a comonomer can run
from one, for a strictly alternating copolymer, to very large numbers for block
copolymers. Additionally, copolymers can have widely varying architectures, such
as combs, stars, dendrimers, and others.
As an initial entry into the field of copolymerization, ACOMP was recently
applied to free radical copolymerization (42). The classical system of polystyrene/
methyl methacrylate was chosen. Exploiting the differences in refractive index
increment and UV absorption between each comonomer and the polymers, it was
possible to obtain a continuous, on-line record of the conversion of each comonomer.
This means that at every instant the remaining concentration of each comonomer is
known, and, from the derivative of these concentrations, the instantaneous rate of
comonomer incorporation into polymer is known. This immediately provides a
record of the average copolymer composition at every instant, so that the entire
average compositional distribution of the copolymer is obtained during the reaction.
Furthermore, by running two or more experiments at different initial relative
comonomer concentrations it is possible to obtain the reactivity ratios of the
comonomers without the need for the many approximations that have often been
made in order to use single point techniques (43,44). Knowledge of the reactivity
ratios, together with the instantaneous comonomer concentrations allows the average
sequence length of the copolymer population also to be followed.
© 2004 by Marcel Dekker, Inc.