Explain the mechanism of reversible addition-fragmentation chain transfer process.
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 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 with conditions to favor low dispersity (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.
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 chain growth occurs at an appropriate rate, the chemical initiator (radical source) delivers radicals at an appropriate rate and 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.
Figure 3. Examples of the major reagents and products in two RAFT polymerizations
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.
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Explain the general atom transfer radical polymerization mechanism.
Conventional free radical polymerization techniques are inherently limited in their ability to synthesize resins with well-defined architectural and structural parameters. Free radical processes have been recently developed which allow for both control over molar masses and for complex architectures. Such processes combine both radical techniques with living supports, permitting reversible termination of propagating radicals. In particular, three controlled free radical polymerizations have been well investigated. Each of these techniques is briefly presented below and all are based upon early work involving the use of initiator-transfer-agent-terminators to control irreversible chain termination of classical free radical process.
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