Bis-RAFT Agents

RAFT Agent Monomers0 RAFT Agent Monomers0 RAFT Agent Monomers  

The Chemistry of Radical Polymerization Table 9.15 Symmetrical Tritbiocarbonate RAFT Agents 

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. 

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Living Radical Polymerization Table 9.13 Bis-RAFT Agents... 

Triblock copolymers can be prepared from diblock copolymers by a third monomer addition. They can also be prepared using a bis-funetional NMP or ATRP initiator or a bis-RAFT agent (for examples, see Table 9.13). Symmetrical trithiocarbonates (Table 9.15) should also be considered as bis-RAFT agents in... 

Mulli-RAFT agents have also been used to prepare segmented block copolymcrs. The molecular weight distributions obtained in these experiments are broad when compared to those obtained using analogous mono- or bis-RAFT agents. 

VPr), and simifar monomers. Other RAFT agents may be required for soiubifity or compatibility with particular polymerization media or to provide specific end-group functionality. Hie rrse of bis- and multi-RAFT agents (see Section 3.07.3.4.4) permits the synthesis of polymers with complex architectmes. Symmetrical trithiocarbonates can be considered as a member of the class of bis-RAFT agents. 

Use of a bis-RAFT agent allows the direct synthesis of triblock copolymers in a one-pot reaction. Bis-RAFT agents are described in Table 12 (bis-dithioesters). Table 16 (bis-trithiocarbonates), and Table 18 (bis-dithiocarbamates). The RAFT agent functionalities may be connected through the Z or R groups to give ABA (Scheme 35) or BAB blocks (Scheme 36), respectively. The limitations of the two forms... 

Scheme 36 BAB tribiock synthesis from R-connected bis-RAFT agent.

Scheme 6.12 ABA triblock synthesis from Z-connected bis-RAFT agents (w= 1) or symmetrical trithiocarbonates n = 0).

Low molecular weight or polymeric ATRP initiators have been converted to dithiobenzoate RAFT agents by reaction with phcnylcthyl dithiobenzoate RAFT agent441,655 or by reaction with bis(thiobenzoyl) disulfides under ATRP conditions.483 It is likely that ATRP initiators can be transformed to other forms of RAFT agent by similar methods. 

Polystyrene prepared by NMP has been reacted with thiuram disulfide compounds (i.e., tetraethyl thiuram disrrUrde and N,N -diethyl-bis[2-(ttimethylsilyl)oxyethyl] thiuram disulfide) to form the corresponding tertiary amine-functionalized polymers. Characterization of the fimrtionalized polymers by MALDl is difficult due to the labile nature of the C-S bond at the chain end, thus ESI was used to demonstrate the high functionality of the resulting polymers. More recently, Favier and co-workers ° have developed a method to convert macro-alkoxyamines to maao-RAFT agents allowing for the preparation of novel block copolymers. 

Radical-induced decomposition of a bis (thioacyl) disulfide (e.g. Scheme 3).[i .i3-i61 is probably the most used method for the synthesis of RAFT agents requiring tertiary R groups. It is also possible to use this chemistry to generate a RAFT agent in situ during polymerization. ... 

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. 

Table 12 Bis-dithioesters (Z = aryl) used as RAFT agents...

For example, the aminolysis of the chain transfer agent end group of PNIPAM prepared by RAFT exposed thiol end groups, which can be reacted with 1,8-bis-maleimidodiethyleneglycol. Ligation of bovine serum albumin and ovalbumin to the maleimide-terminated polymer yielded protein-polymer conjugates. ... 

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