reversible addition-fragmentation chain-transfer polymerization


  • The total number of radicals delivered to the system by the initiator during the course of the polymerization is low compared to the number of RAFT agent molecules, meaning
    that the R group initiated polymer chains from the re-initiation step form the majority of the chains in the system, rather than initiator fragment bearing chains formed in the Initiation step.

  • [11] In contrast to other controlled radical polymerizations (for example ATRP), a RAFT polymerization does not achieve controlled evolution of molecular weight and low polydispersity
    by reducing bi-radical termination events (although in some systems, these events may indeed be reduced somewhat, as outlined above), but rather, by ensuring that most polymer chains start growing at approximately the same time and experience
    equal growth during 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.

  • 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.

  • Taking star polymers as an example, RAFT differs from other forms of living radical polymerization techniques in that either the R- or Z-group may form the core of the star
    (See Figure 10).

  • Products[edit] The desired product of a RAFT polymerization is typically linear polymer with an R-group at one end and a dithiocarbonate moiety at the other end.

  • The pre-equilibrium and re-initiation steps are completed very early in the polymerization meaning that the major product of the reaction (the RAFT polymer chains, RAFT-Pn),
    all start growing at approximately the same time.

  • Main RAFT equilibrium: This is the most important part in the RAFT process,[8] in which, by a process of rapid interchange, the present radicals (and hence opportunities for
    polymer chain growth) are “shared” among all species that have not yet undergone termination (Pn• and.

  • [12][13][14] R group: • It must be a good homolytic leaving group relative to Pn (shifts main equlibrium towards macro-CTA and R radical) • It should reinitiate polymerisation
    efficiently Choice of Z group affects: • Rate of addition of propagating polymer to the thiocarbonyl of intermediate species • Rate of fragmentation of intermediate radicals Figure 6.

  • [13] Important ratios between reaction components[edit] During RAFT synthesis, some ratios between reaction components are important and usually can be used to control or
    set the desired degree of polymerization and polymer molecular weight.

  • [20] RAFT compared to other controlled polymerizations Advantages[edit] Polymerization can be performed in large range of solvents (including water), within a wide temperature
    range, high functional group tolerance and absent of metals for polymerization.

  • If formation of the RAFT adduct radical is sufficiently thermodynamically favorable, the concentration of active species, Pm•, will be reduced to the extent that a reduction
    in the rate of conversion of monomer into polymer is also observed, as compared to an equivalent polymerization without RAFT agent.

  • While utilizing the R-group as the core results in similar structures found using ATRP or NMP, the ability to use the Z-group as the core makes RAFT unique.

  • This may undergo a fragmentation reaction in either direction to yield either the starting species or a radical (R•) and a polymeric RAFT agent.

  • Termination: Chains in their active form react via a process known as bi-radical termination to form chains that cannot react further, known as dead polymer.

  • When the Z-group is used, the reactive polymeric arms are detached from the star’s core during growth and to undergo chain transfer, must once again react at the core.

  • For the first few years addition−fragmentation chain-transfer was used to help synthesize end-functionalized polymers.

  • 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.

  • [18] Smart materials and biological applications[edit] Due to its flexibility with respect to the choice of monomers and reaction conditions, the RAFT process competes favorably
    with other forms of living polymerization for the generation of bio-materials.

  • [3] However, the technique was irreversible, so the transfer reagents could not be used to control radical polymerization at this time.

  • Typically, a RAFT polymerization system consists of: • a radical source (e.g.

  • In such a polymerisation, there is the additional challenge that the RAFT agent for the first monomer must also be suitable for the second monomer, making block copolymerisation
    of monomers of highly disparate character challenging.

  • Decreasing order of reactivity for macro-R groups for the polymerization of block copolymers: Recommended monomers: (partial control); (partial control) For block copolymers,
    different guidelines exist for selecting the macro-R agent for polymerizing the second block (Figure 9).

  • The presence of sulfur and color in the resulting polymer may also be undesirable for some applications; however, this can, to an extent, be eliminated with further chemical
    and physical purification steps.

  • 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.

  • In RAFT polymerizations without rate-retardation, the concentration of the active species P• is close to that in an equivalent conventional polymerization in the absence of
    RAFT agent.

  • [4] Macromonomers were known as reversible chain transfer agents during this time, but had limited applications on controlled radical polymerization.

  • Major product of a RAFT polymerization (left) and other biproducts, arranged in order of decreasing prevalence.

  • As of 2014, the range of commercially available RAFT agents covers close to all the monomer classes that can undergo radical polymerization.

  • Applications RAFT polymerization has been used to synthesize a wide range of polymers with controlled molecular weight and low polydispersities (between 1.05 and 1.4 for many

  • [18] Block copolymers[edit] As with other living radical polymerization techniques, RAFT allows chain extension of a polymer of one monomer with a second type of polymer to
    yield a block copolymer.

  • Disadvantages[edit] A particular RAFT agent is only suitable for a limited set of monomers and the synthesis of a RAFT agent typically requires a multistep synthetic procedure
    and subsequent purification.

  • As such, a RAFT agent must be designed with consideration of the monomer and temperature, since both these parameters also strongly influence the kinetics and thermodynamics
    of the RAFT equilibria.

  • RAFT polymerization can be performed by adding a chosen quantity of an appropriate RAFT agent to a conventional free radical polymerization.

  • Re-initiation: The leaving group radical (R•) then reacts with another monomer species, starting another active polymer chain.

  • These properties make RAFT useful in many types of polymer synthesis.

  • The R group must be able to stabilize a radical such that the right hand side of the pre-equilibrium is favored, but unstable enough that it can reinitiate growth of a new
    polymer chain.

  • As the degassing is decoupled from the polymerization, initiator concentrations can be reduced, allowing for high control and end group fidelity.

  • Monomer to RAFT reagent: gives the expected degree of polymerization (that is, the number of monomer units in each polymer chain) and can be used to estimate the molecular
    weight of the polymer by Equation (1) (see below).

  • Thermodynamics of the main RAFT equilibrium[edit] The position of the main RAFT equilibrium (Figure 5) is affected by the relative stabilities of the RAFT adduct radical and
    its fragmentation products, namely and polymeric radical (Pm•).


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