RAFT (chemistry)
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RAFT or Reversible addition–fragmentation chain transfer is a form of living radical polymerization.
See also
Reversible addition–fragmentation chain transfer polymerization was discovered by the CSIRO in 1998. This is a new method for the synthesis of living radical polymers that may be more versatile than addition transfer radical polymerization (ATRP) or nitroxide-mediated polymerization (NMP). RAFT polymerization uses thiocarbonylthio compounds, such as dithioesters, dithiocarbamates, trithiocarbonates, and xanthates in order to mediate the polymerization via a reversible chain-transfer process.
This allows access to polymers with low polydispersity and high functionality. RAFT also allows for the production of complex architectures such as block, star, graft, comb, and brush (co)polymers.
The RAFT process mediates free radical polymerization through a reversible addition fragmentation process. RAFT polymerization can be described as a conventional free radical polymerization overlayed with a chain transfer pathway. As such, the propagation, initiation and termination steps are identical to a conventional free radical polymerization. The following scheme outlines the mechanism of RAFT.
The reaction steps can be described as follows. The radical generated via the decomposition of initiator reacts with monomer (ki) to form a small polymeric radical. Step II, also called the pre-equililbrium shows this growing polymer chain rapidly adding to the reactive S=C bond in the dithioester (1) to form the radical intermediate (2). Now the radical intermediate can fragment to form either the original polymeric radical and CTA or to form the leaving group R• and the macroCTA (3). Step II can be simplified to a transfer reaction between the polymeric radical and the R group. The R group can then go on to reinitiate polymerization forming a propagating chain. Once all the original CTA has been consumed only macroCTA remains in the system and the core equilibrium (IV) is established. The exchange between propagating polymeric radicals and dormant dithioester capped chain ensures that all chains have equal probability to grow and as such the molecular weight progresses linearly with increasing conversion. Step V shows the termination of propagating radicals which occurs in all free radical polymerizations.
