RAFT: Choosing the Right Agent to Achieve Controlled Polymerization

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.

Dr Graeme Moad, Research Team Leader at CSIRO Materials Science and Engineering — "Sigma-Aldrich Materials Science’s catalogue of diverse RAFT agents enables scientists to rationally design and synthesize novel polymeric materials."

Dr John Chiefari, Principal Research Scientist at CSIRO Materials Science and Engineering — "The variety of RAFT agents currently available allows scientists to explore new application areas as diverse as biomedical, agricultural, personal care and industrial materials."

CSIRO, the Commonwealth Scientific and Industrial Research Organisation, is Australia′s national science agency and one of the largest and most diverse research agencies in the world, and had this to say — "CSIRO’s RAFT technology is a process for making better polymers. Siigma-Aldrich Materials Science (under licence from CSIRO) offers a diverse range of innovative RAFT agents and have now expanded their catalogue to include macro-RAFT agents and copolymers prepared using RAFT technology. CSIRO is excited to note the new custom research & development services that Sigma-Aldrich Material Science offers researchers seeking to evaluate the performance of tailored RAFT polymers."



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

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.

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

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. 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.

 

RAFT Agent to Monomer Compatibility Table

The application of RAFT agents 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, Product 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 with their suitability for various monomer types.

Table 1. A list RAFT agents available from Aldrich® Materials Science with their suitability for various monomer typesCyanomethyl methyl(phenyl)carbamodithioateCyanomethyl dodecyl trithiocarbonate2-Cyano-2-propyl benzodithioate4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid2-Cyano-2-propyl dodecyl trithiocarbonate4-Cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid2-(Dodecylthiocarbonothioylthio)-2-methylpropionic acidstyrenesacrylatesacrylamidesmethacrylatesmethacrylamidesvinyl estersvinyl amides

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

 

 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.