Reversible addition−fragmentation chain-transfer or RAFT polymerization is one of several kinds of reversible-deactivation radical polymerization. It makes use of a chain-transfer agent (CTA) in the form of a thiocarbonylthio compound (or similar, from here on referred to as a RAFT agent, see Figure 1) to afford control over the generated molecular weight and polydispersity during a free-radical polymerization. Discovered at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia in 1998, RAFT polymerization is one of several living or controlled radical polymerization techniques, others being atom transfer radical polymerization (ATRP) and nitroxide-mediated polymerization (NMP), etc. RAFT polymerization uses thiocarbonylthio compounds, such as dithioesters, thiocarbamates, and xanthates, to mediate the polymerization via a reversible chain-transfer process. As with other controlled radical polymerization techniques, RAFT polymerizations can be performed under conditions that favor low dispersity (narrow molecular weight distribution) and a pre-chosen molecular weight. RAFT polymerization can be used to design polymers of complex architectures, such as linear block copolymers, comb-like, star, brush polymers, dendrimers and cross-linked networks.
Overview
History
The addition−fragmentation chain-transfer process was first reported in the early 1970s. However, the technique was irreversible, so the transfer reagents could not be used to control radical polymerization at this time. For the first few years addition−fragmentation chain-transfer was used to help synthesize end-functionalized polymers.
Scientists began to realize the potential of RAFT in controlled radical polymerization in the 1980s. Macromonomers were known as reversible chain transfer agents during this time, but had limited applications on controlled radical polymerization.
In 1995, a key step in the "degenerate" reversible chain transfer step for chain equilibration was brought to attention. The essential feature is that the product of chain transfer is also a chain transfer agent with similar activity to the precursor transfer agent.
RAFT polymerization today is mainly carried out by thiocarbonylthio chain transfer agents. It was first reported by Rizzardo et al. in 1998. RAFT is one of the most versatile methods of controlled radical polymerization because it is tolerant of a very wide range of functionality in the monomer and solvent, including aqueous solutions. RAFT polymerization has also been effectively carried out over a wide temperature range.
Important components of RAFT
Typically, a RAFT polymerization system consists of: a radical source (e.g. thermochemical initiator or the interaction of gamma radiation with some reagent) monomer RAFT agent solvent (not strictly required if the monomer is a liquid)
A temperature is chosen such that (a) chain growth occurs at an appropriate rate, (b) the chemical initiator (radical source) delivers radicals at an appropriate rate and (c) the central RAFT equilibrium (see later) favors the active rather than dormant state to an acceptable extent.
RAFT polymerization can be performed by adding a chosen quantity of an appropriate RAFT agent to a conventional free radical polymerization. Usually the same monomers, initiators, solvents and temperatures can be used.
Radical initiators such as azobisisobutyronitrile (AIBN) and 4,4'-azobis(4-cyanovaleric acid) (ACVA), also called 4,4'-azobis(4-cyanopentanoic acid), are widely used as the initiator in RAFT.
Figure 3 provides a visual description of RAFT polymerizations of poly(methyl methacrylate) and polyacrylic acid using AIBN as the initiator and two RAFT agents.
RAFT polymerization is known for its compatibility with a wide range of monomers compared to other controlled radical polymerizations. These monomers include (meth)acrylates, (meth)acrylamides, acrylonitrile, styrene and derivatives, butadiene, vinyl acetate and N-vinylpyrrolidone. The process is also suitable for use under a wide range of reaction parameters such as temperature or the level of impurities, as compared to NMP or ATRP.