RAFT: Choosing the Right Agent to Achieve Controlled Polymerization

Product Manager

Sandra Forbes

The RAFT Process

RAFT, an acronym standing for Reversible Addition-Fragmentation chain Transfer, represents a unique approach to living radical polymerization. This groundbreaking technique was first unveiled by researchers at CSIRO in 1998 and swiftly garnered significant attention due to its capacity to precisely synthesize macromolecules with intricate architectures, encompassing block copolymers, grafts, combs, and star configurations, all with predefined molecular weights. RAFT polymerization boasts versatility, being compatible with an extensive array of monomers under diverse experimental setups, even facilitating the production of water-soluble materials.

The RAFT methodology entails the conventional free radical polymerization of a substituted monomer, facilitated by the presence of a specially designed chain transfer agent, also known as the RAFT agent or CTA. Among the widely employed RAFT agents are thiocarbonylthio compounds, such as dithioesters, dithiocarbamates, trithiocarbonates, and xanthates. These agents function by orchestrating the polymerization through a reversible chain-transfer mechanism. By selecting an appropriate RAFT agent, researchers can achieve the synthesis of polymers characterized by a low polydispersity index (PDI) and heightened functionality, as illustrated in Figure 1.

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

A typical RAFT chain transfer agent (CTA) features a thiocarbonylthio moiety (S=C-S), adorned with substituents R and Z, which significantly influence the kinetics of the polymerization reaction and, crucially, the level of structural precision achievable. The polymerization process is initiated through conventional means, such as thermal, photochemical, or redox techniques. The success of a RAFT polymerization experiment hinges largely on the judicious selection of RAFT reagents that are compatible with the specific monomer and reaction medium in use (refer to Figure 2 for illustrative purposes).

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

The solubility and reactivity characteristics of a RAFT agent are dictated by the nature of the R and Z groups attached to it. Consequently, various RAFT agents exhibit a preference for specific types of monomers. The primary categories of RAFT agents can be categorized as follows:

Dithiobenzoates

·Exhibiting exceptionally high transfer constants

·Susceptible to hydrolysis reactions

·Potentially leading to retardation effects at elevated concentrations

Trithiocarbonates

·Possessing high transfer constants

·Demonstrating greater hydrolytic stability compared to dithiobenzoates

·Resulting in minimal retardation

Dithiocarbamates

·The activity is governed by the substituents present on the nitrogen atom

·It proves effective when utilized with electron-rich monomers

RAFT Agent to Monomer Compatibility Table

Table 1 displays the application of RAFT agents in conjunction with commonly used monomers in polymerization reactions. The plus and minus symbols indicate the level of compatibility between different monomer classes and a particular RAFT agent. For instance, 2-Cyano-2-propyl dodecyl trithiocarbonate is highly effective for polymerizing styrenes, methacrylates, and methacrylamides, exhibits moderate activity towards acrylates and acrylamides, but is incompatible with vinyl esters and vinyl amides. This table serves as a practical reference for selecting the most suitable RAFT agent tailored to your specific requirements.

Table 1. A list RAFT agents with their suitability for various monomer types.

References

1. Chiefari J, Chong YK(, Ercole F, Krstina J, Jeffery J, Le TPT, Mayadunne RTA, Meijs GF, Moad CL, Moad G, et al. 1998. Living Free-Radical Polymerization by Reversible Addition-Fragmentation Chain Transfer: The RAFT Process. Macromolecules. 31(16):5559-5562. https://doi.org/10.1021/ma9804951

2. Moad G, Rizzardo E, Thang SH. 2005. Living Radical Polymerization by the RAFT Process. Aust. J. Chem.. 58(6):379. https://doi.org/10.1071/ch05072

3. McCormick CL, Lowe AB. 2004. Aqueous RAFT Polymerization: Recent Developments in Synthesis of Functional Water-Soluble (Co)polymers with Controlled Structures. Acc. Chem. Res.. 37(5):312-325. https://doi.org/10.1021/ar0302484

4. Mayadunne RTA, Rizzardo E, Chiefari J, Chong YK, Moad G, Thang SH. 1999. Living Radical Polymerization with Reversible Addition?Fragmentation Chain Transfer (RAFT Polymerization) Using Dithiocarbamates as Chain Transfer Agents. Macromolecules. 32(21):6977-6980. https://doi.org/10.1021/ma9906837

5. Destarac M, Charmot D, Franck X, Zard S. 2000. Macromol. Rapid. Commun. 211035-1039.

6. Mayadunne RTA, Rizzardo E, Chiefari J, Krstina J, Moad G, Postma A, Thang SH. 2000. Living Polymers by the Use of Trithiocarbonates as Reversible Addition-Fragmentation Chain Transfer (RAFT) Agents:  ABA Triblock Copolymers by Radical Polymerization in Two Steps. Macromolecules. 33(2):243-245. https://doi.org/10.1021/ma991451a

7. Francis R, Ajayaghosh A. 2000. Minimization of Homopolymer Formation and Control of Dispersity in Free Radical Induced Graft Polymerization Using Xanthate Derived Macro-photoinitiators. Macromolecules. 33(13):4699-4704. https://doi.org/10.1021/ma991604u

RAFT agents are sold for research purposes only.

Aladdinsci: https://www.aladdinsci.com