Kinetic importance of the missing step in dithiobenzoate-mediated RAFT polymerizations of acrylates

The effect of retardation exhibited in dithiobenzoate-mediated polymerizations is perhaps the most controversial kinetic phenomenon discussed in polymer chemistry in the last 20 years. However, the Missing Step reaction (MSR), which involves the termination between a propagating radical and a 3-arm star product, has emerged as a hypothesis to explain the full kinetic picture for acrylate polymerizations. Here, a kinetic model, the most detailed of its kind, is developed to describe and study the MSR. The model is based on the method of moments and includes RAFT pre-equilibrium and equilibrium, backbiting, cross-termination, MSR and chain-length dependent radical termination. After validation through comparison with experimental data available in the literature, the model is used to demonstrate that the kinetic behavior of the propagating and intermediates radicals can be described by the quasi-steady state assumption and that backbiting does not significantly influence predicted conversion profiles and product molecular weights. It is also shown that the induction period observed experimentally can be represented by either slow initiation/reinitiation of primary radicals or by assuming that radical intermediates formed by addition of primary radicals to the RAFT agent have increased stability; however, the associated rate constants for these reactions had to be adjusted from literature values. A comparison between the MSR and Intermediate Radical Termination (IRT) representations RAFT-mediated acrylate polymerization identifies chain end-group functionality (EGF) as a means to further discriminate between these theories.

Automated summary

This study discusses the controversial phenomenon of retardation in dithiobenzoate-mediated polymerizations, introducing the Missing Step reaction (MSR) hypothesis to explain the kinetic picture. A detailed kinetic model based on the method of moments is developed and validated, demonstrating the accuracy of describing the behavior of propagating and intermediate radicals.