Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization is an extremely powerful tool for polymer chemists as it allows for low molecular weight distribution, controlled molecular weight, and is compatible with a wide variety of functional groups. An unusual phenomenon that is seen in RAFT polymerization is rate retardation which occurs when polymerization deviates from ideal linearity in the form of induction periods and a decrease in the steady state rate. Kinetic models, such as Slow Fragmentation and Intermediate Radical Termination, have been extensively studied and debated over the past twenty years in order to understand why these deviations occur. Most of these experiments used to explore rate retardation used more active monomers (MAMs) with dithiobenzoate chain transfer agents (CTAs). In this paper, we explore a wide variety of different polymeric systems to see how rate retardation occurs beyond the traditional scope of dithiobenzoate CTA with MAMs, while maintaining constant concentrations of radical initiator. We found that rate retardation not only occurs with dithiobenzoates systems, but also for systems that use trithiocarbonates and xanthates CTAs. The data was analyzed using both the Slow Fragmentation and Intermediate Radical Termination models and showed that retardation can be described using a single relationship for a variety of monomer reactivity and corresponding optimized CTA. This provides evidence that the rate retardation is a universal phenomenon amongst a variety of different polymeric systems with varying CTA composition.

CTA concentration effects on rate retardation in RAFT

ACS Fall 2021

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