Dithioester RAFT agents

Living Radical Polymerization Table 9.11 Other Aromatic Dithioester RAFT Agents... 

Scheme 2. Dithioester RAFT agent — general formula.

FIGURE 3.20 RAFT polymerization process based on a dithioester RAFT agent and some examples of RAFT agents (Z = modifies addition and fragmentation rates R is free radical leaving group and defines initiating ability). 

Table 10 Transfer and rate coefficients for dithioester RAFT agents...

A wide range of dithioester RAFT agents has been reported. Common examples of mono-RAFT agents and their application are provided in Tables 11 (Z = aryl) and 13 (Z = alkyl or aralkyl). RAFT agents can contain various unprotected functionality on the R fragment of dithiobenzoate including hydroxy, carboxylic acid/carboxylate, sulfonic acid/sulfonate, olefin, and siloxane. Examples of bis- and multi-dithioester RAFT agents (Z=aryl) that may be used for triblock or star synthesis are shown in Tables 12 and 22, respectively. Bis-dithioesters can be used to synthesize triblock copolymers in a two-step process. 

In solution polymerization the RAFT agent should be selected for solubility in the reaction medium. In protic media some RAFT agents show hydrolytic sensitivity which increases with RAFT agent activity (dithiobenzoates > trithiocarbonates aliphatic dithioesters). " ° " ° RAFT agents are also susceptible to nucleophilic attack by primary and secondary amines and thiols. 

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. 

Reversible addition-fragmentation transfer polymerization (RAFT) typically utilizes a dithioester transfer agent to control the concentration of propagating radicals (equation 97). ... 

An alternative approach uses reversible addition-fragmentation chain transfer (RAFT), which has fewer limitations in the selection of monomers (Chong et al, 1999) so that monomers with hydroxyl, t-amino and acid functionality may be polymerized into narrow-polydispersity block copolymers. The RAFT agent is a dithioester of the general formula S=C(Z)S-R, where Z is usually -Ph (phenyl) and R is chosen from a group of alkyl phenyls (e.g. -C(CH3)2Ph). 

RAFT in miniemulsion has also been reported" and is more readily used to produce polymers with a narrow molecular weight distribution. Moad et used RAFT in miniemulsion to provide narrow dispersity PS in a batch process. Significant retardation w as observed with the dithiobenzoate RAFT agent used. However, this is markedly reduced when aliphatic dithioesters "" or trithiocarbonate RAFT agents are used. " One of the issues with traditional miniemulsion polymerization is the high level of surfactant and co-stabilizer that is typically employed. Pham et have recently described surfactant-free... 

These reagraits have low-ehain transfer eonstants (Q = 0.6-3.5) in eontrast to the dithioesters (Q up to 6000), and this tends to produee polymers with broader moleeular weight distributions. The advantages are that they are more easily synthesized and, more importantly, can be used in emulsion polymerizations, where other RAFT agents have failed. 

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