Abstract
Recent developments in polymerization reactions utilizing thiocarbonylthio compounds have highlighted the surprising versatility of these unique molecules. The increasing popularity of reversible addition–fragmentation chain transfer (RAFT) radical polymerization as a means of producing well‐defined, ‘controlled’ synthetic polymers is largely due to its simplicity of implementation and the availability of a wide range of compatible reagents. However, novel modes of thiocarbonylthio activation can expand the technique beyond the traditional system (i.e., employing a free radical initiator) pushing the applicability and use of thiocarbonylthio compounds even further than previously assumed. The primary advances seen in recent years are a revival in the direct photoactivation of thiocarbonylthio compounds, their activation via photoredox catalysis, and their use in cationic polymerizations. These synthetic approaches and their implications for the synthesis of controlled polymers represent a significant advance in polymer science, with potentially unforeseen benefits and possibilities for further developments still ahead. This Research News aims to highlight key works in this area while also clarifying the differences and similarities of each system.
Highlights
Recent developments in polymerization reactions utilizing thiocarbonylthio the field of ‘reversible deactivation radcompounds have highlighted the surprising versatility of these unique molecules
The use of organic sulphur compounds for the synthesis of carbonylthio (TCT) compounds for the controlled synthesis of synthetic polymers via “living” radical polymerization was pio- polymers via initiation by a conventional radical initiator.[5]
Many of mechanism of reversible addition-fragmentation chain transfer the ideas and concepts proposed by Otsu have greatly influ- (RAFT) was proposed (Figure 1), which can effectively allow for enced the development of radical polymerization techniques control over the active radical species
Summary
The photoactivity of thio-containing compounds is well known,[13] and their use by Otsu as photoiniferters paved the way for their subsequent use in polymerization reactions.[1]. Relatively poor control over the polymerization was commonly observed, especially at moderate to high monomer conversions This was ascribed to the direct photolytic degradation of the fragmented species (i.e., the thiyl radical), resulting in inefficient deactivation and loss of the “living” character.[17] These early investigations introduced a debate over the mechanism for the observed “living” behavior, with the two proposed pathways being reversible termination,[16b,18] and the radical initiation of a RAFT-type degenerative chain transfer process following photolysis of the TCT species.[16a] Key experiments undertaken to elucidate the mechanistic pathway indicate that degenerative chain transfer (i.e., a RAFT process) is likely the main cause of control.[16a] The extent to which reversible termination takes place remains relatively unclear. Regardless, this approach employing solely TCT compounds as the photoactive species and control agent offers the potential for further modification, optimization, as well as a deeper understanding of the competing reaction pathways
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