Number: 2004-034-1-400
Title: Critically evaluated propagation
rate coefficients for free-radical polymerization of water-soluble
monomers polymerized in the aqueous phase
Task Group
Chairman: Igor Lacík
Members: Christopher
Barner-Kowollik, Sabine
Beuermann, Michael
Buback, Patrice
Castignolles, Bernadette
Charleux, Tom Davis,
Robert G. Gilbert,
Klaus-Dieter Hungenberg,
Robin Hutchinson,
Peter Kilz, and Christine
Wandrey
Objective:
To initiate critical evaluation of propagation rate coefficients
for water-soluble monomers polymerized in the aqueous phase. Data
from pulsed-laser initiated polymerization in conjunction with polymer
molecular weight analysis by size-exclusion chromatography will
be considered. The specific role of polymerization conditions with
water being the polymerization medium will be emphasized.
Description:
Knowing the kinetics and mechanism of polymerization provides a
thorough understanding of polymerization processes and allows for
the simulation of polymer properties and polymerization rates. The
precise knowledge of rate coefficients, as is documented by the
activities of the IUPAC Subcommittee on Modeling of Polymerisation
Kinetics and Processes (former
Working Party IV.2.8), was highly insufficient until recently.
The situation has been improved by employing the IUPAC-recommended
PLP-SEC technique which combines pulsed-laser initiated polymerization
(PLP) with size-exclusion chromatography (SEC). This technique provides
benchmark propagation rate coefficient, kp, of
free-radical polymerization that enable quantification also of the
individual termination and transfer rate coefficients for homopolymerization
and copolymerization of a number of hydrophobic monomers, including
styrene, several alkyl methacrylates, functional methacrylates,
and, more recently, butyl acrylate.
The information on individual rate coefficients
for free-radical polymerization of water-soluble monomers polymerized
in the aqueous phase is significantly lagging behind the one for
hydrophobic monomers due to several serious difficulties encountered
in applying the PLP-SEC technique to these polymerization systems.
On the other hand, precise rate coefficients are badly needed, in
particular as aqueous-phase free-radical polymerizations of water-soluble
monomers are important industrial polymerization process, by which
polymers for hydrogels, superabsorbents, cosmetics, membranes, thickeners,
flocculants, etc. are produced.
Recent activities of several groups around
the world demonstrate that careful experiments can provide access
to precise kp values for a wide variety of aqueous-phase
polymerization conditions. Polymerization is strongly influenced
by solvent effects of the hydrogen-bonded monomer, polymer and water
species. Further, it has been recognized that polymerization process
is controlled by monomer concentration, ionic strength, pH (in case
of ionizable monomers), and that characterization of the molecular
weight distribution by SEC may pose problems. Moreover, in case
of the acrylate family of monomers, e.g. acrylic acid and acrylamide,
intra- and intermolecular transfer reactions need to be taken into
account, in particular under technically relevant polymerization
conditions of temperature and conversion.
It is the aim of this project to provide
reliable information on propagation rate coefficients for free-radical
polymerizations in the aqueous phase. The discussion will start
with the critical evaluation of recently obtained PLP-SEC kp
data for methacrylic acid and will be extended to the analysis of
kp values for polymerization of other monomers
in aqueous phase. General recommendations for studying the aqueous
phase kinetics of water-soluble monomers by the PLP-SEC technique
will be another important outcome of this project. Furthermore,
the discussion will provide information on aqueous-phase SEC analysis.