Vol. 27 No. 4
July-August 2005

Towards a Holistic Mechanistic Model for Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerizations

The goals of this project, which emerged from activities of the IUPAC Subcommittee on Modeling of Polymerization Kinetics and Processes, are to develop a detailed understanding of the mechanism of the reversible addition fragmentation chain transfer (RAFT) polymerization and determine the corresponding kinetic coefficients. Whereas reliable information about propagation and—at least to some extent—termination rate coefficients for the modeling of radical polymerization processes has been made available by a number of IUPAC projects, the situation is less satisfying with regard to the mechanism and kinetics governing the RAFT process. Although it is widely used for the generation of both complex and well-defined polymeric materials—especially those employing dithioester compounds as the mediating agents—a complete understanding of the fundamental RAFT 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.

The basic RAFT reaction scheme (middle), which induces an equilibrium between propagating radicals, Pnl and Pml, possibly needs to be extended by reversible and/or irreversible termination reactions of the intermediate radical (center) with either another propagating radical, Pil, (upper part) or with itself (lower part) to correctly describe the kinetics of the process.

A wide variety of advanced techniques have been applied in recent years to elucidate the 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 shows a significant influence on the rate coefficients obtained by the experimental methods presently available. In addition, some of the disagreement in the literature may stem from the fact that vastly differing reaction conditions have been employed in the individual studies.

This project aims to improve the currently obscure situation by assembling a team with expertise in free radical polymerization kinetics, mechanism and synthesis, as well as quantum mechanics. The evidence gathered by different scientific groups and experimental results for various RAFT systems will be collated and critically evaluated. The current situation, including common agreement and outstanding inconsistencies, will be assessed in detail. Subsequently, recommendations will be given on how to rationally perform and present future kinetic RAFT experiments to guarantee comparability.

The project aims to formulate a holistic mechanistic model for dithiobenzoate-based RAFT processes of common monomers and to critically evaluate kinetic parameters for dithiobenzoate mediated polymerizations of styrenics, methacrylates, and acrylates. Dithiobenzoates are important RAFT agents for the generation of well-defined polymers, and reasonable kinetic parameters for these mediating agents are of priority to the scientific community.

For more information and comments, contact the Task Group Chairman Philipp Vana <[email protected]>.

www.iupac.org/projects/2004/2004-040-1-400.html