RAFT Agents

Chain transfer to methacrylate and similar maeromonomers has been discussed in Section 6.2.3.4. The first papers on the use of this process to achieve some of the characteristics of living polymerization appeared in 1995.380 The structure of macromonomer RAFT agents (163) is shown in Figure 9.3. An idealized reaction scheme for the case of a MMA terminated macromonomer is shown in Scheme 9.36. 

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

Living radical polymerization using thiocarbonylthio RAFT agents (including dithioesters, trithiocarbonates and xanthates) was first described in a patent published in 1998.40S The first paper describing the process also appeared in 1998.1R Other patents and papers soon followed. Papers on this method, along with NMP and ATRP, now dominate the literature on radical polymerization. 

For very active RAFT agents, the RAFT agent derived radical (R ) may partition between adding to monomer and reacting with the transfer agent (polymeric or initial). In these circumstances, the transfer constant measured according to the Mayo or related methods will appear to be dependent on the transfer agent concentration and on the monomer conversion. A reverse transfer constant can be defined as follows (eq. 19)... 

Many thiocarbonylthio RAFT agents (164) have now been described. Transfer constants are strongly dependent on the Z and R substituents. For an efficient RAFT polymerization (refer Scheme 9.38 and Figure 9.3) ... 

The dependence of the transfer constant on the Z substituent, summarized in Figure 9.4, is largely based on studies of the apparent transfer constants of benzyl and cyanoisopropyl RAFT agents in S polymerization 4 409 and qualitative observations of other polymerizations/97... 

Figure 9.4 Effect of Z substituent on effectiveness of RAFT agents 164 in various polymerizations. Dashed line implies limited effectiveness with a particular monomer (broad molecular weight distribution).401...

Early reports focused on the dithiobenzoale RAFT agents (Z=Ph e.g. 171-180, Table 9.10).3S2 410 Cumyl dithiobenzoate (175) shows utility with S and (meth)acrylic monomers.38 However, retardation is an issue with the acrylates... 

The trend in relative effectiveness of RAFT agents with varying Z is rationalized in terms of interaction of Z with the C=S double bond to activate or deactivate that group towards free radical addition. Substituents that facilitate addition generally retard fragmentation. O-Alkyl xanthates (Z=0-alkyl, Table... 

The Chemistry of Radical Polymerization Table 9.10 Tertiary Dithiobenzoate RAFT Agents... 

RAFT Agent Monomers0 RAFT Agent Monomers0 RAFT Agent Monomers"... 

Currently, few RAFT agents are commercially available. However, RAFT agents are available in moderate to excellent yields by a variety of methods and syntheses are generally straightforward. 

Retardation is sometimes observed in RAFT polymerizations when high concentrations of RAFT agent are used and/or with inappropriate choice of RAFT agent. Some decrease in polymerization rate is clearly attributable to a mitigation of the gel (or Norrish-Trommsdorf) effect.384" 94 However, it is also clear that other effects are important. 

Retardation has also been observed in polymerizations of S and methacrylates and is pronounced when high concentrations of dithiobenzoate RAFT agent are... 

RAFT polymerization can be performed simply by adding a chosen quantity of an appropriate RAFT agent to an otherwise conventional radical polymerization. Generally, the same monomers, initiators, solvents and temperatures are used. The only commonly encountered functionalities that appear incompatible with RAFT agents are primary and secondary amines and thiols. 

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