Polymer Science RAFT Polymerization

    The RAFT Process
    Classes of RAFT Agents
    RAFT Agent to Monomer Compatibility Table
    Related Content

The RAFT Process

RAFT or Reversible Addition–Fragmentation chain Transfer is a form of living radical polymerization. RAFT polymerization was discovered at CSIRO in 1998.¹ It soon became the focus of intensive research, since the method allows synthetic tailoring of macromolecules with complex architectures including block, graft, comb, and star structures with predetermined molecular weight.² RAFT polymerization is applicable to a very wide range of monomers under a large number of experimental conditions, including the preparation of water-soluble materials.³

The RAFT process involves conventional free radical polymerization of a substituted monomer in the presence of a suitable chain transfer agent (RAFT agent or CTA). Commonly used RAFT agents include thiocarbonylthio compounds such as dithioesters,¹ dithiocarbamates,^4,5 trithiocarbonates,⁶ and xanthates,⁷ which mediate the polymerization via a reversible chain-transfer process. Use of a proper RAFT agent allows synthesis of polymers with low polydispersity index (PDI) and high functionality as shown below in Figure 1.

Figure 1. General comparison of polymers made with traditional radical polymerization against those made using RAFT process.

General structure of a RAFT agent is shown in Figure 2. A RAFT CTA typically has a thiocarbonylthio group (S=C-S) with substituents R and Z that impact the polymerization reaction kinetics and, importantly, the degree of structural control. Initiation of the polymerization reaction is accomplished utilizing conventional thermal, photochemical, or redox methods and it is the proper choice of the RAFT reagents appropriate for a particular monomer and reaction medium that will largely determine success of a RAFT polymerization experiment. This general concept is depicted below in Figure 2.

Figure 2. General structure of a RAFT agent; choice of the RAFT agent is critical to obtain polymers with low PDI and controlled architecture.

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Classes of RAFT Agents

Solubility and reactivity of a RAFT agent depend on the R and Z groups; as a result, different RAFT agents are more suitable for specific classes of monomers. The main classes of RAFT agents are:

Dithiobenzoates Very high transfer constants Prone to hydrolysis May cause retardation under high concentrations
Trithiocarbonates High transfer constants More hydrolytically stable (than dithiobenzoates) Cause less retardation
Dithiocarbamates Activity determined by substituents on N Effective with electron-rich monomers

These three classes of RAFT agents are now available from Aldrich^® Materials Science. For further background information about the mechanism or the RAFT polymerization and several examples of RAFT polymer synthesis, please refer to the RAFT Polymerization article in the recent Material Matters™ v.5.1.

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RAFT Agent to Monomer Compatibility Table

The application of RAFT agents available from Aldrich^® Materials Science with common monomers used in polymerizations is shown in Table 1. The pluses and minuses represent degree of compatibility between monomer classes and a RAFT agent. For example, Aldrich Prod No. 723037 (fifth item down in Table 1) is very useful in polymerizing styrenes, methacrylates and methacrylamides, shows moderate activity for acrylates and acrylamides but cannot be used on vinyl esters or vinyl amides. This table can be used as a guide for selecting the most appropriate RAFT agent for your needs.

Click on the monomer structures to see the list of available products for each monomer class in Table 1. Click on the structures of the RAFT agents to view the detailed product page.

Table 1. A list RAFT agents available from Aldrich^® Materials Science with their suitability for various monomer types. 

RAFT agents are sold for research purposes only: see www.sigmaaldrich.com/raftlicense. Patents WO98/01478, WO99/311444.

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Related Content

• RAFT Polymerization Lecture [VIDEO]
• Living Radical Polymerization Webinar
• RAFT Polymerization Protocol
• Switchable RAFT Polymerization Protocol
• RAFT Polymerization Review Article

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View a list of available monomers
View a list of the available radical initiators suitable for use in the RAFT polymerizations

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References

1. Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A. ; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559–5562.
2. Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005, 58, 379-410.
3. McCormick, C.L.; Lowe, A.B. Acc. Chem. Res. 2004, 37, 312-325.
4. Mayadunne, R.T.A.; Rizzardo, E.; Chiefari, J.; Chong, Y.K.; Moad, G.; Thang, S.H.; Macromolecules 1999, 32, 6977-6980.
5. Destarac. M.; Charmot. D.; Franck, X.; Zard, S. Z. Macromol. Rapid. Commun. 2000, 21, 1035-1039.
6. Mayadunne, R. T. A.; Rizzardo, E.; Chiefari, J.; Kristina, J.; Moad, G.; Postma, A.; Thang, S. H. Macromolecules 2000, 33, 243-245.
7. Francis, R.; Ajayaghosh, A. Macromolecules 2000, 33, 4699-4704.
   

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