Title: Towards a holistic mechanistic
model for reversible addition fragmentation chain transfer (RAFT)
polymerizations: Dithiobenzoates as mediating agents
Coote, Marco Drache,
Andrew Lowe, Graeme
Sanderson, and Matthew
To provide the scientific community involved in performing and modeling
RAFT polymerizations with appropriate kinetic schemes as well as
the best possible and critically evaluated kinetic parameters describing
various RAFT processes.
The reversible addition fragmentation chain transfer (RAFT) polymerization
has witnessed a rapid development during the last years. Although
being widely used for the generation of both complex and well-defined
polymeric materials - especially employing dithioester compounds
as the mediating agents -, a complete understanding of the fundamental
reaction scheme, which induces the equilibrium between dormant and
active radical species, has not yet emerged. A deep understanding
of the RAFT process, however, is mandatory to establish structure/rate
correlations for a specific RAFT agent, which is essential for rational
RAFT agent design delivering novel mediating compounds.
A wide variety of advanced techniques
were applied in the near past to elucidate the detailed mechanism
of RAFT polymerization and to arrive at rate coefficients describing
the RAFT equilibrium reactions. It has been demonstrated that the
choice of the reaction scheme operative in the RAFT process - with
the inclusion of possible side-reactions - shows a major influence
on the rate coefficients obtained by the experimental methods presently
available. Significant experimental evidence has been put forward
for the so-called slow fragmentation model (SFM) and the cross termination
model (CTM), but alternative models such as reversible termination
have also been discussed. Some of the disagreement in the literature
may - however - stem from the fact that vastly differing reaction
conditions have been employed in the individual studies.
The scope of the proposed project is to improve this obscure situation
via jointly discussing the experimental evidence gathered by different
scientific groups. The present proposal puts forward a program that
brings together various research groups - who presently adhere to
different mechanistic pictures - in order to put the scientific
discussion on a common base. This task will be achieved via collating
and comparing experimental results obtained for various RAFT systems.
The current situation will be presented in a joint "dilemma-paper",
in which common agreement, but also outstanding inconsistencies,
will be clarified in detail.
Subsequently, recommendations will be given how to rationally perform
and present future kinetic RAFT experiments to guarantee comparability.
The final task of the project will be compiling and critically
evaluating kinetic parameters for dithiobenzoate mediated RAFT polymerizations
of styrenics, methacrylates and acrylates. Dithiobenzoates are the
most widely used RAFT agents, both in synthetic work, due to their
high reactivities, and in kinetic work, because of interesting rate
altering effects. Reasonable kinetic parameters for these mediating
agents are hence of priority to the scientific community.
This project is envisaged as first part of a series with subsequent
parts considering kinetic data for additional monomer/RAFT agent